ptrace - process tracing and debugging
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);
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.
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
See also the description of PT_READ_I for the
difference
between D and I spaces. 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.
Additionally, machine-specific requests can exist. All
OpenBSD platforms
support the following requests:
PT_GETREGS This request reads the traced process' 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' machine registers from the
``struct reg''
(defined in <machine/reg.h>) pointed to by
addr.
In addition, all platforms but vax support these additional
requests:
PT_GETFPREGS This request reads the traced process' 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' floating-point registers from
the ``struct
fpreg'' (defined in <machine/reg.h>) pointed
to by addr.
Finally, the following request is available on sparc:
PT_WCOOKIE 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.
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.
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
(caddr_t)1 to ptrace(), should be able to sidestep this.
Single-stepping is not available.
OpenBSD 3.6 March 2, 2004
[ Back ] |
