PTRACE(2) OpenBSD Programmer's Manual PTRACE(2)NAMEptrace - 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);
DESCRIPTIONptrace() 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.
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 4.9 September 16, 2008 OpenBSD 4.9
