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NAME
ptrace — process tracing and debugging
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);
DESCRIPTION
The ptrace() system call provides tracing and debugging facilities. It allows one process
(the tracing process) to control another (the traced process). The tracing process must
first attach to the traced process, and then issue a series of ptrace() system calls to
control the execution of the process, as well as access process memory and register state.
For the duration of the tracing session, the traced process will be “re-parented”, with its
parent process ID (and resulting behavior) changed to the tracing process. It is
permissible for a tracing process to attach to more than one other process at a time. When
the tracing process has completed its work, it must detach the traced process; if a tracing
process exits without first detaching all processes it has attached, those processes will be
killed.
Most of the time, the traced process runs normally, but when it receives a signal (see
sigaction(2)), it stops. The tracing process is expected to notice this via wait(2) or the
delivery of a SIGCHLD signal, examine the state of the stopped process, and cause it to
terminate or continue as appropriate. The signal may be a normal process signal, generated
as a result of traced process behavior, or use of the kill(2) system call; alternatively, it
may be generated by the tracing facility as a result of attaching, stepping by the tracing
process, or an event in the traced process. The tracing process may choose to intercept the
signal, using it to observe process behavior (such as SIGTRAP), or forward the signal to the
process if appropriate. The ptrace() system call is the mechanism by which all this
happens.
A traced process may report additional signal stops corresponding to events in the traced
process. These additional signal stops are reported as SIGTRAP or SIGSTOP signals. The
tracing process can use the PT_LWPINFO request to determine which events are associated with
a SIGTRAP or SIGSTOP signal. Note that multiple events may be associated with a single
signal. For example, events indicated by the PL_FLAG_BORN, PL_FLAG_FORKED, and PL_FLAG_EXEC
flags are also reported as a system call exit event (PL_FLAG_SCX). The signal stop for a
new child process enabled via PTRACE_FORK will report a SIGSTOP signal. All other
additional signal stops use SIGTRAP.
Each traced process has a tracing event mask. An event in the traced process only reports a
signal stop if the corresponding flag is set in the tracing event mask. The current set of
tracing event flags include:
PTRACE_EXEC Report a stop for a successful invocation of execve(2). This event is
indicated by the PL_FLAG_EXEC flag in the pl_flags member of struct
ptrace_lwpinfo.
PTRACE_SCE Report a stop on each system call entry. This event is indicated by the
PL_FLAG_SCE flag in the pl_flags member of struct ptrace_lwpinfo.
PTRACE_SCX Report a stop on each system call exit. This event is indicated by the
PL_FLAG_SCX flag in the pl_flags member of struct ptrace_lwpinfo.
PTRACE_SYSCALL Report stops for both system call entry and exit.
PTRACE_FORK This event flag controls tracing for new child processes of a traced
process.
When this event flag is enabled, new child processes will enable tracing
and stop before executing their first instruction. The new child process
will include the PL_FLAG_CHILD flag in the pl_flags member of struct
ptrace_lwpinfo. The traced process will report a stop that includes the
PL_FLAG_FORKED flag. The process ID of the new child process will also
be present in the pl_child_pid member of struct ptrace_lwpinfo. If the
new child process was created via vfork(2), the traced process's stop
will also include the PL_FLAG_VFORKED flag. Note that new child
processes will be attached with the default tracing event mask; they do
not inherit the event mask of the traced process.
When this event flag is not enabled, new child processes will execute
without tracing enabled.
PTRACE_LWP This event flag controls tracing of LWP (kernel thread) creation and
destruction. When this event is enabled, new LWPs will stop and report
an event with PL_FLAG_BORN set before executing their first instruction,
and exiting LWPs will stop and report an event with PL_FLAG_EXITED set
before completing their termination.
Note that new processes do not report an event for the creation of their
initial thread, and exiting processes do not report an event for the
termination of the last thread.
PTRACE_VFORK Report a stop event when a parent process resumes after a vfork(2).
