Provided by: uftrace_0.13-1_amd64 
NAME
uftrace-live - Trace functions in a command during live execution
SYNOPSIS
uftrace [live] [options] COMMAND [command-options]
DESCRIPTION
This command runs COMMAND and prints its functions with time and thread info. This is
basically the same as running the uftrace record and uftrace replay commands in turn, but
it does not save a data file. This command accepts most options that are accepted by the
record or replay commands.
COMMON OPTIONS
-F FUNC, --filter=FUNC
Set filter to trace selected functions and their children functions. This option
can be used more than once. See FILTERS.
-N FUNC, --notrace=FUNC
Set filter not to trace selected functions and their children functions. This
option can be used more than once. See FILTERS.
-C FUNC, --caller-filter=FUNC
Set filter to trace callers of selected functions only. This option can be used
more than once. See FILTERS.
-T TRG, --trigger=TRG
Set trigger on selected functions. This option can be used more than once. See
TRIGGERS.
-D DEPTH, --depth=DEPTH
Set global trace limit in nesting level. See FILTERS.
-t TIME, --time-filter=TIME
Do not show functions which run under the time threshold. If some functions
explicitly have the `trace' trigger applied, those are always traced regardless of
execution time. See FILTERS.
-Z SIZE, --size-filter=SIZE
Do not show functions smaller than SIZE bytes. See FILTERS.
-L LOCATION, --loc-filter=LOCATION
Set filter to trace selected source locations. This option can be used more than
once. Applies to replay command, not record. See FILTERS.
--no-libcall
Do not record library function invocations. Library calls are normally traced by
hooking calls to the resolver function of dynamic linker in the PLT. One can
disable it with this option.
--no-event
Disable event recording which is used by default. Note that explicit event tracing
by --event option is not affected by this. Implies --no-sched.
--no-sched
Disable schedule event recording which is used by default.
--match=TYPE
Use pattern match using TYPE. Possible types are regex and glob. Default is
regex.
--disable
Start uftrace with tracing disabled. This is only meaningful when used with a
trace_on trigger.
--with-syms=DIR
Read symbol data from the .sym files in DIR directory instead of the binary. This
can be useful to deal with stripped binaries. The file name of the main binary
should be the same when saved and used.
LIVE OPTIONS
--list-event
Show available events in the process.
--report
Show live-report before replay.
--record
Do not discard the recorded data.
RECORD OPTIONS
-A SPEC, --argument=SPEC
Record function arguments. This option can be used more than once. See ARGUMENTS.
-R SPEC, --retval=SPEC
Record function return values. This option can be used more than once. See
ARGUMENTS.
-P FUNC, --patch=FUNC
Patch FUNC dynamically. This is only applicable binaries built by gcc with -pg
-mfentry -mnop-mcount or clang with -fxray-instrument. This option can be used
more than once. See DYNAMIC TRACING.
-U FUNC, --unpatch=FUNC
Do not apply dynamic patching for FUNC. This option can be used more than once.
See DYNAMIC TRACING.
-E EVENT, --event=EVENT
Enable event tracing. The event should be available on the system.
-S SCRIPT_PATH, --script=SCRIPT_PATH
Run a given script to do additional work at the entry and exit of function during
target program execution. The type of script is detected by the postfix such as
`.py' for python. See SCRIPT EXECUTION.
-W, --watch=POINT
Add watch point to display POINT if the value is changed. See WATCH POINT.
-a, --auto-args
Automatically record arguments and return values of known functions. These are
usually functions in standard (C language or system) libraries but if debug info is
available it includes functions in the user program.
-l, --nest-libcall
Trace function calls between libraries. By default, uftrace only record library
call from the main executable. Implies --force.
-k, --kernel
Trace kernel functions as well as user functions. Only kernel entry/exit functions
will be traced by default. Use the --kernel-depth option to override this.
-K DEPTH, --kernel-depth=DEPTH
Set kernel max function depth separately. Implies --kernel.
--clock=CLOCK
Set clock source for timestamp recording. CLOCK can be one of `mono', `mono_raw',
or `boot'. Default is `mono'.
