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PTRACE(2) | System Calls Manual | PTRACE(2) |
NAME¶
ptrace — process tracing and debuggingLIBRARY¶
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 aSIGCHLD
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, system calls, or
stepping by the tracing 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.
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.
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 andPT_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
andPT_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 */
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 withSIGKILL
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; };
- 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 succesful 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_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
kind of stop, when pl_flags hasPL_FLAG_SI
set.
- 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.
- 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 trace the specified process on each system call entry.
- PT_TO_SCX
- This request will trace the specified process on each system call exit.
- PT_SYSCALL
- This request will trace the specified process on each system call entry and exit.
- 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; };
RETURN VALUES¶
Some requests can cause ptrace() to return -1 as a non-error value; to disambiguate, errno can be set to 0 before the call and checked afterwards.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
, orPT_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.
- A process attempted to
use
- [
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 underPT_ATTACH
above.
- A request (other than
- [
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 7 AT&T UNIX.BUGS¶
ThePL_FLAG_SCE
, PL_FLAG_SCX
and
PL_FLAG_EXEC
are not implemented for MIPS and ARM
architectures.July 10, 2010 | Debian |