Provided by: systemtap_2.9-2ubuntu2_amd64 
NAME
stap - systemtap script translator/driver
SYNOPSIS
stap [ OPTIONS ] FILENAME [ ARGUMENTS ]
stap [ OPTIONS ] - [ ARGUMENTS ]
stap [ OPTIONS ] -e SCRIPT [ ARGUMENTS ]
stap [ OPTIONS ] -l PROBE [ ARGUMENTS ]
stap [ OPTIONS ] -L PROBE [ ARGUMENTS ]
stap [ OPTIONS ] --dump-probe-types
stap [ OPTIONS ] --dump-probe-aliases
stap [ OPTIONS ] --dump-functions
DESCRIPTION
The stap program is the front-end to the Systemtap tool. It accepts probing instructions written in a
simple domain-specific language, translates those instructions into C code, compiles this C code, and
loads the resulting module into a running Linux kernel or a DynInst user-space mutator, to perform the
requested system trace/probe functions. You can supply the script in a named file (FILENAME), from
standard input (use - instead of FILENAME), or from the command line (using -e SCRIPT). The program runs
until it is interrupted by the user, or if the script voluntarily invokes the exit() function, or by
sufficient number of soft errors.
The language, which is described the SCRIPT LANGUAGE section below, is strictly typed, expressive,
declaration free, procedural, prototyping-friendly, and inspired by awk and C. It allows source code
points or events in the system to be associated with handlers, which are subroutines that are executed
synchronously. It is somewhat similar conceptually to "breakpoint command lists" in the gdb debugger.
OPTIONS
The systemtap translator supports the following options. Any other option prints a list of supported
options. Options may be given on the command line, as usual. If the file $SYSTEMTAP_DIR/rc exist,
options are also loaded from there and interpreted first. ($SYSTEMTAP_DIR defaults to $HOME/.systemtap
if unset.)
- Use standard input instead of a given FILENAME as probe language input, unless -e SCRIPT is given.
-h --help
Show help message.
-V --version
Show version message.
-p NUM Stop after pass NUM. The passes are numbered 1-5: parse, elaborate, translate, compile, run. See
the PROCESSING section for details.
-v Increase verbosity for all passes. Produce a larger volume of informative (?) output each time
option repeated.
--vp ABCDE
Increase verbosity on a per-pass basis. For example, "--vp 002" adds 2 units of verbosity to pass
3 only. The combination "-v --vp 00004" adds 1 unit of verbosity for all passes, and 4 more for
pass 5.
-k Keep the temporary directory after all processing. This may be useful in order to examine the
generated C code, or to reuse the compiled kernel object.
-g Guru mode. Enable parsing of unsafe expert-level constructs like embedded C.
-P Prologue-searching mode. This is equivalent to --prologue-searching=always. Activate heuristics
to work around incorrect debugging information for function parameter $context variables.
-u Unoptimized mode. Disable unused code elision and many other optimizations during elaboration /
translation.
-w Suppressed warnings mode. Disables all warning messages.
-W Treat all warnings as errors.
-b Use bulk mode (percpu files) for kernel-to-user data transfer. Use the stap-merge program to
multiplex them back together later.
-t Collect timing information on the number of times probe executes and average amount of time spent
in each probe-point. Also shows the derivation for each probe-point.
-sNUM Use NUM megabyte buffers for kernel-to-user data transfer. On a multiprocessor in bulk mode, this
is a per-processor amount.
-I DIR Add the given directory to the tapset search directory. See the description of pass 2 for
details.
-D NAME=VALUE
Add the given C preprocessor directive to the module Makefile. These can be used to override
limit parameters described below.
-B NAME=VALUE
Add the given make directive to the kernel module build's make invocation. These can be used to
add or override kconfig options.
-a ARCH
Use a cross-compilation mode for the given target architecture. This requires access to the
cross-compiler and the kernel build tree, and goes along with the -B
CROSS_COMPILE=arch-tool-prefix- and -r /build/tree options.
--modinfo NAME=VALUE
Add the name/value pair as a MODULE_INFO macro call to the generated module. This may be useful
to inform or override various module-related checks in the kernel.
-G NAME=VALUE
Sets the value of global variable NAME to VALUE when staprun is invoked. This applies to scalar
variables declared global in the script/tapset.
-R DIR Look for the systemtap runtime sources in the given directory.
-r /DIR
Build for kernel in given build tree. Can also be set with the SYSTEMTAP_RELEASE environment
variable.
-r RELEASE
Build for kernel in build tree /lib/modules/RELEASE/build. Can also be set with the
SYSTEMTAP_RELEASE environment variable.
-m MODULE
Use the given name for the generated kernel object module, instead of a unique randomized name.
The generated kernel object module is copied to the current directory.
-d MODULE
Add symbol/unwind information for the given module into the kernel object module. This may enable
symbolic tracebacks from those modules/programs, even if they do not have an explicit probe placed
into them.
--ldd Add symbol/unwind information for all shared libraries suspected by ldd to be necessary for user-
space binaries being probe or listed with the -d option. Caution: this can make the probe modules
considerably larger.
--all-modules
Equivalent to specifying "-dkernel" and a "-d" for each kernel module that is currently loaded.
Caution: this can make the probe modules considerably larger.
-o FILE
Send standard output to named file. In bulk mode, percpu files will start with FILE_ (FILE_cpu
with -F) followed by the cpu number. This supports strftime(3) formats for FILE.
-c CMD Start the probes, run CMD, and exit when CMD finishes. This also has the effect of setting
target() to the pid of the command ran.
-x PID Sets target() to PID. This allows scripts to be written that filter on a specific process. Scripts
run independent of the PID's lifespan.
-e SCRIPT
Run the given SCRIPT specified on the command line.
-E SCRIPT
Run the given SCRIPT specified. This SCRIPT is run in addition to the main script specified,
through -e, or as a script file. This option can be repeated to run multiple scripts, and can be
used in listing mode (-l/-L).
-l PROBE
Instead of running a probe script, just list all available probe points matching the given single
probe point. The pattern may include wildcards and aliases, but not comma-separated multiple
probe points. The process result code will indicate failure if there are no matches.
-L PROBE
Similar to "-l", but list probe points and script-level local variables.
-F Without -o option, load module and start probes, then detach from the module leaving the probes
running. With -o option, run staprun in background as a daemon and show its pid.
-S size[,N]
Sets the maximum size of output file and the maximum number of output files. If the size of
output file will exceed size , systemtap switches output file to the next file. And if the number
of output files exceed N , systemtap removes the oldest output file. You can omit the second
argument.
--skip-badvars
Ignore unresolvable or run-time-inaccessible context variables and substitute with 0, without
errors.
--prologue-searching[=WHEN]
Prologue-searching mode. Activate heuristics to work around incorrect debugging information for
function parameter $context variables. WHEN can be either "never", "always", or "auto" (i.e.
enabled by heuristic). If WHEN is missing, then "always" is assumed. If the option is missing,
then "auto" is assumed.
--suppress-handler-errors
Wrap all probe handlers into something like this
try { ... } catch { next }
block, which causes any runtime errors to be quietly suppressed. Suppressed errors do not count
against MAXERRORS limits. In this mode, the MAXSKIPPED limits are also suppressed, so that many
errors and skipped probes may be accumulated during a script's runtime. Any overall counts will
still be reported at shutdown.
--compatible VERSION
Suppress recent script language or tapset changes which are incompatible with given older version
of systemtap. This may be useful if a much older systemtap script fails to run. See the DEPRECA‐
TION section for more details.
--check-version
This option is used to check if the active script has any constructs that may be systemtap version
specific. See the DEPRECATION section for more details.
--clean-cache
This option prunes stale entries from the cache directory. This is normally done automatically
after successful runs, but this option will trigger the cleanup manually and then exit. See the
CACHING section for more details about cache limits.