--signal=TRG
Set trigger on selected signals rather than functions. But there are restrictions
so only a few of trigger actions are support for signals. The available actions
are: trace_on, trace_off, finish. This option can be used more than once. See
TRIGGERS.
--nop Do not record and replay any functions. This is a no-op and only meaningful for
performance comparisons.
--force
Allow running uftrace even if some problems occur. When uftrace record finds no
mcount symbol (which is generated by compiler) in the executable, it quits with an
error message since uftrace can not trace the program. However, it is possible
that the user is only interested in functions within a dynamically-linked library,
in which case this option can be used to cause uftrace to run the program
regardless. Also, the -A/--argument and -R/--retval options work only for binaries
built with -pg, so uftrace will normally exit when it tries to run binaries built
without that option. This option ignores the warning and goes on tracing without
the argument and/or return value.
--time Print running time of children in time(1)-style.
-e, --estimate-return
Record only ENTRY data for each function. This option is useful when the target
program deals with stack in some way. Normally uftrace modifies task’s execution
stack frame to hook return from the function. However sometimes it makes troubles
and it’s hard to handle all the cases properly. This option tells uftrace not to
hook return address in order to prevent those problems. The return time is
estimated as a half of execution time of two consecutive functions.
RECORD CONFIG OPTIONS
--libmcount-path=PATH
Load libmcount libraries from this path. This is mostly for testing purposes.
-b SIZE, --buffer=SIZE
Size of internal buffer in which trace data will be saved. Default size is 128k.
--kernel-buffer=SIZE
Set kernel tracing buffer size. The default value (in the kernel) is 1408k.
--no-pltbind
Do not bind dynamic symbol address. This option uses the LD_BIND_NOT environment
variable to trace library function calls which might be missing due to concurrent
(first) accesses. It is not meaningful to use this option with the --no-libcall
option.
--max-stack=DEPTH
Set the max function stack depth for tracing. Default is 1024.
--num-thread=NUM
Use NUM threads to record trace data. Default is 1/4 of online CPUs (but when full
kernel tracing is enabled, it will use the full number of CPUs).
--libmcount-single
Use single thread version of libmcount for faster recording. This is ignored if
the target program links with the pthread library.
--rt-prio=PRIO
Boost priority of recording threads to real-time (FIFO) with priority of PRIO.
This is particularly useful for high-volume data such as full kernel tracing.
--keep-pid
Retain same pid for traced program. For some daemon processes, it is important to
have same pid when forked. Running under uftrace normally changes pid as it calls
fork() again internally. Note that it might corrupt terminal setting so it’d be
better using it with --no-pager option.
--no-randomize-addr
Disable ASLR (Address Space Layout Randomization). It makes the target process fix
its address space layout.
REPLAY OPTIONS
-f FIELD, --output-fields=FIELD
Customize field in the output. Possible values are: duration, tid, time, delta,
elapsed and addr. Multiple fields can be set by using comma. Special field of
`none' can be used (solely) to hide all fields. Default is `duration,tid'. See
FIELDS.
--flat Print flat format rather than C-like format. This is usually for debugging and
testing purpose.
--column-view
Show each task in separate column. This makes easy to distinguish functions in
different tasks.
--column-offset=DEPTH
When --column-view option is used, this option specifies the amount of offset
between each task. Default is 8.
--task-newline
Interleave a new line when task is changed. This makes easy to distinguish
functions in different tasks.
--no-comment
Do not show comments of returned functions.
--libname
Show library name along with function name.
--no-args
Do not show function arguments and return value.
COMMON ANALYSIS OPTIONS
-H FUNC, --hide=FUNC
Set filter not to trace selected functions. It doesn’t affect their subtrees, but
hides only the given functions. This option can be used more than once. See
FILTERS.
--kernel-full
Show all kernel functions called outside of user functions.
--kernel-only
Show kernel functions only without user functions.
--event-full
Show all (user) events outside of user functions.
--demangle=TYPE
Demangle C++ symbol names. Possible values are “full”, “simple” and “no”. Default
is “simple” which ignores function arguments and template parameters.
-r RANGE, --time-range=RANGE
Only show functions executed within the time RANGE. The RANGE can be
<start>~<stop> (separated by “~”) and one of <start> and <stop> can be omitted.
