Messages in this thread | | | Date | Fri, 18 Dec 2020 15:10:33 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH] ptrace: make ptrace() fail if the tracee changed its pid unexpectedly |
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On 12/17, Eric W. Biederman wrote: > > Oleg Nesterov <oleg@redhat.com> writes: > > > Suppose we have 2 threads, the group-leader L and a sub-theread T, > > both parked in ptrace_stop(). Debugger tries to resume both threads > > and does > > > > ptrace(PTRACE_CONT, T); > > ptrace(PTRACE_CONT, L); > > > > If the sub-thread T execs in between, the 2nd PTRACE_CONT doesn not > > resume the old leader L, it resumes the post-exec thread T which was > > actually now stopped in PTHREAD_EVENT_EXEC. In this case the > > PTHREAD_EVENT_EXEC event is lost, and the tracer can't know that the > > tracee changed its pid. > > The change seems sensible. I don't expect this is common but it looks > painful to deal with if it happens.
Yes, this is not a bug, but gdb can't handle this case without some help from the kernel.
> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Thanks!
> I am wondering if this should be expanded to all ptrace types for > consistency. Or maybe we should set a flag to make this happen for > all ptrace events.
But for what? ptrace is the very old API, I don't think we want to suddenly enforce the rule that every reported event must be wait()'ed. Plus this needs some complications to support WNOWAIT.
I would like to kill JOBCTL_TRAPPING_BIT which ensures that the tracer does NOT need wait() after PTRACE_ATTACH(stopped-task) (see wait_on_bit() in ptrace_attach()). I think this makes no sense but who knows, perhaps even this change can break something.
> It just seems really odd to only worry about missing this event.
Agreed,
> I admit this a threaded PTRACE_EVENT_EXEC is the only event we are > likely to miss but still.
Yes, this is the only event debugger can miss even if it uses wait() correctly.
> Do you by any chance have any debugger/strace test cases? > > I would think that would be the way to test to see if this breaks > anything. I think I remember strace having a good test suite.
Heh. You can never know what other people do with ptrace ;) For example, see
fab840fc2d54 ptrace: PTRACE_DETACH should do flush_ptrace_hw_breakpoint(child) 35114fcbe0b9 Revert "ptrace: PTRACE_DETACH should do flush_ptrace_hw_breakpoint(child)"
Oleg.
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