When a thread in the traced process creates a new child process via
vfork(2), the stop that reports PL_FLAG_FORKED and PL_FLAG_SCX occurs
just after the child process is created, but before the thread waits for
the child process to stop sharing process memory. If a debugger is not
tracing the new child process, it must ensure that no breakpoints are
enabled in the shared process memory before detaching from the new child
process. This means that no breakpoints are enabled in the parent
process either.
The PTRACE_VFORK flag enables a new stop that indicates when the new
child process stops sharing the process memory of the parent process. A
debugger can reinsert breakpoints in the parent process and resume it in
response to this event. This event is indicated by setting the
PL_FLAG_VFORK_DONE flag.
The default tracing event mask when attaching to a process via PT_ATTACH, PT_TRACE_ME, or
PTRACE_FORK includes only PTRACE_EXEC events. All other event flags are disabled.
The request argument specifies what operation is being performed; the meaning of the rest of
the arguments depends on the operation, but except for one special case noted below, all
ptrace() calls are made by the tracing process, and the pid argument specifies the process
ID of the traced process or a corresponding thread ID. The request argument can be:
PT_TRACE_ME This request is the only one used by the traced process; it declares
that the process expects to be traced by its parent. All the other
arguments are ignored. (If the parent process does not expect to
trace the child, it will probably be rather confused by the results;
once the traced process stops, it cannot be made to continue except
via ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as execv(3)), it
will stop before executing the first instruction of the new image.
Also, any setuid or setgid bits on the executable being executed will
be ignored. If the child was created by vfork(2) system call or
rfork(2) call with the RFMEM flag specified, the debugging events are
reported to the parent only after the execve(2) is executed.
PT_READ_I, PT_READ_D These requests read a single int of data from the traced process's
address space. Traditionally, ptrace() has allowed for machines with
distinct address spaces for instruction and data, which is why there
are two requests: conceptually, PT_READ_I reads from the instruction
space and PT_READ_D reads from the data space. In the current FreeBSD
implementation, these two requests are completely identical. The addr
argument specifies the address (in the traced process's virtual
address space) at which the read is to be done. This address does not
have to meet any alignment constraints. The value read is returned as
the return value from ptrace().
PT_WRITE_I, PT_WRITE_D
These requests parallel PT_READ_I and PT_READ_D, except that they
write rather than read. The data argument supplies the value to be
written.
PT_IO This request allows reading and writing arbitrary amounts of data in
the traced process's address space. The addr argument specifies a
pointer to a struct ptrace_io_desc, which is defined as follows:
struct ptrace_io_desc {
int piod_op; /* I/O operation */
void *piod_offs; /* child offset */
void *piod_addr; /* parent offset */
size_t piod_len; /* request length */
};
/*
* Operations in piod_op.
*/
#define PIOD_READ_D 1 /* Read from D space */
#define PIOD_WRITE_D 2 /* Write to D space */
#define PIOD_READ_I 3 /* Read from I space */
#define PIOD_WRITE_I 4 /* Write to I space */
The data argument is ignored. The actual number of bytes read or
written is stored in piod_len upon return.
PT_CONTINUE The traced process continues execution. The addr argument is an
address specifying the place where execution is to be resumed (a new
value for the program counter), or (caddr_t)1 to indicate that
execution is to pick up where it left off. The data argument provides
a signal number to be delivered to the traced process as it resumes
execution, or 0 if no signal is to be sent.
PT_STEP The traced process is single stepped one instruction. The addr
argument should be passed (caddr_t)1. The data argument provides a
signal number to be delivered to the traced process as it resumes
execution, or 0 if no signal is to be sent.
PT_KILL The traced process terminates, as if PT_CONTINUE had been used with
SIGKILL given as the signal to be delivered.
PT_ATTACH This request allows a process to gain control of an otherwise
unrelated process and begin tracing it. It does not need any
cooperation from the to-be-traced process. In this case, pid
specifies the process ID of the to-be-traced process, and the other
two arguments are ignored. This request requires that the target
process must have the same real UID as the tracing process, and that
it must not be executing a setuid or setgid executable. (If the
tracing process is running as root, these restrictions do not apply.)
The tracing process will see the newly-traced process stop and may
then control it as if it had been traced all along.