The <start> and <stop> are timestamp or elapsed time if they have <time_unit>
postfix, for example `100us'. However, it is highly recommended to use only
elapsed time because there is no way to know the timestamp before actually running
the program. The timestamp or elapsed time can be shown with -f time or -f elapsed
option respectively.
FILTERS
The uftrace tool supports filtering out uninteresting functions. Filtering is highly
recommended since it helps users focus on the interesting functions and reduces the data
size. When uftrace is called, it receives two types of function filter; an opt-in filter
with -F/--filter and an opt-out filter with -N/--notrace. These filters can be applied
either at record time or replay time.
The first one is an opt-in filter. By default, it doesn’t trace anything. But when one
of the specified functions is executed, tracing is started. When the function returns,
tracing is stopped again.
For example, consider a simple program which calls a(), b() and c() in turn.
$ cat abc.c
void c(void) {
/* do nothing */
}
void b(void) {
c();
}
void a(void) {
b();
}
int main(void) {
a();
return 0;
}
$ gcc -pg -o abc abc.c
Normally uftrace will trace all the functions from main() to c().
$ uftrace live ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
[ 1234] | a() {
[ 1234] | b() {
3.880 us [ 1234] | c();
5.475 us [ 1234] | } /* b */
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
In the above example, the command name live is explicitly used, but it can be omitted
because uftrace uses live command by default. So the above command can be reused as
uftrace ./abc in short.
But when the -F b filter option is used, it will not trace main() or a() but only b() and
c().
$ uftrace -F b ./abc
# DURATION TID FUNCTION
[ 1234] | b() {
3.880 us [ 1234] | c();
5.475 us [ 1234] | } /* b */
The second type of filter is opt-out. By default, everything is traced, but when one of
the specified functions is executed, tracing stops. When the excluded function returns,
tracing is started again.
In the above example, you can omit the function b() and all calls it makes with the -N
option.
$ uftrace -N b ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
6.448 us [ 1234] | a();
8.631 us [ 1234] | } /* main */
You can hide the function b() only without affecting the calls it makes in its subtree
functions with -H option.
$ uftrace -H b ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
[ 1234] | a() {
3.880 us [ 1234] | c();
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
The above -H option is especially useful when hiding std namespace functions in C++
programs by using -H ^std:: option setting.
If users only care about specific functions and want to know how they are called, one can
use the caller filter. It makes the function as leaf and records the parent functions to
the function.
$ uftrace -C b ./abc
# DURATION TID FUNCTION
[ 1234] | main() {
[ 1234] | a() {
5.475 us [ 1234] | b();
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
In the above example, functions not in the calling path were not shown. Also the function
`c' - which is a child of the function `b' - is also hidden.
In addition, you can limit the nesting level of functions with the -D option.
$ uftrace -D 3 ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
[ 1234] | a() {
5.475 us [ 1234] | b();
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
In the above example, uftrace only prints functions up to a depth of 3, so leaf function
c() was omitted. Note that the -D option works with -F.
Sometimes, it’s useful to see long-running functions only. This is good because there are
usually many tiny functions that are not interesting. The -t/--time-filter option
implements the time-based filter that only records functions which run longer than the
given threshold. In the above example, the user might want to see functions running more
than 5 micro-seconds like below:
$ uftrace -t 5us ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
[ 1234] | a() {
5.475 us [ 1234] | b();
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
In addition, you can set filter to record selected source locations with -L option.
$ uftrace -L s-libmain.c --srcline t-lib
# DURATION TID FUNCTION [SOURCE]
[ 5043] | main() { /* /home/uftrace/tests/s-libmain.c:16 */
6.998 us [ 5043] | foo(); /* /home/uftrace/tests/s-libmain.c:11 */
9.393 us [ 5043] | } /* main */
You can set filter with the @hide suffix not to record selected source locations.
$ uftrace -L s-libmain.c@hide --srcline t-lib
# DURATION TID FUNCTION [SOURCE]
[ 14688] | lib_a() { /* /home/uftrace/tests/s-lib.c:10 */
[ 14688] | lib_b() { /* /home/uftrace/tests/s-lib.c:15 */
1.505 us [ 14688] | lib_c(); /* /home/uftrace/tests/s-lib.c:20 */
2.816 us [ 14688] | } /* lib_b */
3.181 us [ 14688] | } /* lib_a */
The -Z/--size-filter option is to filter functions that has small sizes. It reads ELF
symbols size and compare it with the given value. The PLT functions may have no symbol
size in the ELF format, in that case the PLT entry size will be used as the size of the
function.
