PTRACE(2)

NAME
ptrace - process tracing and debugging

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, void *addr, int data);

DESCRIPTION
ptrace() provides tracing and debugging facilities. It allows one process (the tracing process) to control another (the traced process). 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 ( see siginfo(2)), examine the state of the stopped process, and cause it to terminate or continue as appropriate. ptrace() is the mechanism by which all this happens.

When a process that is traced by a debugger requests 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 and emit SIGTRAP with si_code set to TRAP_EXEC. If a program is traced with the PT_SYSCALL option enabled, this event notifier is disabled. If a traced program calls execve(2) any setuid or setgid bits on the executable being executed will be ignored.

Program (software) breakpoints are reported with SIGTRAP and the si_code value set to TRAP_BKPT. These breakpoints are machine specific instructions that interrupt the process. In order to put a trap by a tracer into the tracee's program, debugger must violate the PaX MPROTECT restrictions. For details check the security.pax.mprotect.ptrace option described in sysctl(7). When a tracee is interrupted by a trap, the trap is not removed by the kernel and it must be handled by a debugger.

If a program is traced with single steps (PT_STEP) it reports each step with SIGTRAP with si_code set to TRAP_TRACE. This event is not maskable PT_SET_EVENT_MASK.

Child program traps are reported with SIGTRAP and the si_code value set to TRAP_CHLD. These events are by default disabled and can be configured with PT_SET_EVENT_MASK. If this event occurs, check with PT_GET_PROCESS_STATE the details of the process state associated with this event.

Design choices for Debug Register accessors

-   exec() (TRAP_EXEC event) must remove debug registers from LWP

-   debug registers are only per-LWP, not per-process globally

-   debug registers must not be inherited after (v)forking a process

-   debug registers must not be inherited after forking a thread

-   a debugger is responsible to set global watchpoints/breakpoints with the debug registers, to achieve this PTRACE_LWP_CREATE / PTRACE_LWP_EXIT event monitoring function is designed to be used

-   debug register traps must generate SIGTRAP with si_code TRAP_DBREG

-   debugger is responsible to retrieve debug register state to distinguish the exact debug register trap

-   kernel must not remove debug register traps after triggering a trap event; a debugger is responsible to detach this trap with appropriate PT_SETDBREGS call

-   debug registers must not be exposed in mcontext

-   userland must not be allowed to set a trap on the kernel

A debugger might reuse port specific symbols, to help writing portable code as described in the port specific part of the <sys/ptrace.h> header. Among these symbols, there are:

-   PTRACE_REG_PC

-   PTRACE_REG_SET_PC

-   PTRACE_REG_SP

-   PTRACE_REG_INTRV

-   PTRACE_BREAKPOINT

-   PTRACE_BREAKPOINT_SIZE

-   PTRACE_BREAKPOINT_ADJ

The request argument of ptrace 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. request 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().

    This call does not stop the process neither emit SIGSTOP to parent.

PT_READ_I, PT_READ_D
    These requests read a single int of data from the traced process' 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 NetBSD implementation, these two requests are completely identical. The addr argument specifies the address (in the traced process' 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_CONTINUE
    The traced process continues execution. addr is an address specifying the place where execution is to be resumed (a new value for the program counter), or (void *)1 to indicate that execution is to pick up where it left off. data provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent. If a negative value is supplied, that is the negative of the LWP ID of the thread to be resumed, and only that thread executes.

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. It means that the SIGSTOP signal is emitted to tracer. It is different behavior to the one from PT_TRACE_ME.

    Three other restrictions apply to all tracing processes, even those running as root. First, no process may trace a system process. Second, no process may trace the process running init(8). Third, if a process has its root directory set with chroot(2), it may not trace another process unless that process' root directory is at or below the tracing process' root.

PT_DETACH
    This request is like PT_CONTINUE, except that after it succeeds, the traced process is no longer traced and continues execution normally.

