Messages in this thread Patch in this message | | | Date | Wed, 26 Feb 2014 17:56:07 +0100 | From | Oleg Nesterov <> | Subject | [PATCH 2/2] wait: WSTOPPED|WCONTINUED doesn't work if a zombie leader is traced by another process |
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Even if the main thread is dead the process still can stop/continue. However, if the leader is ptraced wait_consider_task(ptrace => false) always skips wait_task_stopped/wait_task_continued, so WSTOPPED or WCONTINUED can never work for the natural parent in this case.
Move the "A zombie ptracee is only visible to its ptracer" check into the "if (!delay_group_leader(p))" block. ->notask_error is cleared by the "fall through" code below.
This depends on the previous change, wait_task_stopped/continued must be avoided if !delay_group_leader() and the tracer is ->real_parent. Otherwise WSTOPPED|WEXITED could wrongly report "stopped" when the child is already dead (single-threaded or not). If it is traced by another task then the "stopped" state is fine until the debugger detaches and reveals a zombie state.
Stupid test-case:
void *tfunc(void *arg) { sleep(1); // wait for zombie leader raise(SIGSTOP); exit(0x13); return NULL; }
int run_child(void) { pthread_t thread;
if (!fork()) { int tracee = getppid();
assert(ptrace(PTRACE_ATTACH, tracee, 0,0) == 0); do ptrace(PTRACE_CONT, tracee, 0,0); while (wait(NULL) > 0);
return 0; }
sleep(1); // wait for PTRACE_ATTACH assert(pthread_create(&thread, NULL, tfunc, NULL) == 0); pthread_exit(NULL); }
int main(void) { int child, stat;
child = fork(); if (!child) return run_child();
assert(child == waitpid(-1, &stat, WSTOPPED)); assert(stat == 0x137f);
kill(child, SIGCONT);
assert(child == waitpid(-1, &stat, WCONTINUED)); assert(stat == 0xffff);
assert(child == waitpid(-1, &stat, 0)); assert(stat == 0x1300);
return 0; }
Without this patch it hangs in waitpid(WSTOPPED), wait_task_stopped() is never called.
Note: this doesn't fix all problems with a zombie delay_group_leader(), WCONTINUED | WEXITED check is not exactly right. debugger can't assume it will be notified if another thread reaps the whole thread group.
Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- kernel/exit.c | 22 +++++++++------------- 1 files changed, 9 insertions(+), 13 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c index 56c5ac3..790b73c 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1382,20 +1382,16 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, /* slay zombie? */ if (p->exit_state == EXIT_ZOMBIE) { - /* - * A zombie ptracee is only visible to its ptracer. - * Notification and reaping will be cascaded to the real - * parent when the ptracer detaches. - */ - if (likely(!ptrace) && unlikely(p->ptrace)) { - /* it will become visible, clear notask_error */ - wo->notask_error = 0; - return 0; - } - /* we don't reap group leaders with subthreads */ - if (!delay_group_leader(p)) - return wait_task_zombie(wo, p); + if (!delay_group_leader(p)) { + /* + * A zombie ptracee is only visible to its ptracer. + * Notification and reaping will be cascaded to the + * real parent when the ptracer detaches. + */ + if (unlikely(ptrace) || likely(!p->ptrace)) + return wait_task_zombie(wo, p); + } /* * Allow access to stopped/continued state via zombie by -- 1.5.5.1
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