$ uftrace -Z 100 t-arg
# DURATION TID FUNCTION
[162500] | main() {
12.486 us [162500] | foo();
0.505 us [162500] | many();
[162500] | pass() {
0.283 us [162500] | check();
1.449 us [162500] | } /* pass */
18.478 us [162500] | } /* main */
You can also set triggers on filtered functions. See TRIGGERS section below for details.
When kernel function tracing is enabled, you can also set the filters on kernel functions
by marking the symbol with the @kernel modifier. The following example will show all user
functions and the (kernel) page fault handler.
$ sudo uftrace -k -F '.*page_fault@kernel' ./abc
# DURATION TID FUNCTION
[14721] | main() {
7.713 us [14721] | __do_page_fault();
6.600 us [14721] | __do_page_fault();
6.544 us [14721] | __do_page_fault();
[14721] | a() {
[14721] | b() {
[14721] | c() {
0.860 us [14721] | getpid();
2.346 us [14721] | } /* c */
2.956 us [14721] | } /* b */
3.340 us [14721] | } /* a */
79.086 us [14721] | } /* main */
TRIGGERS
The uftrace tool supports triggering actions on selected function calls (with or without
filters) and/or signals. Currently supported triggers are listed below. The BNF for
trigger specification is as follows:
<trigger> := <symbol> "@" <actions>
<actions> := <action> | <action> "," <actions>
<action> := "depth="<num> | "backtrace" | "trace" | "trace_on" | "trace_off" |
"recover" | "color="<color> | "time="<time_spec> | "read="<read_spec> |
"finish" | "filter" | "notrace" | "hide"
<time_spec> := <num> [ <time_unit> ]
<time_unit> := "ns" | "nsec" | "us" | "usec" | "ms" | "msec" | "s" | "sec" | "m" | "min"
<read_spec> := "proc/statm" | "page-fault" | "pmu-cycle" | "pmu-cache" | "pmu-branch"
The depth trigger is to change filter depth during execution of the function. It can be
used to apply different filter depths for different functions. And the backtrace trigger
is used to print a stack backtrace at replay time.
The color trigger is to change the color of the function in replay output. The supported
colors are red, green, blue, yellow, magenta, cyan, bold, and gray.
The following example shows how triggers work. The global filter maximum depth is 5, but
when function b() is called, it is changed to 1, so functions below b() will not be shown.
$ uftrace -D 5 -T 'b@depth=1' ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main() {
[ 1234] | a() {
5.475 us [ 1234] | b();
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } /* main */
The backtrace trigger is only meaningful in the replay command.
The trace_on and trace_off actions (the _ can be omitted as traceon and traceoff) control
whether uftrace records the specified functions or not.
The recover trigger is for some corner cases in which the process accesses the callstack
directly. During tracing of the v8 javascript engine, for example, it kept getting
segfaults in the garbage collection stage. It was because v8 incorporates the return
address into compiled code objects(?). The recover trigger restores the original return
address at the function entry point and resets to the uftrace return hook address again at
function exit. This was used to work around segfaults by setting the recover trigger on
the related function (specifically ExitFrame::Iterate)
The time trigger is to change time filter setting during execution of the function. It
can be used to apply different time filter for different functions.
The read trigger is to read some information at runtime. The result will be recorded as
(builtin) events at the beginning and the end of a given function. As of now, the
following events are supported:
• “proc/statm”: process memory stat from /proc filesystem
• “page-fault”: number of page faults using getrusage(2)
• “pmu-cycle”: cpu cycles and instructions using Linux perf-event syscall
• “pmu-cache”: (cpu) cache-references and misses using Linux perf-event syscall
• “pmu-branch”: branch instructions and misses using Linux perf-event syscall
The results are printed as events (comments) like below.