--color[=WHEN], --colour[=WHEN]
This option controls coloring of error messages. WHEN can be either "never", "always", or "auto"
(i.e. enable only if at a terminal). If WHEN is missing, then "always" is assumed. If the option
is missing, then "auto" is assumed.
Colors can be modified using the SYSTEMTAP_COLORS environment variable. The format must be of the
form key1=val1:key2=val2:key3=val3 ...etc. Valid keys are "error", "warning", "source", "caret",
and "token". Values constitute Select Graphic Rendition (SGR) parameter(s). Consult the documen‐
tation of your terminal for the SGRs it supports. As an example, the default colors would be ex‐
pressed as error=01;31:warning=00;33:source=00;34:caret=01:token=01. If SYSTEMTAP_COLORS is ab‐
sent, the default colors will be used. If it is empty or invalid, coloring is turned off.
--disable-cache
This option disables all use of the cache directory. No files will be either read from or written
to the cache.
--poison-cache
This option treats files in the cache directory as invalid. No files will be read from the cache,
but resulting files from this run will still be written to the cache. This is meant as a trou‐
bleshooting aid when stap's cached behavior seems to be misbehaving.
--privilege[=stapusr | =stapsys | =stapdev]
This option instructs stap to examine the script looking for constructs which are not allowed for
the specified privilege level (see UNPRIVILEGED USERS). Compilation fails if any such constructs
are used. If stapusr or stapsys are specified when using a compile server (see --use-server), the
server will examine the script and, if compilation succeeds, the server will cryptographically
sign the resulting kernel module, certifying that is it safe for use by users at the specified
privilege level.
If --privilege has not been specified, -pN has not been specified with N < 5, and the invoking
user is not root, and is not a member of the group stapdev, then stap will automatically add the
appropriate --privilege option to the options already specified.
--unprivileged
This option is equivalent to --privilege=stapusr.
--use-server[=HOSTNAME[:PORT] | =IP_ADDRESS[:PORT] | =CERT_SERIAL]
Specify compile-server(s) to be used for compilation and/or in conjunction with --list-servers and
--trust-servers (see below). If no argument is supplied, then the default in unprivileged mode
(see --privilege) is to select compatible servers which are trusted as SSL peers and as module
signers and currently online. Otherwise the default is to select compatible servers which are
trusted as SSL peers and currently online. --use-server may be specified more than once, in which
case a list of servers is accumulated in the order specified. Servers may be specified by host
name, ip address, or by certificate serial number (obtained using --list-servers). The latter is
most commonly used when adding or revoking trust in a server (see --trust-servers below). If a
server is specified by host name or ip address, then an optional port number may be specified.
This is useful for accessing servers which are not on the local network or to specify a particular
server.
IP addresses may be IPv4 or IPv6 addresses.
If a particular IPv6 address is link local and exists on more than one interface, the intended in‐
terface may be specified by appending the address with a percent sign (%) followed by the intended
interface name. For example, "fe80::5eff:35ff:fe07:55ca%eth0".
In order to specify a port number with an IPv6 address, it is necessary to enclose the IPv6 ad‐
dress in square brackets ([]) in order to separate the port number from the rest of the address.
For example, "[fe80::5eff:35ff:fe07:55ca]:5000" or "[fe80::5eff:35ff:fe07:55ca%eth0]:5000".
If --use-server has not been specified, -pN has not been specified with N < 5, and the invoking
user not root, is not a member of the group stapdev, but is a member of the group stapusr, then
stap will automatically add --use-server to the options already specified.
--use-server-on-error[=yes|=no]
Instructs stap to retry compilation of a script using a compile server if compilation on the local
host fails in a manner which suggests that it might succeed using a server. If this option is not
specified, the default is no. If no argument is provided, then the default is yes. Compilation
will be retried for certain types of errors (e.g. insufficient data or resources) which may not
occur during re-compilation by a compile server. Compile servers will be selected automatically
for the re-compilation attempt as if --use-server was specified with no arguments.
--list-servers[=SERVERS]
Display the status of the requested SERVERS, where SERVERS is a comma-separated list of server at‐
tributes. The list of attributes is combined to filter the list of servers displayed. Supported
attributes are:
all specifies all known servers (trusted SSL peers, trusted module signers, online servers).
specified
specifies servers specified using --use-server.
online filters the output by retaining information about servers which are currently online.
trusted
filters the output by retaining information about servers which are trusted as SSL peers.
signer filters the output by retaining information about servers which are trusted as module sign‐
ers (see --privilege).
compatible
filters the output by retaining information about servers which are compatible with the
current kernel release and architecture.
If no argument is provided, then the default is specified. If no servers were specified using
--use-server, then the default servers for --use-server are listed.
Note that --list-servers uses the avahi-daemon service to detect online servers. If this service
is not available, then --list-servers will fail to detect any online servers. In order for
--list-servers to detect servers listening on IPv6 addresses, the avahi-daemon configuration file
/etc/avahi/avahi-daemon.conf must contain an active "use-ipv6=yes" line. The service must be
restarted after adding this line in order for IPv6 to be enabled.
--trust-servers[=TRUST_SPEC]
Grant or revoke trust in compile-servers, specified using --use-server as specified by TRUST_SPEC,
where TRUST_SPEC is a comma-separated list specifying the trust which is to be granted or revoked.
Supported elements are:
ssl trust the specified servers as SSL peers.
signer trust the specified servers as module signers (see --privilege). Only root can specify
signer.
all-users
grant trust as an ssl peer for all users on the local host. The default is to grant trust
as an ssl peer for the current user only. Trust as a module signer is always granted for
all users. Only root can specify all-users.
revoke revoke the specified trust. The default is to grant it.
no-prompt
do not prompt the user for confirmation before carrying out the requested action. The de‐
fault is to prompt the user for confirmation.
If no argument is provided, then the default is ssl. If no servers were specified using
--use-server, then no trust will be granted or revoked.
Unless no-prompt has been specified, the user will be prompted to confirm the trust to be granted
or revoked before the operation is performed.
--dump-probe-types
Dumps a list of supported probe types and exits. If --privilege=stapusr is also specified, the
list will be limited to probe types available to unprivileged users.
--dump-probe-aliases
Dumps a list of all probe aliases found in library files and exits.
--dump-functions
Dumps a list of all functions found in library files and exits. Also includes their parameters and
types. A function of type 'unknown' indicates a function that does not return a value. Note that
not all function/parameter types may be resolved (these are also shown by 'unknown'). This fea‐
tures is very memory-intensive and thus may not work properly with --use-server if the target
server imposes an rlimit on process memory (i.e. through the ~stap-server/.systemtap/rc configura‐
tion file, see stap-server(8)).
--remote URL
Set the execution target to the given host. This option may be repeated to target multiple execu‐
tion targets. Passes 1-4 are completed locally as normal to build the script, and then pass 5
will copy the module to the target and run it. Acceptable URL forms include:
[USER@]HOSTNAME, ssh://[USER@]HOSTNAME
This mode uses ssh, optionally using a username not matching your own. If a custom ssh_con‐
fig file is in use, add SendEnv LANG to retain internationalization functionality.
libvirt://DOMAIN, libvirt://DOMAIN/LIBVIRT_URI
This mode uses stapvirt to execute the script on a domain managed by libvirt. Optionally,
LIBVIRT_URI may be specified to connect to a specific driver and/or a remote host. For ex‐
ample, to connect to the local privileged QEMU driver, use:
--remote libvirt://MyDomain/qemu:///system
See the page at <http://libvirt.org/uri.html> for supported URIs. Also see stapvirt(1) for
more information on how to prepare the domain for stap probing.
unix:PATH
This mode connects to a UNIX socket. This can be used with a QEMU virtio-serial port for
executing scripts inside a running virtual machine.
direct://
Special loopback mode to run on the local host.
