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PTRACE(2) |
FreeBSD System Calls Manual |
PTRACE(2) |
ptrace —
process tracing and debugging
Standard C Library (libc, -lc)
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 .
Normally, exiting tracing process should wait for all pending debugging events
and then detach from all alive traced processes before exiting using
PT_DETACH request. If tracing process exits without
detaching, for instance due to abnormal termination, the destiny of the traced
children processes is determined by the
kern.kill_on_debugger_exit sysctl control.
If the control is set to the default value 1, such traced
processes are terminated. If set to zero, kernel implicitly detaches traced
processes. Traced processes are un-stopped if needed, and then continue the
execution without tracing. Kernel drops any SIGTRAP
signals queued to the traced children, which could be either generated by
not yet consumed debug events, or sent by other means, the later should not
be done anyway.
The ptrace subsystem provides rich facilities to
manipulate other processes state. Sometimes it may be desirable to disallow it
either completely, or limit its scope. The following controls are provided for
this:
security.bsd.allow_ptrace
- Setting this sysctl to zero value makes
ptrace(2)
return
ENOSYS always as if the syscall is not
implemented by the kernel.
security.bsd.unprivileged_proc_debug
- Setting this sysctl to zero disallows use of
ptrace () by unprivileged processes.
security.bsd.see_other_uids
- Setting this sysctl to zero value disallows
ptrace () requests from targeting processes with
the real user identifier different from the real user identifier of the
caller. The requests return ESRCH if policy is not
met.
security.bsd.see_other_gids
- Setting this sysctl to zero value disallows
ptrace () requests from process belonging to a
group that is not also one of the group of the target process. The
requests return ESRCH if policy is not met.
securelevel
and init
- The
init(1)
process can only be traced with
ptrace if
securelevel is zero.
procctl(2)
PROC_TRACE_CTL
- Process can deny attempts to trace itself with
procctl(2)
PROC_TRACE_CTL request. In this case requests
return
EPERM
error.
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.
PT_COREDUMP
- This request creates a coredump for the stopped program. The
addr argument specifies a pointer to a
struct ptrace_coredump, which is defined as follows:
struct ptrace_coredump {
int pc_fd;
uint32_t pc_flags;
off_t pc_limit;
};
The fields of the structure are:
pc_fd
- File descriptor to write the dump to. It must refer to a regular file,
opened for writing.
pc_flags
- Flags. The following flags are defined:
PC_COMPRESS
- Request compression of the dump.
PC_ALL
- Include non-dumpable entries into the dump. The dumper ignores
MAP_NOCORE flag of the process map entry,
but device mappings are not dumped even with
PC_ALL set.
pc_limit
- Maximum size of the coredump. Specify zero for no limit.
The size of struct ptrace_coredump must
be passed in data.
The process must be stopped before dumping core. A single
thread in the target process is temporarily unsuspended in kernel to
write the dump. If the ptrace call fails before
a thread is unsuspended, there is no event to
waitpid(2)
for. If a thread was unsuspended, it will stop again before the
ptrace call returns, and the process must be
waited upon using
waitpid(2)
to consume the new stop event. Since it is hard to deduce whether a
thread was unsuspended before an error occurred, it is recommended to
unconditionally perform
waitpid(2)
with WNOHANG flag after
PT_COREDUMP , and silently accept zero result
from it.
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.
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.
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.
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.
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.
The ptrace () function appeared in
Version 6 AT&T UNIX.
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