$ uftrace -T a@read=proc/statm ./abc
# DURATION TID FUNCTION
[ 1234] | main() {
[ 1234] | a() {
[ 1234] | /* read:proc/statm (size=6808KB, rss=776KB, shared=712KB) */
[ 1234] | b() {
[ 1234] | c() {
1.448 us [ 1234] | getpid();
10.270 us [ 1234] | } /* c */
11.250 us [ 1234] | } /* b */
[ 1234] | /* diff:proc/statm (size=+4KB, rss=+0KB, shared=+0KB) */
18.380 us [ 1234] | } /* a */
19.537 us [ 1234] | } /* main */
The finish trigger is to end recording. The process can still run, which can be useful to
trace non-terminating processes like daemon.
The filter and notrace triggers have same effect as -F/--filter and -N/--notrace options
respectively.
The hide trigger has the same effect as -H/--hide option that hides the given functions,
but does not affect to the functions in their subtree unlike the notrace trigger.
Triggers only work for user-level functions for now.
The trigger can be used for signals as well. This is done by signal trigger with --signal
option. The syntax is similar to function trigger but only “trace_on”, “trace_off” and
“finish” trigger actions are supported.
$ uftrace --signal 'SIGUSR1@finish' ./some-daemon
ARGUMENTS
The uftrace tool supports recording function arguments and/or return values using the
-A/--argument and -R/--retval options respectively. The syntax is very similar to that of
triggers:
<argument> := <symbol> [ "@" <specs> ]
<specs> := <spec> | <spec> "," <spec>
<spec> := ( <int_spec> | <float_spec> | <ret_spec> )
<int_spec> := "arg" N [ "/" <format> [ <size> ] ] [ "%" ( <reg> | <stack> ) ]
<float_spec> := "fparg" N [ "/" ( <size> | "80" ) ] [ "%" ( <reg> | <stack> ) ]
<ret_spec> := "retval" [ "/" <format> [ <size> ] ]
<format> := "d" | "i" | "u" | "x" | "s" | "c" | "f" | "S" | "p"
<size> := "8" | "16" | "32" | "64"
<reg> := <arch-specific register name> # "rdi", "xmm0", "r0", ...
<stack> := "stack" [ "+" ] <offset>
The -A/--argument option takes a symbol name pattern and its optional specs. The spec is
started by argN where N is an index of the arguments. The index starts from 1 and
corresponds to the argument passing order of the calling convention on the system. Note
that the indexes of arguments are separately counted for integer (or pointer) and
floating-point type, and they can interfere depending on the calling convention. The argN
is for integer arguments and fpargN is for floating-point arguments.
Users can optionally specify a format and size for the arguments and/or return values.
The “d” format or without format field, uftrace treats them as `long int' type for
integers and `double' for floating-point numbers. The “i” format makes it signed integer
type and “u” format is for unsigned type. Both are printed as decimal while “x” format
makes it printed as hexadecimal. The “s” format is for null-terminated string type and
“c” format is for character type. The “f” format is for floating-point type and is
meaningful only for return value (generally). Note that fpargN doesn’t take the format
field since it’s always floating-point. The “S” format is for std::string, but it only
supports libstdc++ library as of yet. Finally, the “p” format is for function pointer.
Once the target address is recorded, it will be displayed as function name.
Please beware when using string type arguments since it can crash the program if the
(pointer) value is invalid. Actually uftrace tries to keep track of valid ranges of
process address space but it might miss some corner cases.
It is also possible to specify a certain register name or stack offset for arguments (but
not for return value). The following register names can be used for argument:
• x86: rdi, rsi, rdx, rcx, r8, r9 (for integer), xmm[0-7] (for floating-point)
• arm: r[0-3] (for integer), s[0-15] or d[0-7] (for floating-point)
Examples are shown below:
$ uftrace -A main@arg1/x -R main@retval/i32 ./abc
# DURATION TID FUNCTION
138.494 us [ 1234] | __cxa_atexit();
[ 1234] | main(0x1) {
[ 1234] | a() {
[ 1234] | b() {
3.880 us [ 1234] | c();
5.475 us [ 1234] | } /* b */
6.448 us [ 1234] | } /* a */
8.631 us [ 1234] | } = 0; /* main */
$ uftrace -A puts@arg1/s -R puts@retval ./hello
Hello world
# DURATION TID FUNCTION
1.457 us [21534] | __monstartup();
0.997 us [21534] | __cxa_atexit();
[21534] | main() {
7.226 us [21534] | puts("Hello world") = 12;
8.708 us [21534] | } /* main */
Note that these arguments and return value are recorded only if the executable was built
with the -pg option. Executables built with -finstrument-functions will ignore it except
for library calls. Recording of arguments and return values only works with user-level
functions for now.