--remote-prefix
Prefix each line of remote output with "N: ", where N is the index of the remote execution target
from which the given line originated.
--download-debuginfo[=OPTION]
Enable, disable or set a timeout for the automatic debuginfo downloading feature offered by abrt
as specified by OPTION, where OPTION is one of the following:
yes enable automatic downloading of debuginfo with no timeout. This is the same as not provid‐
ing an OPTION value to --download-debuginfo
no explicitly disable automatic downloading of debuginfo. This is the same as not using the
option at all.
ask show abrt output, and ask before continuing download. No timeout will be set.
<timeout>
specify a timeout as a positive number to stop the download if it is taking too long.
--rlimit-as=NUM
Specify the maximum size of the process's virtual memory (address space), in bytes. If nothing is
specified, no limits are imposed.
--rlimit-cpu=NUM
Specify the CPU time limit, in seconds. If nothing is specified, no limits are imposed.
--rlimit-nproc=NUM
Specify the maximum number of processes that can be created. If nothing is specified, no limits
are imposed.
--rlimit-stack=NUM
Specify the maximum size of the process stack, in bytes. If nothing is specified, no limits are
imposed.
--rlimit-fsize=NUM
Specify the maximum size of files that the process may create, in bytes. If nothing is specified,
no limits are imposed.
--sysroot=DIR
Specify sysroot directory where target files (executables, libraries, etc.) are located. With -r
RELEASE, the sysroot will be searched for the appropriate kernel build directory. With -r /DIR,
however, the sysroot will not be used to find the kernel build.
--sysenv=VAR=VALUE
Provide an alternate value for an environment variable where the value on a remote system differs.
Path variables (e.g. PATH, LD_LIBRARY_PATH) are assumed to be relative to the directory provided
by --sysroot, if provided.
--suppress-time-limits
Disable -DSTP_OVERLOAD related options as well as -DMAXACTION and -DMAXTRYLOCK. This option re‐
quires guru mode.
--runtime=MODE
Set the pass-5 runtime mode. Valid options are kernel (default) and dyninst. See ALTERNATE RUN‐
TIMES below for more information.
--dyninst
Shorthand for --runtime=dyninst.
--save-uprobes
On machines that require SystemTap to build its own uprobes module (kernels prior to version 3.5),
this option instructs SystemTap to also save a copy of the module in the current directory (creat‐
ing a new "uprobes" directory first).
--target-namespaces=PID
Allow for a set of target namespaces to be set based on the namespaces the given PID is in. This
is for namespace-aware tapset functions. If the target namespaces was not set, the target defaults
to the stap process' namespaces.
ARGUMENTS
Any additional arguments on the command line are passed to the script parser for substitution. See be‐
low.
SCRIPT LANGUAGE
The systemtap script language resembles awk and C. There are two main outermost constructs: probes and
functions. Within these, statements and expressions use C-like operator syntax and precedence.
GENERAL SYNTAX
Whitespace is ignored. Three forms of comments are supported:
# ... shell style, to the end of line, except for $# and @#
// ... C++ style, to the end of line
/* ... C style ... */
Literals are either strings enclosed in double-quotes (passing through the usual C escape codes with
backslashes, and with adjacent string literals glued together, also as in C), or integers (in decimal,
hexadecimal, or octal, using the same notation as in C). All strings are limited in length to some rea‐
sonable value (a few hundred bytes). Integers are 64-bit signed quantities, although the parser also ac‐
cepts (and wraps around) values above positive 2**63.
In addition, script arguments given at the end of the command line may be inserted. Use $1 ... $<NN> for
insertion unquoted, @1 ... @<NN> for insertion as a string literal. The number of arguments may be ac‐
cessed through $# (as an unquoted number) or through @# (as a quoted number). These may be used at any
place a token may begin, including within the preprocessing stage. Reference to an argument number be‐
yond what was actually given is an error.
PREPROCESSING
A simple conditional preprocessing stage is run as a part of parsing. The general form is similar to the
cond ? exp1 : exp2 ternary operator:
%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)
The CONDITION is either an expression whose format is determined by its first keyword, or a string liter‐
als comparison or a numeric literals comparison. It can be also composed of many alternatives and con‐
junctions of CONDITIONs (meant as in previous sentence) using || and && respectively. However, parenthe‐
ses are not supported yet, so remembering that conjunction takes precedence over alternative is impor‐
tant.
If the first part is the identifier kernel_vr or kernel_v to refer to the kernel version number, with
("2.6.13-1.322FC3smp") or without ("2.6.13") the release code suffix, then the second part is one of the
six standard numeric comparison operators <, <=, ==, !=, >, and >=, and the third part is a string liter‐
al that contains an RPM-style version-release value. The condition is deemed satisfied if the version of
the target kernel (as optionally overridden by the -r option) compares to the given version string. The
comparison is performed by the glibc function strverscmp. As a special case, if the operator is for sim‐
ple equality (==), or inequality (!=), and the third part contains any wildcard characters (* or ? or [),
then the expression is treated as a wildcard (mis)match as evaluated by fnmatch.
If, on the other hand, the first part is the identifier arch to refer to the processor architecture (as
named by the kernel build system ARCH/SUBARCH), then the second part is one of the two string comparison
operators == or !=, and the third part is a string literal for matching it. This comparison is a wild‐
card (mis)match.
Similarly, if the first part is an identifier like CONFIG_something to refer to a kernel configuration
option, then the second part is == or !=, and the third part is a string literal for matching the value
(commonly "y" or "m"). Nonexistent or unset kernel configuration options are represented by the empty
string. This comparison is also a wildcard (mis)match.
If the first part is the identifier systemtap_v, the test refers to the systemtap compatibility version,
which may be overridden for old scripts with the --compatible flag. The comparison operator is as is for
kernel_v and the right operand is a version string. See also the DEPRECATION section below.
If the first part is the identifier systemtap_privilege, the test refers to the privilege level that the
systemtap script is compiled with. Here the second part is == or !=, and the third part is a string lit‐
eral, either "stapusr" or "stapsys" or "stapdev".
If the first part is the identifier guru_mode, the test refers to if the systemtap script is compiled
with guru_mode. Here the second part is == or !=, and the third part is a number, either 1 or 0.
If the first part is the identifier runtime, the test refers to the systemtap runtime mode. See ALTER‐
NATE RUNTIMES below for more information on runtimes. The second part is one of the two string compari‐
son operators == or !=, and the third part is a string literal for matching it. This comparison is a
wildcard (mis)match.
Otherwise, the CONDITION is expected to be a comparison between two string literals or two numeric liter‐
als. In this case, the arguments are the only variables usable.
The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens (possibly including nested pre‐
processor conditionals), and are passed into the input stream if the condition is true or false. For ex‐
ample, the following code induces a parse error unless the target kernel version is newer than 2.6.5:
%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence
The following code might adapt to hypothetical kernel version drift:
probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13*smp" %? "do_page_fault" %:
UNSUPPORTED %) %)
) { /* ... */ }
%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)
PREPROCESSOR MACROS
The preprocessor also supports a simple macro facility, run as a separate pass before conditional prepro‐
cessing.
Macros are defined using the following construct:
@define NAME %( BODY %)
@define NAME(PARAM_1, PARAM_2, ...) %( BODY %)
Macros, and parameters inside a macro body, are both invoked by prefixing the macro name with an @ sym‐
bol:
@define foo %( x %)
@define add(a,b) %( ((@a)+(@b)) %)
@foo = @add(2,2)
Macro expansion is currently performed in a separate pass before conditional compilation. Therefore, both
TRUE- and FALSE-tokens in conditional expressions will be macroexpanded regardless of how the condition
is evaluated. This can sometimes lead to errors:
// The following results in a conflict:
%( CONFIG_UTRACE == "y" %?
