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

[Mike Galbraith]

On Tue, 2013-07-23 at 17:58 +0200, 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.

Thanks for confirmation.  The man page was pretty clear (read it after
slogging through source/traces, oh well, educational;) that -ESRCH was
expected, but I wanted to be sure about tracee state thereafter.

-Mike

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