PT_DETACH This request is like PT_CONTINUE, except that it does not allow
specifying an alternate place to continue execution, and after it
succeeds, the traced process is no longer traced and continues
execution normally.
PT_GETREGS This request reads the traced process's machine registers into the
“struct reg” (defined in <machine/reg.h>) pointed to by addr.
PT_SETREGS This request is the converse of PT_GETREGS; it loads the traced
process's machine registers from the “struct reg” (defined in
<machine/reg.h>) pointed to by addr.
PT_GETFPREGS This request reads the traced process's floating-point registers into
the “struct fpreg” (defined in <machine/reg.h>) pointed to by addr.
PT_SETFPREGS This request is the converse of PT_GETFPREGS; it loads the traced
process's floating-point registers from the “struct fpreg” (defined in
<machine/reg.h>) pointed to by addr.
PT_GETDBREGS This request reads the traced process's debug registers into the
“struct dbreg” (defined in <machine/reg.h>) pointed to by addr.
PT_SETDBREGS This request is the converse of PT_GETDBREGS; it loads the traced
process's debug registers from the “struct dbreg” (defined in
<machine/reg.h>) pointed to by addr.
PT_LWPINFO This request can be used to obtain information about the kernel
thread, also known as light-weight process, that caused the traced
process to stop. The addr argument specifies a pointer to a struct
ptrace_lwpinfo, which is defined as follows:
struct ptrace_lwpinfo {
lwpid_t pl_lwpid;
int pl_event;
int pl_flags;
sigset_t pl_sigmask;
sigset_t pl_siglist;
siginfo_t pl_siginfo;
char pl_tdname[MAXCOMLEN + 1];
pid_t pl_child_pid;
u_int pl_syscall_code;
u_int pl_syscall_narg;
};
The data argument is to be set to the size of the structure known to
the caller. This allows the structure to grow without affecting older
programs.
The fields in the struct ptrace_lwpinfo have the following meaning:
pl_lwpid
LWP id of the thread
pl_event
Event that caused the stop. Currently defined events are:
PL_EVENT_NONE No reason given
PL_EVENT_SIGNAL Thread stopped due to the pending signal
pl_flags
Flags that specify additional details about observed stop.
Currently defined flags are:
PL_FLAG_SCE
The thread stopped due to system call entry, right
after the kernel is entered. The debugger may examine
syscall arguments that are stored in memory and
registers according to the ABI of the current process,
and modify them, if needed.
PL_FLAG_SCX
The thread is stopped immediately before syscall is
returning to the usermode. The debugger may examine
system call return values in the ABI-defined registers
and/or memory.
PL_FLAG_EXEC
When PL_FLAG_SCX is set, this flag may be additionally
specified to inform that the program being executed by
debuggee process has been changed by successful
execution of a system call from the execve(2) family.
PL_FLAG_SI
Indicates that pl_siginfo member of struct
ptrace_lwpinfo contains valid information.
PL_FLAG_FORKED
Indicates that the process is returning from a call to
fork(2) that created a new child process. The process
identifier of the new process is available in the
pl_child_pid member of struct ptrace_lwpinfo.
PL_FLAG_CHILD
The flag is set for first event reported from a new
child which is automatically attached when PTRACE_FORK
is enabled.
PL_FLAG_BORN
This flag is set for the first event reported from a
new LWP when PTRACE_LWP is enabled. It is reported
along with PL_FLAG_SCX.
PL_FLAG_EXITED
This flag is set for the last event reported by an
exiting LWP when PTRACE_LWP is enabled. Note that
this event is not reported when the last LWP in a
process exits. The termination of the last thread is
reported via a normal process exit event.
PL_FLAG_VFORKED
Indicates that the thread is returning from a call to
vfork(2) that created a new child process. This flag
is set in addition to PL_FLAG_FORKED.