@define foo %( process.syscall %)
%:
@define foo %( **ERROR** %)
%)
// The following works properly as expected:
@define foo %(
%( CONFIG_UTRACE == "y" %? process.syscall %: **ERROR** %)
%)
The first example is incorrect because both @defines are evaluated in a pass prior to the conditional be‐
ing evaluated.
Normally, a macro definition is local to the file it occurs in. Thus, defining a macro in a tapset does
not make it available to the user of the tapset. Publically available library macros can be defined by
including .stpm files on the tapset search path. These files may only contain @define constructs, which
become visible across all tapsets and user scripts.
VARIABLES
Identifiers for variables and functions are an alphanumeric sequence, and may include _ and $ characters.
They may not start with a plain digit, as in C. Each variable is by default local to the probe or func‐
tion statement block within which it is mentioned, and therefore its scope and lifetime is limited to a
particular probe or function invocation.
Scalar variables are implicitly typed as either string or integer. Associative arrays also have a string
or integer value, and a tuple of strings and/or integers serving as a key. Here are a few basic expres‐
sions.
var1 = 5
var2 = "bar"
array1 [pid()] = "name" # single numeric key
array2 ["foo",4,i++] += 5 # vector of string/num/num keys
if (["hello",5,4] in array2) println ("yes") # membership test
The translator performs type inference on all identifiers, including array indexes and function parame‐
ters. Inconsistent type-related use of identifiers signals an error.
Variables may be declared global, so that they are shared amongst all probes and live as long as the en‐
tire systemtap session. There is one namespace for all global variables, regardless of which script file
they are found within. Concurrent access to global variables is automatically protected with locks, see
the SAFETY AND SECURITY section for more details. A global declaration may be written at the outermost
level anywhere, not within a block of code. Global variables which are written but never read will be
displayed automatically at session shutdown. The translator will infer for each its value type, and if
it is used as an array, its key types. Optionally, scalar globals may be initialized with a string or
number literal. The following declaration marks variables as global.
global var1, var2, var3=4
Global variables can also be set as module options. One can do this by either using the -G option, or the
module must first be compiled using stap -p4. Global variables can then be set on the command line when
calling staprun on the module generated by stap -p4. See staprun(8) for more information.
Arrays are limited in size by the MAXMAPENTRIES variable -- see the SAFETY AND SECURITY section for de‐
tails. Optionally, global arrays may be declared with a maximum size in brackets, overriding MAXMAPEN‐
TRIES for that array only. Note that this doesn't indicate the type of keys for the array, just the
size.
global tiny_array[10], normal_array, big_array[50000]
Arrays may be configured for wrapping using the '%' suffix. This causes older elements to be overwritten
if more elements are inserted than the array can hold. This works for both associative and statistics
typed arrays.
global wrapped_array1%[10], wrapped_array2%
Many types of probe points provide context variables, which are run-time values, safely extracted from
the kernel or userspace program being probed. These are prefixed with the $ character. The CONTEXT
VARIABLES section in stapprobes(3stap) lists what is available for each type of probe point. These con‐
text variables become normal string or numeric scalars once they are stored in normal script variables.
See the TYPECASTING section below on how to to turn them back into typed pointers for further processing
as context variables.
STATEMENTS
Statements enable procedural control flow. They may occur within functions and probe handlers. The to‐
tal number of statements executed in response to any single probe event is limited to some number defined
by a macro in the translated C code, and is in the neighbourhood of 1000.
EXP Execute the string- or integer-valued expression and throw away the value.
{ STMT1 STMT2 ... }
Execute each statement in sequence in this block. Note that separators or terminators are gener‐
ally not necessary between statements.
; Null statement, do nothing. It is useful as an optional separator between statements to improve
syntax-error detection and to handle certain grammar ambiguities.
if (EXP) STMT1 [ else STMT2 ]
Compare integer-valued EXP to zero. Execute the first (non-zero) or second STMT (zero).
while (EXP) STMT
While integer-valued EXP evaluates to non-zero, execute STMT.
for (EXP1; EXP2; EXP3) STMT
Execute EXP1 as initialization. While EXP2 is non-zero, execute STMT, then the iteration expres‐
sion EXP3.
foreach (VAR in ARRAY [ limit EXP ]) STMT
Loop over each element of the named global array, assigning current key to VAR. The array may not
be modified within the statement. By adding a single + or - operator after the VAR or the ARRAY
identifier, the iteration will proceed in a sorted order, by ascending or descending index or val‐
ue. If the array contains statistics aggregates, adding the desired @operator between the ARRAY
identifier and the + or - will specify the sorting aggregate function. See the STATISTICS section
below for the ones available. Default is @count. Using the optional limit keyword limits the
number of loop iterations to EXP times. EXP is evaluated once at the beginning of the loop.
foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT
Same as above, used when the array is indexed with a tuple of keys. A sorting suffix may be used
on at most one VAR or ARRAY identifier.
foreach ([VAR1, VAR2, ...] in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT
Same as above, where iterations are limited to elements in the array where the keys match the in‐
dex values specified. The symbol * can be used to specify an index and will be treated as a wild‐
card.
foreach (VALUE = VAR in ARRAY [ limit EXP ]) STMT
This variant of foreach saves current value into VALUE on each iteration, so it is the same as AR‐
RAY[VAR]. This also works with a tuple of keys. Sorting suffixes on VALUE have the same effect
as on ARRAY.
foreach (VALUE = VAR in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT
Same as above, where iterations are limited to elements in the array where the keys match the in‐
dex values specified. The symbol * can be used to specify an index and will be treated as a wild‐
card.
break, continue
Exit or iterate the innermost nesting loop (while or for or foreach) statement.
return EXP
Return EXP value from enclosing function. If the function's value is not taken anywhere, then a
return statement is not needed, and the function will have a special "unknown" type with no return
value.
next Return now from enclosing probe handler. This is especially useful in probe aliases that apply
event filtering predicates.
try { STMT1 } catch { STMT2 }
Run the statements in the first block. Upon any run-time errors, abort STMT1 and start executing
STMT2. Any errors in STMT2 will propagate to outer try/catch blocks, if any.
try { STMT1 } catch(VAR) { STMT2 }
Same as above, plus assign the error message to the string scalar variable VAR.
delete ARRAY[INDEX1, INDEX2, ...]
Remove from ARRAY the element specified by the index tuple. If the index tuple contains a * in
place of an index, the * is treated as a wildcard and all elements with keys that match the index
tuple will be removed from ARRAY. The value will no longer be available, and subsequent itera‐
tions will not report the element. It is not an error to delete an element that does not exist.
delete ARRAY
Remove all elements from ARRAY.
delete SCALAR
Removes the value of SCALAR. Integers and strings are cleared to 0 and "" respectively, while
statistics are reset to the initial empty state.
EXPRESSIONS
Systemtap supports a number of operators that have the same general syntax, semantics, and precedence as
in C and awk. Arithmetic is performed as per typical C rules for signed integers. Division by zero or
overflow is detected and results in an error.
binary numeric operators
* / % + - >> << & ^ | && ||
binary string operators
. (string concatenation)
numeric assignment operators
= *= /= %= += -= >>= <<= &= ^= |=
string assignment operators
= .=
unary numeric operators
+ - ! ~ ++ --
binary numeric, string comparison or regex matching operators
< > <= >= == != =~ !~
ternary operator
cond ? exp1 : exp2
grouping operator
( exp )
function call
fn ([ arg1, arg2, ... ])
array membership check
exp in array
[exp1, exp2, ...] in array
[*, *, ... ]in array
REGULAR EXPRESSION MATCHING
The scripting language supports regular expression matching. The basic syntax is as follows:
exp =~ regex
exp !~ regex
(The first operand must be an expression evaluating to a string; the second operand must be a string lit‐
eral containing a syntactically valid regular expression.)
The regular expression syntax supports most of the features of POSIX Extended Regular Expressions, except
for subexpression reuse ("\1") functionality. The ability to capture and extract the contents of the
matched string and subexpressions has not yet been implemented.
