PTRACE(2)                 NetBSD System Calls Manual                 PTRACE(2)

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, 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 instruc-
     tions that interrput 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 interrputed 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 distin-
         guish 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 per-
     formed; the meaning of the rest of the arguments depends on the opera-
     tion, 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 par-
                   ent.  All the other arguments are ignored.  If the parent
                   process does not expect to trace the child, it will proba-
                   bly 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 align-
                   ment 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 sup-
                   plies 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 indi-
                   cate 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 nega-
                   tive 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 other-
                   wise 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 direc-
                   tory 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 suc-
                   ceeds, 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 off-
                   set 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 argu-
                   ment 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 success-
                   fully 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 con-
                   tain 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 speci-
                   fied 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.  Pos-
                   sible 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 exe-
                   cuted, 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) oper-
                   ation.  If enabled, the child is also traced by the debug-
                   ger 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 struc-
                   ture.  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_SET_SIGMASK
                   This request loads the traced process' signal mask from
                   ``sigset_t'' (defined in <sys/sigtypes.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_GET_SIGMASK
                   This request is the converse of PT_SET_SIGMASK; it reads
                   the traced process' signal mask into ``sigset_t'' (defined
                   in <sys/sigtypes.h>) pointed to by addr.  The data argument
                   contains the LWP ID of the thread whose mask is to be read.
                   If zero is supplied, the first thread of the process is
                   read.

     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 instruc-
                   tion, 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 reg-
                   isters 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 sup-
                   plied, 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 sup-
                   plied, 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 after-
     wards.  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 archi-
                   tecture.
                  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 viola-
                   tion 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 Version 7 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.

NetBSD 7.0                       April 7, 2017                      NetBSD 7.0

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©1996-2017 Modified for NetBSD by Kimmo Suominen