PL_FLAG_VFORK_DONE
Indicates that the thread has resumed after a child
process created via vfork(2) has stopped sharing its
address space with the traced process.
pl_sigmask
The current signal mask of the LWP
pl_siglist
The current pending set of signals for the LWP. Note that
signals that are delivered to the process would not appear on
an LWP siglist until the thread is selected for delivery.
pl_siginfo
The siginfo that accompanies the signal pending. Only valid
for PL_EVENT_SIGNAL stop when PL_FLAG_SI is set in pl_flags.
pl_tdname
The name of the thread.
pl_child_pid
The process identifier of the new child process. Only valid
for a PL_EVENT_SIGNAL stop when PL_FLAG_FORKED is set in
pl_flags.
pl_syscall_code
The ABI-specific identifier of the current system call. Note
that for indirect system calls this field reports the
indirected system call. Only valid when PL_FLAG_SCE or
PL_FLAG_SCX is set in pl_flags.
pl_syscall_narg
The number of arguments passed to the current system call not
counting the system call identifier. Note that for indirect
system calls this field reports the arguments passed to the
indirected system call. Only valid when PL_FLAG_SCE or
PL_FLAG_SCX is set in pl_flags.
PT_GETNUMLWPS This request returns the number of kernel threads associated with the
traced process.
PT_GETLWPLIST This request can be used to get the current thread list. A pointer to
an array of type lwpid_t should be passed in addr, with the array size
specified by data. The return value from ptrace() is the count of
array entries filled in.
PT_SETSTEP This request will turn on single stepping of the specified process.
Stepping is automatically disabled when a single step trap is caught.
PT_CLEARSTEP This request will turn off single stepping of the specified process.
PT_SUSPEND This request will suspend the specified thread.
PT_RESUME This request will resume the specified thread.
PT_TO_SCE This request will set the PTRACE_SCE event flag to trace all future
system call entries and continue the process. The addr and data
arguments are used the same as for PT_CONTINUE.
PT_TO_SCX This request will set the PTRACE_SCX event flag to trace all future
system call exits and continue the process. The addr and data
arguments are used the same as for PT_CONTINUE.
PT_SYSCALL This request will set the PTRACE_SYSCALL event flag to trace all
future system call entries and exits and continue the process. The
addr and data arguments are used the same as for PT_CONTINUE.
PT_GET_SC_ARGS For the thread which is stopped in either PL_FLAG_SCE or PL_FLAG_SCX
state, that is, on entry or exit to a syscall, this request fetches
the syscall arguments.
The arguments are copied out into the buffer pointed to by the addr
pointer, sequentially. Each syscall argument is stored as the machine
word. Kernel copies out as many arguments as the syscall accepts, see
the pl_syscall_narg member of the struct ptrace_lwpinfo, but not more
than the data bytes in total are copied.
PT_GET_SC_RET Fetch the system call return values on exit from a syscall. This
request is only valid for threads stopped in a syscall exit (the
PL_FLAG_SCX state). The addr argument specifies a pointer to a struct
ptrace_sc_ret, which is defined as follows:
struct ptrace_sc_ret {
register_t sr_retval[2];
int sr_error;
};
The data argument is set to the size of the structure.
If the system call completed successfully, sr_error is set to zero and
the return values of the system call are saved in sr_retval. If the
system call failed to execute, sr_error field is set to a positive
errno(2) value. If the system call completed in an unusual fashion,
sr_error is set to a negative value:
ERESTART System call will be restarted.
EJUSTRETURN System call completed sucessfully but did not set a
return value (for example, setcontext(2) and
sigreturn(2)).
PT_FOLLOW_FORK This request controls tracing for new child processes of a traced
process. If data is non-zero, PTRACE_FORK is set in the traced
process's event tracing mask. If data is zero, PTRACE_FORK is cleared
from the traced process's event tracing mask.
PT_LWP_EVENTS This request controls tracing of LWP creation and destruction. If
data is non-zero, PTRACE_LWP is set in the traced process's event
tracing mask. If data is zero, PTRACE_LWP is cleared from the traced
process's event tracing mask.
PT_GET_EVENT_MASK This request reads the traced process's event tracing mask into the
integer pointed to by addr. The size of the integer must be passed in
data.