PROBES
The main construct in the scripting language identifies probes. Probes associate abstract events with a
statement block ("probe handler") that is to be executed when any of those events occur. The general
syntax is as follows:
probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
probe PROBEPOINT [, PROBEPOINT] if (CONDITION) { [STMT ...] }
Events are specified in a special syntax called "probe points". There are several varieties of probe
points defined by the translator, and tapset scripts may define further ones using aliases. Probe points
may be wildcarded, grouped, or listed in preference sequences, or declared optional. More details on
probe point syntax and semantics are listed on the stapprobes(3stap) manual page.
The probe handler is interpreted relative to the context of each event. For events associated with ker‐
nel code, this context may include variables defined in the source code at that spot. These "context
variables" are presented to the script as variables whose names are prefixed with "$". They may be ac‐
cessed only if the kernel's compiler preserved them despite optimization. This is the same constraint
that a debugger user faces when working with optimized code. In addition, the objects must exist in
paged-in memory at the moment of the systemtap probe handler's execution, because systemtap must not
cause (suppresses) any additional paging. Some probe types have very little context. See the stap‐
probes(3stap) man pages to see the kinds of context variables available at each kind of probe point.
Probes may be decorated with an arming condition, consisting of a simple boolean expression on read-only
global script variables. While disarmed (condition evaluates to false), some probe types reduce or elim‐
inate their run-time overheads. When an arming condition evaluates to true, probes will be soon re-
armed, and their probe handlers will start getting called as the events fire. (Some events may be lost
during the arming interval. If this is unacceptable, do not use arming conditions for those probes.)
New probe points may be defined using "aliases". Probe point aliases look similar to probe definitions,
but instead of activating a probe at the given point, it just defines a new probe point name as an alias
to an existing one. There are two types of alias, i.e. the prologue style and the epilogue style which
are identified by "=" and "+=" respectively.
For prologue style alias, the statement block that follows an alias definition is implicitly added as a
prologue to any probe that refers to the alias. While for the epilogue style alias, the statement block
that follows an alias definition is implicitly added as an epilogue to any probe that refers to the
alias. For example:
probe syscall.read = kernel.function("sys_read") {
fildes = $fd
if (execname() == "init") next # skip rest of probe
}
defines a new probe point syscall.read, which expands to kernel.function("sys_read"), with the given
statement as a prologue, which is useful to predefine some variables for the alias user and/or to skip
probe processing entirely based on some conditions. And
probe syscall.read += kernel.function("sys_read") {
if (tracethis) println ($fd)
}
defines a new probe point with the given statement as an epilogue, which is useful to take actions based
upon variables set or left over by the the alias user. Please note that in each case, the statements in
the alias handler block are treated ordinarily, so that variables assigned there constitute mere initial‐
ization, not a macro substitution.
An alias is used just like a built-in probe type.
probe syscall.read {
printf("reading fd=%d\n", fildes)
if (fildes > 10) tracethis = 1
}
FUNCTIONS
Systemtap scripts may define subroutines to factor out common work. Functions take any number of scalar
(integer or string) arguments, and must return a single scalar (integer or string). An example function
declaration looks like this:
function thisfn (arg1, arg2) {
return arg1 + arg2
}
Note the general absence of type declarations, which are instead inferred by the translator. However, if
desired, a function definition may include explicit type declarations for its return value and/or its ar‐
guments. This is especially helpful for embedded-C functions. In the following example, the type infer‐
ence engine need only infer type type of arg2 (a string).
function thatfn:string (arg1:long, arg2) {
return sprint(arg1) . arg2
}
Functions may call others or themselves recursively, up to a fixed nesting limit. This limit is defined
by a macro in the translated C code and is in the neighbourhood of 10.
PRINTING
There are a set of function names that are specially treated by the translator. They format values for
printing to the standard systemtap output stream in a more convenient way. The sprint* variants return
the formatted string instead of printing it.
print, sprint
Print one or more values of any type, concatenated directly together.
println, sprintln
Print values like print and sprint, but also append a newline.
printd, sprintd
Take a string delimiter and two or more values of any type, and print the values with the delim‐
iter interposed. The delimiter must be a literal string constant.
printdln, sprintdln
Print values with a delimiter like printd and sprintd, but also append a newline.
printf, sprintf
Take a formatting string and a number of values of corresponding types, and print them all. The
format must be a literal string constant.
The printf formatting directives similar to those of C, except that they are fully type-checked by the
translator:
%b Writes a binary blob of the value given, instead of ASCII text. The width specifier deter‐
mines the number of bytes to write; valid specifiers are %b %1b %2b %4b %8b. Default (%b)
is 8 bytes.
%c Character.
%d,%i Signed decimal.
%m Safely reads kernel memory at the given address, outputs its content. The optional preci‐
sion specifier (not field width) determines the number of bytes to read - default is 1
byte. %10.4m prints 4 bytes of the memory in a 10-character-wide field.
%M Same as %m, but outputs in hexadecimal. The minimal size of output is double the optional
precision specifier - default is 1 byte (2 hex chars). %10.4M prints 4 bytes of the memory
as 8 hexadecimal characters in a 10-character-wide field.
%o Unsigned octal.
%p Unsigned pointer address.
%s String.
%u Unsigned decimal.
%x Unsigned hex value, in all lower-case.
%X Unsigned hex value, in all upper-case.
%% Writes a %.
The # flag selects the alternate forms. For octal, this prefixes a 0. For hex, this prefixes 0x or 0X,
depending on case. For characters, this escapes non-printing values with either C-like escapes or raw
octal.
Examples:
a = "alice", b = "bob", p = 0x1234abcd, i = 123, j = -1, id[a] = 1234, id[b] = 4567
print("hello")
Prints: hello
println(b)
Prints: bob\n
println(a . " is " . sprint(16))
Prints: alice is 16
foreach (name in id) printdln("|", strlen(name), name, id[name])
Prints: 5|alice|1234\n3|bob|4567
printf("%c is %s; %x or %X or %p; %d or %u\n",97,a,p,p,p,j,j)
Prints: a is alice; 1234abcd or 1234ABCD or 0x1234abcd; -1 or 18446744073709551615\n
printf("2 bytes of kernel buffer at address %p: %2m", p, p)
Prints: 2 byte of kernel buffer at address 0x1234abcd: <binary data>
printf("%4b", p)
Prints (these values as binary data): 0x1234abcd
printf("%#o %#x %#X\n", 1, 2, 3)
Prints: 01 0x2 0X3
printf("%#c %#c %#c\n", 0, 9, 42)
Prints: \000 \t *
STATISTICS
It is often desirable to collect statistics in a way that avoids the penalties of repeatedly exclusive
locking the global variables those numbers are being put into. Systemtap provides a solution using a
special operator to accumulate values, and several pseudo-functions to extract the statistical aggre‐
gates.
The aggregation operator is <<<, and resembles an assignment, or a C++ output-streaming operation. The
left operand specifies a scalar or array-index lvalue, which must be declared global. The right operand
is a numeric expression. The meaning is intuitive: add the given number to the pile of numbers to com‐
pute statistics of. (The specific list of statistics to gather is given separately, by the extraction
functions.)
foo <<< 1
stats[pid()] <<< memsize
The extraction functions are also special. For each appearance of a distinct extraction function operat‐
ing on a given identifier, the translator arranges to compute a set of statistics that satisfy it. The
statistics system is thereby "on-demand". Each execution of an extraction function causes the aggrega‐
tion to be computed for that moment across all processors.
Here is the set of extractor functions. The first argument of each is the same style of lvalue used on
the left hand side of the accumulate operation. The @count(v), @sum(v), @min(v), @max(v), @avg(v) ex‐
tractor functions compute the number/total/minimum/maximum/average of all accumulated values. The re‐
sulting values are all simple integers. Arrays containing aggregates may be sorted and iterated. See
the foreach construct above.
