Messages in this thread | | | Date | Tue, 23 Jul 2013 18:38:06 +0200 | From | Oleg Nesterov <> | Subject | Re: ptrace(PTRACE_ATTACH) [no intervering wait] ptrace(PTRACE_DETACH) may leave tracee stuck |
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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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