PT_SET_EVENT_MASK This request sets the traced process's event tracing mask from the
integer pointed to by addr. The size of the integer must be passed in
data.
PT_VM_TIMESTAMP This request returns the generation number or timestamp of the memory
map of the traced process as the return value from ptrace(). This
provides a low-cost way for the tracing process to determine if the VM
map changed since the last time this request was made.
PT_VM_ENTRY This request is used to iterate over the entries of the VM map of the
traced process. The addr argument specifies a pointer to a struct
ptrace_vm_entry, which is defined as follows:
struct ptrace_vm_entry {
int pve_entry;
int pve_timestamp;
u_long pve_start;
u_long pve_end;
u_long pve_offset;
u_int pve_prot;
u_int pve_pathlen;
long pve_fileid;
uint32_t pve_fsid;
char *pve_path;
};
The first entry is returned by setting pve_entry to zero. Subsequent
entries are returned by leaving pve_entry unmodified from the value
returned by previous requests. The pve_timestamp field can be used to
detect changes to the VM map while iterating over the entries. The
tracing process can then take appropriate action, such as restarting.
By setting pve_pathlen to a non-zero value on entry, the pathname of
the backing object is returned in the buffer pointed to by pve_path,
provided the entry is backed by a vnode. The pve_pathlen field is
updated with the actual length of the pathname (including the
terminating null character). The pve_offset field is the offset
within the backing object at which the range starts. The range is
located in the VM space at pve_start and extends up to pve_end
(inclusive).
The data argument is ignored.
ARM MACHINE-SPECIFIC REQUESTS
PT_GETVFPREGS Return the thread's VFP machine state in the buffer pointed to by addr.
The data argument is ignored.
PT_SETVFPREGS Set the thread's VFP machine state from the buffer pointed to by addr.
The data argument is ignored.
x86 MACHINE-SPECIFIC REQUESTS
PT_GETXMMREGS Copy the XMM FPU state into the buffer pointed to by the argument
addr. The buffer has the same layout as the 32-bit save buffer for
the machine instruction FXSAVE.
This request is only valid for i386 programs, both on native 32-bit
systems and on amd64 kernels. For 64-bit amd64 programs, the XMM
state is reported as part of the FPU state returned by the
PT_GETFPREGS request.
The data argument is ignored.
PT_SETXMMREGS Load the XMM FPU state for the thread from the buffer pointed to by
the argument addr. The buffer has the same layout as the 32-bit load
buffer for the machine instruction FXRSTOR.
As with PT_GETXMMREGS, this request is only valid for i386 programs.
The data argument is ignored.
PT_GETXSTATE_INFO Report which XSAVE FPU extensions are supported by the CPU and allowed
in userspace programs. The addr argument must point to a variable of
type struct ptrace_xstate_info, which contains the information on the
request return. struct ptrace_xstate_info is defined as follows:
struct ptrace_xstate_info {
uint64_t xsave_mask;
uint32_t xsave_len;
};
The xsave_mask field is a bitmask of the currently enabled extensions.
The meaning of the bits is defined in the Intel and AMD processor
documentation. The xsave_len field reports the length of the XSAVE
area for storing the hardware state for currently enabled extensions
in the format defined by the x86 XSAVE machine instruction.
The data argument value must be equal to the size of the struct
ptrace_xstate_info.
PT_GETXSTATE Return the content of the XSAVE area for the thread. The addr
argument points to the buffer where the content is copied, and the
data argument specifies the size of the buffer. The kernel copies out
as much content as allowed by the buffer size. The buffer layout is
specified by the layout of the save area for the XSAVE machine
instruction.
PT_SETXSTATE Load the XSAVE state for the thread from the buffer specified by the
addr pointer. The buffer size is passed in the data argument. The
buffer must be at least as large as the struct savefpu (defined in
x86/fpu.h) to allow the complete x87 FPU and XMM state load. It must
not be larger than the XSAVE state length, as reported by the
xsave_len field from the struct ptrace_xstate_info of the
PT_GETXSTATE_INFO request. Layout of the buffer is identical to the
layout of the load area for the XRSTOR machine instruction.