Histograms are also available, but are more complicated because they have a vector rather than scalar
value. @hist_linear(v,start,stop,interval) represents a linear histogram from "start" to "stop" by in‐
crements of "interval". The interval must be positive. Similarly, @hist_log(v) represents a base-2 loga‐
rithmic histogram. Printing a histogram with the print family of functions renders a histogram object as
a tabular "ASCII art" bar chart.
probe timer.profile {
x[1] <<< pid()
x[2] <<< uid()
y <<< tid()
}
global x // an array containing aggregates
global y // a scalar
probe end {
foreach ([i] in x @count+) {
printf ("x[%d]: avg %d = sum %d / count %d\n",
i, @avg(x[i]), @sum(x[i]), @count(x[i]))
println (@hist_log(x[i]))
}
println ("y:")
println (@hist_log(y))
}
TYPECASTING
Once a pointer (see the CONTEXT VARIABLES section of stapprobes(3stap)) has been saved into a script in‐
teger variable, the translator loses the type information necessary to access members from that pointer.
Using the @cast() operator tells the translator how to interpret the number as a typed pointer.
@cast(p, "type_name"[, "module"])->member
This will interpret p as a pointer to a struct/union named type_name and dereference the member value.
Further ->subfield expressions may be appended to dereference more levels. NOTE: the same dereferencing
operator -> is used to refer to both direct containment or pointer indirection. Systemtap automatically
determines which. The optional module tells the translator where to look for information about that
type. Multiple modules may be specified as a list with : separators. If the module is not specified, it
will default either to the probe module for dwarf probes, or to "kernel" for functions and all other
probes types.
The translator can create its own module with type information from a header surrounded by angle brack‐
ets, in case normal debuginfo is not available. For kernel headers, prefix it with "kernel" to use the
appropriate build system. All other headers are build with default GCC parameters into a user module.
Multiple headers may be specified in sequence to resolve a codependency.
@cast(tv, "timeval", "<sys/time.h>")->tv_sec
@cast(task, "task_struct", "kernel<linux/sched.h>")->tgid
@cast(task, "task_struct",
"kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask
Values acquired by @cast may be pretty-printed by the $ and $$ suffix operators, the same way as de‐
scribed in the CONTEXT VARIABLES section of the stapprobes(3stap) manual page.
When in guru mode, the translator will also allow scripts to assign new values to members of typecasted
pointers.
Typecasting is also useful in the case of void* members whose type may be determinable at runtime.
probe foo {
if ($var->type == 1) {
value = @cast($var->data, "type1")->bar
} else {
value = @cast($var->data, "type2")->baz
}
print(value)
}
EMBEDDED C
When in guru mode, the translator accepts embedded code in the top level of the script. Such code is en‐
closed between %{ and %} markers, and is transcribed verbatim, without analysis, in some sequence, into
the top level of the generated C code. At the outermost level, this may be useful to add #include in‐
structions, and any auxiliary definitions for use by other embedded code.
Another place where embedded code is permitted is as a function body. In this case, the script language
body is replaced entirely by a piece of C code enclosed again between %{ and %} markers. This C code may
do anything reasonable and safe. There are a number of undocumented but complex safety constraints on
atomicity, concurrency, resource consumption, and run time limits, so this is an advanced technique.
The memory locations set aside for input and output values are made available to it using macros
STAP_ARG_* and STAP_RETVALUE. Errors may be signalled with STAP_ERROR. Output may be written with
STAP_PRINTF. The function may return early with STAP_RETURN. Here are some examples:
function integer_ops (val) %{
STAP_PRINTF("%d\n", STAP_ARG_val);
STAP_RETVALUE = STAP_ARG_val + 1;
if (STAP_RETVALUE == 4)
STAP_ERROR("wrong guess: %d", (int) STAP_RETVALUE);
if (STAP_RETVALUE == 3)
STAP_RETURN(0);
STAP_RETVALUE ++;
%}
function string_ops (val) %{
strlcpy (STAP_RETVALUE, STAP_ARG_val, MAXSTRINGLEN);
strlcat (STAP_RETVALUE, "one", MAXSTRINGLEN);
if (strcmp (STAP_RETVALUE, "three-two-one"))
STAP_RETURN("parameter should be three-two-");
%}
function no_ops () %{
STAP_RETURN(); /* function inferred with no return value */
%}
The function argument and return value types have to be inferred by the translator from the call sites in
order for this to work. The user should examine C code generated for ordinary script-language functions
in order to write compatible embedded-C ones.
The last place where embedded code is permitted is as an expression rvalue. In this case, the C code en‐
closed between %{ and %} markers is interpreted as an ordinary expression value. It is assumed to be a
normal 64-bit signed number, unless the marker /* string */ is included, in which case it's treated as a
string.
function add_one (val) {
return val + %{ 1 %}
}
function add_string_two (val) {
return val . %{ /* string */ "two" %}
}
The embedded-C code may contain markers to assert optimization and safety properties.
/* pure */
means that the C code has no side effects and may be elided entirely if its value is not used by
script code.
/* stable */
means that the C code always has the same value (in any given probe handler invocation), so re‐
peated calls may be automatically replaced by memoized values. Such functions must take no para‐
meters, and also be pure.
/* unprivileged */
means that the C code is so safe that even unprivileged users are permitted to use it.
/* myproc-unprivileged */
means that the C code is so safe that even unprivileged users are permitted to use it, provided
that the target of the current probe is within the user's own process.
/* guru */
means that the C code is so unsafe that a systemtap user must specify -g (guru mode) to use this.
/* unmangled */
in an embedded-C function, means that the legacy (pre-1.8) argument access syntax should be made
available inside the function. Hence, in addition to STAP_ARG_foo and STAP_RETVALUE one can use
THIS->foo and THIS->__retvalue respectively inside the function. This is useful for quickly mi‐
grating code written for SystemTap version 1.7 and earlier.
/* unmodified-fnargs */
in an embedded-C function, means that the function arguments are not modified inside the function
body.
/* string */
in embedded-C expressions only, means that the expression has const char * type and should be
treated as a string value, instead of the default long numeric.
BUILT-INS
A set of builtin probe point aliases are provided by the scripts installed in the directory specified in
the stappaths(7) manual page. The functions are described in the stapprobes(3stap) manual page.
PROCESSING
The translator begins pass 1 by parsing the given input script, and all scripts (files named *.stp) found
in a tapset directory. The directories listed with -I are processed in sequence, each processed in "guru
mode". For each directory, a number of subdirectories are also searched. These subdirectories are de‐
rived from the selected kernel version (the -R option), in order to allow more kernel-version-specific
scripts to override less specific ones. For example, for a kernel version 2.6.12-23.FC3 the following
patterns would be searched, in sequence: 2.6.12-23.FC3/*.stp, 2.6.12/*.stp, 2.6/*.stp, and finally *.stp.
Stopping the translator after pass 1 causes it to print the parse trees.
In pass 2, the translator analyzes the input script to resolve symbols and types. References to vari‐
ables, functions, and probe aliases that are unresolved internally are satisfied by searching through the
parsed tapset script files. If any tapset script file is selected because it defines an unresolved sym‐
bol, then the entirety of that file is added to the translator's resolution queue. This process iterates
until all symbols are resolved and a subset of tapset script files is selected.
Next, all probe point descriptions are validated against the wide variety supported by the translator.
Probe points that refer to code locations ("synchronous probe points") require the appropriate kernel de‐
bugging information to be installed. In the associated probe handlers, target-side variables (whose
names begin with "$") are found and have their run-time locations decoded.
Next, all probes and functions are analyzed for optimization opportunities, in order to remove variables,
expressions, and functions that have no useful value and no side-effect. Embedded-C functions are as‐
sumed to have side-effects unless they include the magic string /* pure */. Since this optimization can
hide latent code errors such as type mismatches or invalid $context variables, it sometimes may be useful
to disable the optimizations with the -u option.