PT_IO
    This request is a more general interface that can be used instead of PT_READ_D, PT_WRITE_D, PT_READ_I, and PT_WRITE_I. The I/O request is encoded in a “struct ptrace_io_desc” defined as:

        struct ptrace_io_desc {
        	int	piod_op;
        	void	*piod_offs;
        	void	*piod_addr;
        	size_t	piod_len;
        };

    where piod_offs is the offset within the traced process where the I/O operation should take place, piod_addr is the buffer in the tracing process, and piod_len is the length of the I/O request. The piod_op field specifies which type of I/O operation to perform. Possible values are:

    PIOD_READ_D
    PIOD_WRITE_D
    PIOD_READ_I
    PIOD_WRITE_I
    PIOD_READ_AUXV

    See the description of PT_READ_I for the difference between I and D spaces.

    The PIOD_READ_AUXV operation can be used to read from the ELF auxiliary vector. The piod_offs argument sets the offset within the tracee's vector. To read from the beginning of it, this value must be set to 0 and cast to (void *).

    A pointer to the I/O descriptor is passed in the addr argument to ptrace(). On return, the piod_len field in the I/O descriptor will be updated with the actual number of bytes transferred. If the requested I/O could not be successfully performed, ptrace() will return -1 and set errno.

PT_DUMPCORE
    Makes the process specified in the pid pid generate a core dump. The addr argument should contain the name of the core file to be generated and the data argument should contain the length of the core filename.

PT_LWPINFO
    Returns information about a thread from the list of threads for the process specified in the pid argument. The addr argument should contain a “struct ptrace_lwpinfo” defined as:

        struct ptrace_lwpinfo {
        	lwpid_t pl_lwpid;
        	int pl_event;
        };

    where pl_lwpid contains a thread LWP ID. Information is returned for the thread following the one with the specified ID in the process thread list, or for the first thread if pl_lwpid is 0. Upon return pl_lwpid contains the LWP ID of the thread that was found, or 0 if there is no thread after the one whose LWP ID was supplied in the call. pl_event contains the event that stopped the thread. Possible values are:

    PL_EVENT_NONE
    PL_EVENT_SIGNAL
    PL_EVENT_SUSPENDED

    The data argument should contain “sizeof(struct ptrace_lwpinfo)”.

PT_SYSCALL
    Stops a process before and after executing each system call. Otherwise this operation is the same as PT_CONTINUE.

PT_SYSCALLEMU
    Intercept and ignore a system call before it has been executed, for use with PT_SYSCALL. This operation shall be called for syscall entry trap from PT_SYSCALL. To resume execution after intercepting the system call, another PT_SYSCALL shall be used.

PT_SET_EVENT_MASK
    This request can be used to specify which events in the traced process should be reported to the tracing process. These events are specified in a “struct ptrace_event” defined as:

        typedef struct ptrace_event {
        	int	pe_set_event;
        } ptrace_event_t;

    pe_set_event is the set of events to be reported. This set is formed by OR'ing together the following values:

    PTRACE_FORK
        Report fork(2).

    PTRACE_VFORK
        Report vfork(2).

    PTRACE_VFORK_DONE
        Report parent resumed after vfork(2).

    PTRACE_LWP_CREATE
        Report thread birth.

    PTRACE_LWP_EXIT
        Report thread termination.

    The fork(2) and vfork(2) events can occur with similar operations, like clone(2) or posix_spawn(3). The PTRACE_FORK value means that process gives birth to its child without pending on its termination or execve(2) operation. If enabled, the child is also traced by the debugger and SIGRAP is generated twice, first for the parent and second for the child. The PTRACE_VFORK event is the same as PTRACE_FORK, but the parent blocks after giving birth to the child. The PTRACE_VFORK_DONE event can be used to report unblocking of the parent.

    A pointer to this structure is passed in addr. The data argument should be set to sizeof(struct ptrace_event).

PT_GET_EVENT_MASK
    This request can be used to determine which events in the traced process will be reported. The information is read into the “struct ptrace_event” pointed to by addr. The data argument should be set to sizeof(struct ptrace_event).

PT_GET_PROCESS_STATE
    This request reads the state information associated with the event that stopped the traced process. The information is reported in a “struct ptrace_state” defined as:

        typedef struct ptrace_state {
        	int	pe_report_event;
        	pid_t	pe_other_pid;
        } ptrace_state_t;

    A pointer to this structure is passed in addr. The data argument should be set to sizeof(struct ptrace_state).