If the target program is built with debug info like DWARF, uftrace can identify number of
arguments and their types automatically (when built with libdw). Also arguments and
return value of some well-known library functions are provided even if the debug info is
not available. In these cases user don’t need to specify spec of the arguments and return
value manually - just a function name (or pattern) is enough. In fact, manual argspec
will suppress the automatic argspec.
For example, the above example can be written like below:
$ uftrace -A . -R main -F main ./hello
Hello world
# DURATION TID FUNCTION
[ 18948] | main(1, 0x7ffeeb7590b8) {
7.183 us [ 18948] | puts("Hello world");
9.832 us [ 18948] | } = 0; /* main */
Note that argument pattern (“.”) matches to any character so it recorded all (supported)
functions. It shows two arguments for “main” and a single string argument for “puts”. If
you simply want to see all arguments and return values of every functions (if supported),
use -a/--auto-args option.
FIELDS
The uftrace allows for user to customize the replay output with a couple of fields. Here
the field means info on the left side of the pipe (|) character. By default it uses
duration and tid fields, but you can use other fields in any order like:
$ uftrace -f time,delta,duration,tid,addr ./abc
# TIMESTAMP TIMEDELTA DURATION TID ADDRESS FUNCTION
75059.205379813 1.374 us [27804] 4004d0 | __monstartup();
75059.205384184 4.371 us 0.737 us [27804] 4004f0 | __cxa_atexit();
75059.205386655 2.471 us [27804] 4006b1 | main() {
75059.205386838 0.183 us [27804] 400656 | a() {
75059.205386961 0.123 us [27804] 400669 | b() {
75059.205387078 0.117 us [27804] 40067c | c() {
75059.205387264 0.186 us 0.643 us [27804] 4004b0 | getpid();
75059.205388501 1.237 us 1.423 us [27804] 40067c | } /* c */
75059.205388724 0.223 us 1.763 us [27804] 400669 | } /* b */
75059.205388878 0.154 us 2.040 us [27804] 400656 | } /* a */
75059.205389030 0.152 us 2.375 us [27804] 4006b1 | } /* main */
Each field has following meaning:
• tid: task id (obtained by gettid(2))
• duration: function execution time
• time: timestamp at the execution
• delta: difference between two timestamp in a task
• elapsed: elapsed time from the first timestamp
• addr: address of the function
• task: task name (comm)
• module: library or executable name of the function
The default value is `duration,tid'. If given field name starts with “+”, then it’ll be
appended to the default fields. So “-f +time” is as same as “-f duration,tid,time”. And
it also accepts a special field name of `none' which disables the field display and shows
function output only.
DYNAMIC TRACING
FULL DYNAMIC TRACING
The uftrace tool supports dynamic function tracing which can be enabled at runtime (load-
time, to be precise) on x86_64 and AArch64. Before recording functions, normally you need
to build the target program with -pg (or -finstrument-functions), incurring some
performance impact because all functions call mcount().
With dynamic tracing, you can trace specific functions only given by the -P/--patch option
and can also disable specific functions given by the -U/--unpatch option. With capstone
disassembly engine, you don’t even need to (re)compile the target with the option above.
Now uftrace can analyze the instructions and (if possible) it can copy them to a different
place and rewrite it to call mcount() function so that it can be traced by uftrace. After
that, the control is passed to the copied instructions and then returned back to the
remaining instructions.
The following example shows an error message when normally running uftrace. Because the
binary doesn’t call any instrumentation code (i.e. `mcount').
$ gcc -o abc tests/s-abc.c
$ uftrace abc
uftrace: /home/namhyung/project/uftrace/cmd-record.c:1305:check_binary
ERROR: Can't find 'mcount' symbol in the 'abc'.