Finally, all variable, function, parameter, array, and index types are inferred from context (literals
and operators). Stopping the translator after pass 2 causes it to list all the probes, functions, and
variables, along with all inferred types. Any inconsistent or unresolved types cause an error.
In pass 3, the translator writes C code that represents the actions of all selected script files, and
creates a Makefile to build that into a kernel object. These files are placed into a temporary directo‐
ry. Stopping the translator at this point causes it to print the contents of the C file.
In pass 4, the translator invokes the Linux kernel build system to create the actual kernel object file.
This involves running make in the temporary directory, and requires a kernel module build system (head‐
ers, config and Makefiles) to be installed in the usual spot /lib/modules/VERSION/build. Stopping the
translator after pass 4 is the last chance before running the kernel object. This may be useful if you
want to archive the file.
In pass 5, the translator invokes the systemtap auxiliary program staprun program for the given kernel
object. This program arranges to load the module then communicates with it, copying trace data from the
kernel into temporary files, until the user sends an interrupt signal. Any run-time error encountered by
the probe handlers, such as running out of memory, division by zero, exceeding nesting or runtime limits,
results in a soft error indication. Soft errors in excess of MAXERRORS block of all subsequent probes
(except error-handling probes), and terminate the session. Finally, staprun unloads the module, and
cleans up.
ABNORMAL TERMINATION
One should avoid killing the stap process forcibly, for example with SIGKILL, because the stapio process
(a child process of the stap process) and the loaded module may be left running on the system. If this
happens, send SIGTERM or SIGINT to any remaining stapio processes, then use rmmod to unload the systemtap
module.
EXAMPLES
See the stapex(3stap) manual page for a brief collection of samples, or a large set of installed samples
under the systemtap documentation/testsuite directories. See stappaths(7stap) for the likely location of
these on the system.
CACHING
The systemtap translator caches the pass 3 output (the generated C code) and the pass 4 output (the com‐
piled kernel module) if pass 4 completes successfully. This cached output is reused if the same script
is translated again assuming the same conditions exist (same kernel version, same systemtap version,
etc.). Cached files are stored in the $SYSTEMTAP_DIR/cache directory. The cache can be limited by having
the file cache_mb_limit placed in the cache directory (shown above) containing only an ASCII integer rep‐
resenting how many MiB the cache should not exceed. In the absence of this file, a default will be creat‐
ed with the limit set to 256MiB. This is a 'soft' limit in that the cache will be cleaned after a new
entry is added if the cache clean interval is exceeded, so the total cache size may temporarily exceed
this limit. This interval can be specified by having the file cache_clean_interval_s placed in the cache
directory (shown above) containing only an ASCII integer representing the interval in seconds. In the ab‐
sence of this file, a default will be created with the interval set to 300 s.
SAFETY AND SECURITY
Systemtap is may be used as a powerful administrative tool. It can expose kernel internal data struc‐
tures and potentially private user information. (In dyninst runtime mode, this is not the case, see the
ALTERNATE RUNTIMES section below.)
The translator asserts many safety constraints during compilation and more during run-time. It aims to
ensure that no handler routine can run for very long, allocate boundless memory, perform unsafe opera‐
tions, or in unintentionally interfere with the system. Uses of script global variables are automatical‐
ly read/write locked as appropriate, to protect against manipulation by concurrent probe handlers.
(Deadlocks are detected with timeouts. Use the -t flag to receive reports of excessive lock contention.)
Experimenting with scripts is therefore generally safe. The guru-mode -g option allows administrators to
bypass most safety measures, which permits invasive or state-changing operations, embedded-C code, and
increases the risk of upset. By default, overload prevention is turned on for all modules. If you would
like to disable overload processing, use the --suppress-time-limits option.
Errors that are caught at run time normally result in a clean script shutdown and a pass-5 error message.
The --suppress-handler-errors option lets scripts tolerate soft errors without shutting down.
PERMISSIONS
For the normal linux-kernel-module runtime, to run the kernel objects systemtap builds, a user must be
one of the following:
• the root user;
• a member of the stapdev and stapusr groups;
• a member of the stapsys and stapusr groups; or
• a member of the stapusr group.
The root user or a user who is a member of both the stapdev and stapusr groups can build and run any sys‐
temtap script.
A user who is a member of both the stapsys and stapusr groups can only use pre-built modules under the
following conditions:
• The module has been signed by a trusted signer. Trusted signers are normally systemtap com‐
pile-servers which sign modules when the --privilege option is specified by the client. See the
stap-server(8) manual page for more information.
• The module was built using the --privilege=stapsys or the --privilege=stapusr options.
Members of only the stapusr group can only use pre-built modules under the following conditions:
• The module is located in the /lib/modules/VERSION/systemtap directory. This directory must be owned
by root and not be world writable.
or
• The module has been signed by a trusted signer. Trusted signers are normally systemtap com‐
pile-servers which sign modules when the --privilege option is specified by the client. See the
stap-server(8) manual page for more information.
• The module was built using the --privilege=stapusr option.
The kernel modules generated by stap program are run by the staprun program. The latter is a part of the
Systemtap package, dedicated to module loading and unloading (but only in the white zone), and kernel-to-
user data transfer. Since staprun does not perform any additional security checks on the kernel objects
it is given, it would be unwise for a system administrator to add untrusted users to the stapdev or sta‐
pusr groups.
SECUREBOOT
If the current system has SecureBoot turned on in the UEFI firmware, all kernel modules must be signed.
(Some kernels may allow disabling SecureBoot long after booting with a key sequence such as SysRq-X, mak‐
ing it unnecessary to sign modules.) The systemtap compile server can sign modules with a MOK (Machine
Owner Key) that it has in common with a client system. See the following wiki page for more details:
https://sourceware.org/systemtap/wiki/SecureBoot
RESOURCE LIMITS
Many resource use limits are set by macros in the generated C code. These may be overridden with -D
flags. A selection of these is as follows:
MAXNESTING
Maximum number of nested function calls. Default determined by script analysis, with a bonus 10
slots added for recursive scripts.
MAXSTRINGLEN
Maximum length of strings, default 128.
MAXTRYLOCK
Maximum number of iterations to wait for locks on global variables before declaring possible dead‐
lock and skipping the probe, default 1000.
MAXACTION
Maximum number of statements to execute during any single probe hit (with interrupts disabled),
default 1000. Note that for straight-through probe handlers lacking loops or recursion, due to
optimization, this parameter may be interpreted too conservatively.
MAXACTION_INTERRUPTIBLE
Maximum number of statements to execute during any single probe hit which is executed with inter‐
rupts enabled (such as begin/end probes), default (MAXACTION * 10).
MAXBACKTRACE
Maximum number of stack frames that will be be processed by the stap runtime unwinder as produced
by the backtrace functions in the [u]context-unwind.stp tapsets, default 20.
MAXMAPENTRIES
Default maximum number of rows in any single global array, default 2048. Individual arrays may be
declared with a larger or smaller limit instead:
global big[10000],little[5]
or denoted with % to make them wrap-around automatically.
MAXERRORS
Maximum number of soft errors before an exit is triggered, default 0, which means that the first
error will exit the script. Note that with the --suppress-handler-errors option, this limit is
not enforced.
MAXSKIPPED
Maximum number of skipped probes before an exit is triggered, default 100. Running systemtap with
-t (timing) mode gives more details about skipped probes. With the default -DINTERRUPTIBLE=1 set‐
ting, probes skipped due to reentrancy are not accumulated against this limit. Note that with the
--suppress-handler-errors option, this limit is not enforced.
MINSTACKSPACE
Minimum number of free kernel stack bytes required in order to run a probe handler, default 1024.
This number should be large enough for the probe handler's own needs, plus a safety margin.
