Re: ptrace(PTRACE_ATTACH) [no intervering wait] ptrace(PTRACE_DETACH) may leave tracee stuck

[Oleg Nesterov <oleg@redhat.com>]

On 07/23, Oleg Nesterov wrote:
>
> On 07/23, Mike Galbraith wrote:
> >
> > I received a report that glibc:elf/pldd hangs occasionally, and indeed..
> >
> >   for i in `seq 1 1000`; do taskset -c 3 pldd $$ > /dev/null 2>&1; done
> >
> > ..will do so.  Rummage.....
> >
> > ptrace(PTRACE_DETACH) returns -ESRCH when the trap hasn't happened yet,
> > which happens because pldd doesn't wait() before ptrace(PTRACE_DETACH).
> >
> > pldd source:
> >
> [...snip...]
> >
> > Seems this usually works only because cycles expended between attach and
> > detach is usually enough to let trap happen so tracee can set its state
> > to TASK_TRACED as PTRACE_DETACH expects it to be.
> >
> > Is this expected behavior?
>
> Yes. PTRACE_ATTACH + PTRACE_DETACH is not correct without wait() in
> between, this is expected.
>
> PTRACE_DETACH like (almost) any other ptrace request needs the stopped
> tracee. Otherwise, say, ptrace_disable() or flush_ptrace_hw_breakpoint()
> are not safe.

I have found the source of pldd.c. It seems that it has another reason
for waitpid().

	/* Stop all threads since otherwise the list of loaded modules might
	   change while we are reading it. */

Yes, but without waitpid() we can't know if it was actually stopped.

OTOH, in this particular case pldd.c doesn't really need PTRACE_DETACH,
it can simply exit.

Oleg.