It seems not to be compiled with -pg or -finstrument-functions flag
which generates traceable code. Please check your binary file.
But when the -P a patch option is used, uftrace can dynamically trace a().
$ uftrace --no-libcall -P a abc
# DURATION TID FUNCTION
0.923 us [19379] | a();
In addition, you can enable all functions using `.' (for glob, ’*’) that matches to any
character in a regex pattern with P option.
$ uftrace --no-libcall -P . abc
# DURATION TID FUNCTION
[19387] | main() {
[19387] | a() {
[19387] | b() {
0.940 us [19387] | c();
2.030 us [19387] | } /* b */
2.451 us [19387] | } /* a */
3.289 us [19387] | } /* main */
Note that -U option has the opposite effect of -P option so users can use both for fine-
control. The option that comes later will override the formers. For example if you want
to trace all functions but `a' in the above:
$ uftrace --no-libcall -P . -U a abc
# DURATION TID FUNCTION
[19390] | main() {
[19390] | b() {
0.983 us [19390] | c();
2.012 us [19390] | } /* b */
3.373 us [19390] | } /* main */
The order of the options is important. If you change it like -U a -P . then it will trace
all the functions since -P . will take precedence and match everything.
GCC FENTRY
If the capstone is not available, you need to add some more compiler (gcc) options when
building the target program. The gcc 5.1 or more recent versions provide -mfentry and
-mnop-mcount options which add instrumentation code (i.e. calling mcount() function) at
the very beginning of a function and convert the instruction to a NOP. Then it has almost
zero performance overhead when running in a normal condition. The uftrace can selectively
convert it back to call mcount() using -P option.
$ gcc -pg -mfentry -mnop-mcount -o abc-fentry tests/s-abc.c
$ uftrace -P . --no-libcall abc-fentry
# DURATION TID FUNCTION
[ 18973] | main() {
[ 18973] | a() {
[ 18973] | b() {
0.852 us [ 18973] | c();
2.378 us [ 18973] | } /* b */
2.909 us [ 18973] | } /* a */
3.756 us [ 18973] | } /* main */
CLANG XRAY
Clang/LLVM 4.0 provides a dynamic instrumentation technique called X-ray
(http://llvm.org/docs/XRay.html). It’s similar to a combination of gcc -mfentry -mnop-
mcount and -finstrument-functions. The uftrace also supports dynamic tracing on the
executables built with the X-ray.
For example, you can build the target program by clang with the below option and equally
use -P option for dynamic tracing like below:
$ clang -fxray-instrument -fxray-instruction-threshold=1 -o abc-xray tests/s-abc.c
$ uftrace -P main abc-xray
# DURATION TID FUNCTION
[11093] | main() {
1.659 us [11093] | getpid();
5.963 us [11093] | } /* main */
$ uftrace -P . abc-xray
# DURATION TID FUNCTION
[11098] | main() {
[11098] | a() {
[11098] | b() {
[11098] | c() {
0.753 us [11098] | getpid();
1.430 us [11098] | } /* c */
1.915 us [11098] | } /* b */
2.405 us [11098] | } /* a */
3.005 us [11098] | } /* main */
PATCHABLE FUNCTION ENTRY
Recent compilers in both gcc and clang support another useful option -fpatchable-function-
entry=N[,M] that generates M NOPs before the function entry and N-M NOPs after the
function entry. We can simply use the case when M is 0 so -fpatchable-function-entry=N is
enough. The number of NOPs required for dynamic tracing depends on the architecture but
x86_64 requires 5 NOPs and AArch64 requires 2 NOPs to dynamically patch a call instruction
for uftrace recording.
For example in x86_64, you can build the target program and trace as follows.
