Ptrace segfault

[Gunnar Hinriksson]

2010/4/29 Bob Bishop <rb at gid.co.uk>:
> Hi,
>
> On 29 Apr 2010, at 22:37, Garrett Cooper wrote:
>
>> On Thu, Apr 29, 2010 at 12:06 PM, Gunnar Hinriksson <tomtinn at gmail.com> wrote:
>>> Hello
>>>
>>> Im having a little problem using ptrace on my system.
>>> If I use ptrace to attach to another process the child process
>>> segfaults once I detach.
>>> For example using this simple program.
>>>
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <sys/types.h>
>>> #include <sys/ptrace.h>
>>> #include <sys/wait.h>
>>>
>>> int main(int argc, char *argv[])
>>> {
>>>        int pid = atoi(argv[1]);
>>>        ptrace(PT_ATTACH, pid, 0, 0);
>>>        wait(NULL);
>>>        ptrace(PT_DETACH, pid, 0, 0);
>>>        return 0;
>>> }
>>>
>>> Am I using ptrace incorrectly or is there perhaps a bug in ptrace that
>>> causes the child to always segfault ?
>>
>>    Nope -- it's a bug in your code. From ptrace(2):
>>
>>     PT_CONTINUE   The traced process continues execution.  The addr argument
>>                   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.  The data argument 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_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.
>>
>>    Note very carefully the fact that PT_DETACH is like PT_CONTINUE,
>> and that PT_CONTINUE says that addr references the memory where the
>> execution is going to be resumed.
>
> Looks to me like a bug in ptrace(PT_DETACH,...) which to agree with ptrace(2) ought either to
> (a) fail (EINVAL?) if addr != (caddr_t)1, or
> (b) ignore addr entirely; it's not clear which.
>
> OP inferred (b) which is reasonable.
>
>> HTH,
>> -Garrett
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>
>
> --
> Bob Bishop          +44 (0)118 940 1243
> rb at gid.co.uk    fax +44 (0)118 940 1295
>             mobile +44 (0)783 626 4518
>
>
>
>
>
>

Hello

I didn't want to make the code to big so I omitted any tests.
About wait(), you are supposed to use wait to check if the child
process has stopped successfully.
I guess the correct usage would be something like this if im
understanding the documentation correctly.
if ( wait(WIFSTOPPED(SIGSTOP)) == pid )
Im not sure if there is any posix way of describing how ptrace should
be implemented but I know linux ptrace ignores the addr on DETACH.
So my vote would be that ptrace ignores addr on detach to make it
compatible with code for linux.

With Regards

Gunnar