MAXUPROBES
Maximum number of concurrently armed user-space probes (uprobes), default somewhat larger than the
number of user-space probe points named in the script. This pool needs to be potentially large
because individual uprobe objects (about 64 bytes each) are allocated for each process for each
matching script-level probe.
STP_MAXMEMORY
Maximum amount of memory (in kilobytes) that the systemtap module should use, default unlimited.
The memory size includes the size of the module itself, plus any additional allocations. This on‐
ly tracks direct allocations by the systemtap runtime. This does not track indirect allocations
(as done by kprobes/uprobes/etc. internals).
STP_OVEROAD_THRESHOLD, STP_OVERLOAD_INTERVAL
Maximum number of machine cycles spent in probes on any cpu per given interval, before an overload
condition is declared and the script shut down. The defaults are 500 million and 1 billion, so as
to limit stap script cpu consumption at around 50%.
STP_PROCFS_BUFSIZE
Size of procfs probe read buffers (in bytes). Defaults to MAXSTRINGLEN. This value can be over‐
ridden on a per-procfs file basis using the procfs read probe .maxsize(MAXSIZE) parameter.
With scripts that contain probes on any interrupt path, it is possible that those interrupts may occur in
the middle of another probe handler. The probe in the interrupt handler would be skipped in this case to
avoid reentrance. To work around this issue, execute stap with the option -DINTERRUPTIBLE=0 to mask in‐
terrupts throughout the probe handler. This does add some extra overhead to the probes, but it may pre‐
vent reentrance for common problem cases. However, probes in NMI handlers and in the callpath of the
stap runtime may still be skipped due to reentrance.
Multiple scripts can write data into a relay buffer concurrently. A host script provides an interface for
accessing its relay buffer to guest scripts. Then, the output of the guests are merged into the output
of the host. To run a script as a host, execute stap with -DRELAYHOST[=name] option. The name identifies
your host script among several hosts. While running the host, execute stap with -DRELAYGUEST[=name] to
add a guest script to the host. Note that you must unload guests before unloading a host. If there are
some guests connected to the host, unloading the host will be failed.
In case something goes wrong with stap or staprun after a probe has already started running, one may
safely kill both user processes, and remove the active probe kernel module with rmmod. Any pending trace
messages may be lost.
UNPRIVILEGED USERS
Systemtap exposes kernel internal data structures and potentially private user information. Because of
this, use of systemtap's full capabilities are restricted to root and to users who are members of the
groups stapdev and stapusr.
However, a restricted set of systemtap's features can be made available to trusted, unprivileged users.
These users are members of the group stapusr only, or members of the groups stapusr and stapsys. These
users can load systemtap modules which have been compiled and certified by a trusted systemtap com‐
pile-server. See the descriptions of the options --privilege and --use-server. See README.unprivileged in
the systemtap source code for information about setting up a trusted compile server.
The restrictions enforced when --privilege=stapsys is specified are designed to prevent unprivileged
users from:
• harming the system maliciously.
The restrictions enforced when --privilege=stapusr is specified are designed to prevent unprivileged
users from:
• harming the system maliciously.
• gaining access to information which would not normally be available to an unprivileged user.
• disrupting the performance of processes owned by other users of the system. Some overhead to
the system in general is unavoidable since the unprivileged user's probes will be triggered
at the appropriate times. What we would like to avoid is targeted interruption of another
user's processes which would not normally be possible by an unprivileged user.
PROBE RESTRICTIONS
A member of the groups stapusr and stapsys may use all probe points.
A member of only the group stapusr may use only the following probes:
• begin, begin(n)
• end, end(n)
• error(n)
• never
• process.*, where the target process is owned by the user.
• timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*
• timer.hz(n)
SCRIPT LANGUAGE RESTRICTIONS
The following scripting language features are unavailable to all unprivileged users:
• any feature enabled by the Guru Mode (-g) option.
• embedded C code.
RUNTIME RESTRICTIONS
The following runtime restrictions are placed upon all unprivileged users:
• Only the default runtime code (see -R) may be used.
Additional restrictions are placed on members of only the group stapusr:
• Probing of processes owned by other users is not permitted.
• Access of kernel memory (read and write) is not permitted.
COMMAND LINE OPTION RESTRICTIONS
Some command line options provide access to features which must not be available to all unprivileged
users:
• -g may not be specified.
• The following options may not be used by the compile-server client:
-a, -B, -D, -I, -r, -R
ENVIRONMENT RESTRICTIONS
The following environment variables must not be set for all unprivileged users:
SYSTEMTAP_RUNTIME
SYSTEMTAP_TAPSET
SYSTEMTAP_DEBUGINFO_PATH
TAPSET RESTRICTIONS
In general, tapset functions are only available for members of the group stapusr when they do not gather
information that an ordinary program running with that user's privileges would be denied access to.
There are two categories of unprivileged tapset functions. The first category consists of utility func‐
tions that are unconditionally available to all users; these include such things as:
cpu:long ()
exit ()
str_replace:string (prnt_str:string, srch_str:string, rplc_str:string)
The second category consists of so-called myproc-unprivileged functions that can only gather information
within their own processes. Scripts that wish to use these functions must test the result of the tapset
function is_myproc and only call these functions if the result is 1. The script will exit immediately if
any of these functions are called by an unprivileged user within a probe within a process which is not
owned by that user. Examples of myproc-unprivileged functions include:
print_usyms (stk:string)
user_int:long (addr:long)
usymname:string (addr:long)
A compile error is triggered when any function not in either of the above categories is used by members
of only the group stapusr.
No other built-in tapset functions may be used by members of only the group stapusr.
ALTERNATE RUNTIMES
As described above, systemtap's default runtime mode involves building and loading kernel modules, with
various security tradeoffs presented. Systemtap now includes a new prototype backend, selected with
--runtime=dyninst, which uses Dyninst to instrument a user's own processes at runtime. This backend does
not use kernel modules, and does not require root privileges, but is restricted with respect to the kinds
of probes and other constructs that a script may use.
The dyninst runtime operates in target-attach mode, so it does require a -c COMMAND or -x PID process.
For example:
stap --runtime=dyninst -c 'stap -V' \
-e 'probe process.function("main")
{ println("hi from dyninst!") }'
It may be necessary to disable a conflicting selinux check with
# setsebool allow_execstack 1
EXIT STATUS
The systemtap translator generally returns with a success code of 0 if the requested script was processed
and executed successfully through the requested pass. Otherwise, errors may be printed to stderr and a
failure code is returned. Use -v or -vp N to increase (global or per-pass) verbosity to identify the
source of the trouble.
In listings mode (-l and -L), error messages are normally suppressed. A success code of 0 is returned if
at least one matching probe was found.
A script executing in pass 5 that is interrupted with ^C / SIGINT is considered to be successful.
DEPRECATION
Over time, some features of the script language and the tapset library may undergo incompatible changes,
so that a script written against an old version of systemtap may no longer run. In these cases, it may
help to run systemtap with the --compatible VERSION flag, specifying the last known working version.
Running systemtap with the --check-version flag will output a warning if any possible incompatible ele‐
ments have been parsed. Deprecation historical details may be found in the NEWS file.
FILES
Important files and their corresponding paths can be located in the
stappaths (7) manual page.
SEE ALSO
stapprobes(3stap),
function::*(3stap),
probe::*(3stap),
tapset::*(3stap),
stappaths(7),
staprun(8),
stapdyn(8),
systemtap(8),
stapvars(3stap),
stapex(3stap),
stap-server(8),
stap-prep(1),
stapref(1),
awk(1),
gdb(1)
BUGS
Use the Bugzilla link of the project web page or our mailing list. http://sourceware.org/systemtap/,
<systemtap@sourceware.org>.
error::reporting(7stap), https://sourceware.org/systemtap/wiki/HowToReportBugs
STAP(1)