$ gcc -fpatchable-function-entry=5 -o abc-fpatchable tests/s-abc.c
$ uftrace -P . abc-fpatchable
# DURATION TID FUNCTION
[ 6818] | main() {
[ 6818] | a() {
[ 6818] | b() {
[ 6818] | c() {
0.926 us [ 6818] | getpid();
4.158 us [ 6818] | } /* c */
4.590 us [ 6818] | } /* b */
4.957 us [ 6818] | } /* a */
5.593 us [ 6818] | } /* main */
This feature can also be used by explicitly adding compiler attribute to some specific
functions with __attribute__ ((patchable_function_entry (N,M))). For example, the
`tests/s-abc.c' program can be modified as follows.
static int c(void)
{
return 100000;
}
__attribute__((patchable_function_entry(5)))
static int b(void)
{
return c() + 1;
}
static int a(void)
{
return b() - 1;
}
__attribute__((patchable_function_entry(5)))
int main(void)
{
int ret = 0;
ret += a();
return ret ? 0 : 1;
}
The attribute is added to function `main' and `b' only and this program can normally be
compiled without any additional compiler options, but the compiler detects the attributes
and adds 5 NOPs at the entry of `main' and `b'.
$ gcc -o abc tests/s-patchable-abc.c
$ uftrace -P . abc
# DURATION TID FUNCTION
[ 20803] | main() {
0.342 us [ 20803] | b();
1.608 us [ 20803] | } /* main */
With this way, uftrace can selectively trace only the functions user wants by explicitly
adding the attribute. This approach can collect trace records in a much less intrusive
way compared to tracing the entire functions enabled by compiler flags.
-fpatchable-function-entry=N[,M] option and its attribute are supported since gcc-8.1 and
clang-10. This dynamic tracing feature can be used in both x86_64 and AArch64 as of now.
SCRIPT EXECUTION
The uftrace tool supports script execution for each function entry and exit. The
supported script types are Python 2.7, Python 3 and Lua 5.1 as of now.
The user can write four functions. `uftrace_entry' and `uftrace_exit' are executed
whenever each function is executed at the entry and exit. However `uftrace_begin' and
`uftrace_end' are only executed once when the target program begins and ends.
$ cat scripts/simple.py
def uftrace_begin(ctx):
print("program begins...")
def uftrace_entry(ctx):
func = ctx["name"]
print("entry : " + func + "()")
def uftrace_exit(ctx):
func = ctx["name"]
print("exit : " + func + "()")
def uftrace_end():
print("program is finished")
The above script can be executed in record time as follows:
$ uftrace -S scripts/simple.py -F main tests/t-abc
program begins...
entry : main()
entry : a()
entry : b()
entry : c()
entry : getpid()
exit : getpid()
exit : c()
exit : b()
exit : a()
exit : main()
program is finished
# DURATION TID FUNCTION
[10929] | main() {
[10929] | a() {
[10929] | b() {
[10929] | c() {
4.293 us [10929] | getpid();
19.017 us [10929] | } /* c */
27.710 us [10929] | } /* b */
37.007 us [10929] | } /* a */
55.260 us [10929] | } /* main */
The `ctx' variable is a dictionary type that contains the below information.
/* context information passed to uftrace_entry(ctx) and uftrace_exit(ctx) */
script_context = {
int tid;
int depth;
long timestamp;
long duration; # exit only
long address;
string name;
list args; # entry only (if available)
value retval; # exit only (if available)
};
/* context information passed to uftrace_begin(ctx) */
script_context = {
bool record; # True if it runs at record time, otherwise False
string version; # uftrace version info
list cmds; # execution commands
};
Each field in `script_context' can be read inside the script. Please see uftrace-
script(1) for details about scripting.
WATCH POINT
The uftrace watch point is to display certain value only if it’s changed. It’s
conceptually the same as that of a debugger’s but only works at function entry and exit so
it might miss some updates.
As of now, the following watch points are supported:
• “cpu” : cpu number current task is running on
Like read triggers, the result is displayed as event (comment):
$ uftrace -W cpu tests/t-abc
# DURATION TID FUNCTION
[ 19060] | main() {
[ 19060] | /* watch:cpu (cpu=8) */
[ 19060] | a() {
[ 19060] | b() {
[ 19060] | c() {
2.365 us [ 19060] | getpid();
8.002 us [ 19060] | } /* c */
8.690 us [ 19060] | } /* b */
9.350 us [ 19060] | } /* a */
12.479 us [ 19060] | } /* main */
SEE ALSO
uftrace-record(1), uftrace-replay(1), uftrace-report(1), uftrace-script(1)
AUTHORS
Namhyung Kim <namhyung@gmail.com>.