PT_GETFSBASE Return the value of the base used when doing segmented memory
addressing using the %fs segment register. The addr argument points
to an unsigned long variable where the base value is stored.
The data argument is ignored.
PT_GETGSBASE Like the PT_GETFSBASE request, but returns the base for the %gs
segment register.
PT_SETFSBASE Set the base for the %fs segment register to the value pointed to by
the addr argument. addr must point to the unsigned long variable
containing the new base.
The data argument is ignored.
PT_SETGSBASE Like the PT_SETFSBASE request, but sets the base for the %gs segment
register.
PowerPC MACHINE-SPECIFIC REQUESTS
PT_GETVRREGS Return the thread's ALTIVEC machine state in the buffer pointed to by addr.
The data argument is ignored.
PT_SETVRREGS Set the thread's ALTIVEC machine state from the buffer pointed to by addr.
The data argument is ignored.
PT_GETVSRREGS Return doubleword 1 of the thread's VSX registers VSR0-VSR31 in the buffer
pointed to by addr.
The data argument is ignored.
PT_SETVSRREGS Set doubleword 1 of the thread's VSX registers VSR0-VSR31 from the buffer
pointed to by addr.
The data argument is ignored.
Additionally, other machine-specific requests can exist.
RETURN VALUES
Most requests return 0 on success and -1 on error. Some requests can cause ptrace() to
return -1 as a non-error value, among them are PT_READ_I and PT_READ_D, which return the
value read from the process memory on success. To disambiguate, errno can be set to 0
before the call and checked afterwards.
The current ptrace() implementation always sets errno to 0 before calling into the kernel,
both for historic reasons and for consistency with other operating systems. It is
recommended to assign zero to errno explicitly for forward compatibility.
ERRORS
The ptrace() system call may fail if:
[ESRCH]
• No process having the specified process ID exists.
[EINVAL]
• A process attempted to use PT_ATTACH on itself.
• The request argument was not one of the legal requests.
• The signal number (in data) to PT_CONTINUE was neither 0 nor a legal
signal number.
• PT_GETREGS, PT_SETREGS, PT_GETFPREGS, PT_SETFPREGS, PT_GETDBREGS, or
PT_SETDBREGS was attempted on a process with no valid register set.
(This is normally true only of system processes.)
• PT_VM_ENTRY was given an invalid value for pve_entry. This can also
be caused by changes to the VM map of the process.
• The size (in data) provided to PT_LWPINFO was less than or equal to
zero, or larger than the ptrace_lwpinfo structure known to the
kernel.
• The size (in data) provided to the x86-specific PT_GETXSTATE_INFO
request was not equal to the size of the struct ptrace_xstate_info.
• The size (in data) provided to the x86-specific PT_SETXSTATE request
was less than the size of the x87 plus the XMM save area.
• The size (in data) provided to the x86-specific PT_SETXSTATE request
was larger than returned in the xsave_len member of the struct
ptrace_xstate_info from the PT_GETXSTATE_INFO request.
• The base value, provided to the amd64-specific requests PT_SETFSBASE
or PT_SETGSBASE, pointed outside of the valid user address space.
This error will not occur in 32-bit programs.
[EBUSY]
• PT_ATTACH was attempted on a process that was already being traced.
• A request attempted to manipulate a process that was being traced by
some process other than the one making the request.
• A request (other than PT_ATTACH) specified a process that was not
stopped.
[EPERM]
• A request (other than PT_ATTACH) attempted to manipulate a process
that was not being traced at all.
• An attempt was made to use PT_ATTACH on a process in violation of the
requirements listed under PT_ATTACH above.
[ENOENT]
• PT_VM_ENTRY previously returned the last entry of the memory map. No
more entries exist.
[ENAMETOOLONG]
• PT_VM_ENTRY cannot return the pathname of the backing object because
the buffer is not big enough. pve_pathlen holds the minimum buffer
size required on return.
SEE ALSO
execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3), i386_set_watch(3)
HISTORY
The ptrace() function appeared in Version 6 AT&T UNIX.