On Wed, Jun 19, 2013 at 01:11:56PM +0200, Denys Vlasenko wrote:
> Dmitry, I am a bit worried about the flow in this function still.
> Let's take a look:
>
> error = ptrace(PTRACE_DETACH, tcp->pid, 0, 0);
> if (error == 0) {
> /* On a clear day, you can see forever. */
> }
> else if (errno != ESRCH) {
> /* Shouldn't happen. */
> perror_msg("detach: ptrace(PTRACE_DETACH, ...)");
> }
> else
> /* ESRCH: process is either not stopped or doesn't exist. */
> if (my_tkill(tcp->pid, 0) < 0) {
> if (errno != ESRCH)
> /* Shouldn't happen. */
> perror_msg("detach: checking sanity");
> /* else: process doesn't exist. */
> ^^^^^^^^^^^^^^^^^
> Well, it may not exist already, but was it *waited for*?
> IOW: we may still need to enter waitpid loop.
> This may rarely trigger - say, we do "strace -p PROCESS",
> and process exits just as we ^C the strace,
> and we may end up here.
> OTOH, not-waited-for child reparents to init when we exit,
> so... do we ever detach() NOT not strace exit, where dead
> children are a problem? I see one location:
> if (event == PTRACE_EVENT_EXEC) {
> if (detach_on_execve && !skip_one_b_execve)
> detach(tcp); /* do "-b execve" thingy */
> Maybe in the name of correctness we should wait for the process
> if we see ESRCH? Possibly with WHOHANG for paranoid reasons.In case of "-b execve", the tracee is in syscall-stop state already, so PTRACE_DETACH should succeed and there should be no need to wait (and if PTRACE_DETACH failed, then the tracee is no more so strace is expected to wait for it). I see no reason why detach() called from cleanup() should have to call PTRACE_DETACH for TCB_ATTACHED tracees. At strace exit, those tracees that have TCB_IGNORE_ONE_SIGSTOP bit set (can happen in !use_seize case) have to be waited for SIGSTOP and PTRACE_CONT'ed (otherwise they would be left stopped after strace exit), all the rest TCB_ATTACHED tracees should be fine as they are. -- ldv
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