PT_SET_SIGINFO
    This request can be used to specify signal information emitted to tracee. This signal information is specified in “struct ptrace_siginfo” defined as:

        typedef struct ptrace_siginfo {
        	siginfo_t	psi_siginfo;
        	lwpid_t		psi_lwpid;
        } ptrace_siginfo_t;

    Where psi_siginfo is the set to signal information structure. The psi_lwpid field describes LWP address of the signal. Value 0 means the whole process (route signal to all LWPs).

    A pointer to this structure is passed in addr. The data argument should be set to sizeof(struct ptrace_siginfo).

    In order to pass faked signal to the tracee, the signal type must match the signal passed to the process with PT_CONTINUE or PT_SYSCALL.

PT_GET_SIGINFO
    This request can be used to determine signal information that was received by a debugger ( see siginfo(2)). The information is read into the “struct ptrace_siginfo” pointed to by addr. The data argument should be set to sizeof(struct ptrace_siginfo).

PT_RESUME
    Allow execution of a specified thread, change its state from suspended to continued. The addr argument is unused. The data argument specifies the LWP ID.

    This call is equivalent to _lwp_continue(2) called by a traced process. This call does not change the general process state from stopped to continued.

PT_SUSPEND
    Prevent execution of a specified thread, change its state from continued to suspended. The addr argument is unused. The data argument specifies the requested LWP ID.

    This call is equivalent to _lwp_suspend(2) called by a traced process. This call does not change the general process state from continued to stopped.

Additionally, the following requests exist but are not available on all machine architectures. The file <machine/ptrace.h> lists which requests exist on a given machine.

PT_STEP
    Execution continues as in request PT_CONTINUE; however as soon as possible after execution of at least one instruction, execution stops again. If the data argument is greater than 0, it contains the LWP ID of the thread to be stepped, and any other threads are continued. If the data argument is less than zero, it contains the negative of the LWP ID of the thread to be stepped, and only that thread executes.

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_GETREGS
    This request reads the traced process' machine registers into the “struct reg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.

PT_SETREGS
    This request is the converse of PT_GETREGS; it loads the traced process' machine registers from the “struct reg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.

PT_GETFPREGS
    This request reads the traced process' floating-point registers into the “struct fpreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.

PT_SETFPREGS
    This request is the converse of PT_GETFPREGS; it loads the traced process' floating-point registers from the “struct fpreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.

PT_GETDBREGS
    This request reads the traced process' debug registers into the “struct dbreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.

PT_SETDBREGS
    This request is the converse of PT_GETDBREGS; it loads the traced process' debug registers from the “struct dbreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.

PT_GETXMMREGS
    This request reads the traced process' XMM registers into the “struct xmmregs” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.

PT_SETXMMREGS
    This request is the converse of PT_GETXMMREGS; it loads the traced process' XMM registers from the “struct xmmregs” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.

PT_GETVECREGS
    This request reads the traced process' vector registers into the “struct vreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.

PT_SETVECREGS
    This request is the converse of PT_GETVECREGS; it loads the traced process' vector registers from the “struct vreg” (defined in <machine/reg.h>) pointed to by addr. The data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.

ERRORS
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. The possible errors are:

[EAGAIN]
    Process is currently exec'ing and cannot be traced.

[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 wasn't stopped.

[EDEADLK]
    An attempt to unstop a process with locked threads.

[EINVAL]
    •   A process attempted to use PT_ATTACH on itself.
    •   The request was not a legal request on this machine architecture.
    •   The signal number (in data) to PT_CONTINUE was neither 0 nor a legal signal number.
    •   PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was attempted on a process with no valid register set. (This is normally true only of system processes.)

[EPERM]
    •   A request (other than PT_ATTACH) attempted to manipulate a process that wasn't 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.

[ESRCH]
    No process having the specified process ID exists.

SEE ALSO
sigaction(2), signal(7)

HISTORY
The ptrace() function appeared in AT&T UNIX.

BUGS
On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and similar calls, but the NPC is set willy-nilly to 4 greater than the PC value. Using PT_GETREGS and PT_SETREGS to modify the PC, passing (void *)1 to ptrace(), should be able to sidestep this.

PTRACE_VFORK is currently unimplemented and it will return ENOTSUP.

PT_SET_SIGINFO, PT_RESUME and PT_SUSPEND can change the image of process returned by PT_LWPINFO.