ptrace



PTRACE(2)                 OpenBSD Programmer's Manual                PTRACE(2)


NAME

     ptrace - process tracing and debugging


SYNOPSIS

     #include <sys/types.h>
     #include <sys/ptrace.h>

     int
     ptrace(int request, pid_t pid, caddr_t 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, 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.
     ptrace() is only available on kernels compiled with the PTRACE option.

     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.  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().) 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' 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
                   OpenBSD 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 (caddr_t)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.

     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 set-user-ID or set-group-ID 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_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 be made, piod_addr is the
                   buffer in the parent and piod_len is the length of the I/O
                   request.  The piod_op member specifies what operation needs
                   to be done.  Possible values are:

                         PIOD_READ_D
                         PIOD_WRITE_D
                         PIOD_READ_I
                         PIOD_WRITE_I
                         PIOD_READ_AUXV

                   See also the description of PT_READ_I for the difference
                   between D and I spaces.  The PIOD_READ_AUXV operation can
                   be used to read from the ELF auxiliary vector.  A pointer
                   to the descriptor is passed in addr.  On return the
                   piod_len field in the descriptor will be updated with the
                   actual number of bytes transferred.  If the requested I/O
                   couldn't be successfully performed ptrace() will return -1
                   and set errno.

     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;

                   Where 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).

                   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;

                   Where pe_report_event is the event being reported.  If the
                   event being reported is PTRACE_FORK, pe_other_pid will be
                   set to the process ID of the other end of the fork.  A
                   pointer to this structure is passed in addr.  The data
                   argument should be set to sizeof(struct ptrace_state).

     Additionally, machine-specific requests can exist.  Depending on the
     architecture, the following requests may be available under OpenBSD:

     PT_GETREGS (all platforms)
                   This request reads the traced process' machine registers
                   into the ``struct reg'' (defined in <machine/reg.h>)
                   pointed to by addr.

     PT_SETREGS (all platforms)
                   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.

     PT_STEP (not available on sparc and sparc64)
                   The traced process continues execution, as in request
                   PT_CONTINUE; however, execution stops as soon as possible
                   after execution of at least one instruction (single-step).

     PT_GETFPREGS (not available on aviion, luna88k, mvme88k, sgi and vax)
                   This request reads the traced process' floating-point
                   registers into the ``struct fpreg'' (defined in
                   <machine/reg.h>) pointed to by addr.

     PT_SETFPREGS (not available on aviion, luna88k, mvme88k, sgi and vax)
                   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.

     PT_GETXMMREGS (i386 only)
                   This request reads the traced process' XMM registers into
                   the ``struct xmmregs'' (defined in <machine/reg.h>) pointed
                   to by addr.

     PT_SETXMMREGS (i386 only)
                   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.

     PT_WCOOKIE (sparc and sparc64 only)
                   This request reads the traced process' `window cookie' into
                   the int pointed to by addr.  The window cookie needs to be
                   `XOR'ed' to stack-saved program counters.


ERRORS

     Some requests can cause ptrace() to return -1 as a non-error value; to
     disambiguate, errno is set to zero and this should be checked.  The
     possible errors are:

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

     [EINVAL]
           o   A process attempted to use PT_ATTACH on itself.
           o   The request was not one of the legal requests.
           o   The signal number (in data) to PT_CONTINUE was neither 0 nor a
               legal signal number.
           o   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.)

     [EBUSY]
           o   PT_ATTACH was attempted on a process that was already being
               traced.
           o   A request attempted to manipulate a process that was being
               traced by some process other than the one making the request.
           o   A request (other than PT_ATTACH) specified a process that
               wasn't stopped.

     [EPERM]
           o   A request (other than PT_ATTACH) attempted to manipulate a
               process that wasn't being traced at all.
           o   An attempt was made to use PT_ATTACH on a process in violation
               of the requirements listed under PT_ATTACH above.
           o   An attempt was made to use PT_ATTACH on a system process.


HISTORY

     The ptrace() system call first appeared in Version 6 AT&T UNIX.


BUGS

     On several RISC architectures (such as aviion, luna88k, mvme88k, sparc
     and sparc64), the PC is set to the provided PC value for PT_CONTINUE and
     similar calls, and the remainder of the execution pipeline registers are
     set to the following instructions, even if the instruction at PC is a
     branch instruction.  Using PT_GETREGS and PT_SETREGS to modify the PC,
     passing (caddr_t)1 to ptrace(), should be able to sidestep this.

OpenBSD 5.4                      July 17, 2013                     OpenBSD 5.4

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