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



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