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authorTejun Heo <tj@kernel.org>2011-03-23 10:37:01 +0100
committerTejun Heo <tj@kernel.org>2011-03-23 10:37:01 +0100
commit9b84cca2564b9a5b2d064fb44d2a55a5b44473a0 (patch)
tree376808993d708f23830ff069fdf157e99bfb7516 /kernel/exit.c
parent823b018e5b1196d810790559357447948f644548 (diff)
job control: Fix ptracer wait(2) hang and explain notask_error clearing
wait(2) and friends allow access to stopped/continued states through zombies, which is required as the states are process-wide and should be accessible whether the leader task is alive or undead. wait_consider_task() implements this by always clearing notask_error and going through wait_task_stopped/continued() for unreaped zombies. However, while ptraced, the stopped state is per-task and as such if the ptracee became a zombie, there's no further stopped event to listen to and wait(2) and friends should return -ECHILD on the tracee. Fix it by clearing notask_error only if WCONTINUED | WEXITED is set for ptraced zombies. While at it, document why clearing notask_error is safe for each case. Test case follows. #include <stdio.h> #include <unistd.h> #include <pthread.h> #include <time.h> #include <sys/types.h> #include <sys/ptrace.h> #include <sys/wait.h> static void *nooper(void *arg) { pause(); return NULL; } int main(void) { const struct timespec ts1s = { .tv_sec = 1 }; pid_t tracee, tracer; siginfo_t si; tracee = fork(); if (tracee == 0) { pthread_t thr; pthread_create(&thr, NULL, nooper, NULL); nanosleep(&ts1s, NULL); printf("tracee exiting\n"); pthread_exit(NULL); /* let subthread run */ } tracer = fork(); if (tracer == 0) { ptrace(PTRACE_ATTACH, tracee, NULL, NULL); while (1) { if (waitid(P_PID, tracee, &si, WSTOPPED) < 0) { perror("waitid"); break; } ptrace(PTRACE_CONT, tracee, NULL, (void *)(long)si.si_status); } return 0; } waitid(P_PID, tracer, &si, WEXITED); kill(tracee, SIGKILL); return 0; } Before the patch, after the tracee becomes a zombie, the tracer's waitid(WSTOPPED) never returns and the program doesn't terminate. tracee exiting ^C After the patch, tracee exiting triggers waitid() to fail. tracee exiting waitid: No child processes -v2: Oleg pointed out that exited in addition to continued can happen for ptraced dead group leader. Clear notask_error for ptraced child on WEXITED too. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Oleg Nesterov <oleg@redhat.com>
Diffstat (limited to 'kernel/exit.c')
-rw-r--r--kernel/exit.c44
1 files changed, 34 insertions, 10 deletions
diff --git a/kernel/exit.c b/kernel/exit.c
index b4a935c72159..84d13d6bb30b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1550,17 +1550,41 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
return 0;
}
- /*
- * We don't reap group leaders with subthreads.
- */
- if (p->exit_state == EXIT_ZOMBIE && !delay_group_leader(p))
- return wait_task_zombie(wo, p);
+ /* slay zombie? */
+ if (p->exit_state == EXIT_ZOMBIE) {
+ /* we don't reap group leaders with subthreads */
+ if (!delay_group_leader(p))
+ return wait_task_zombie(wo, p);
- /*
- * It's stopped or running now, so it might
- * later continue, exit, or stop again.
- */
- wo->notask_error = 0;
+ /*
+ * Allow access to stopped/continued state via zombie by
+ * falling through. Clearing of notask_error is complex.
+ *
+ * When !@ptrace:
+ *
+ * If WEXITED is set, notask_error should naturally be
+ * cleared. If not, subset of WSTOPPED|WCONTINUED is set,
+ * so, if there are live subthreads, there are events to
+ * wait for. If all subthreads are dead, it's still safe
+ * to clear - this function will be called again in finite
+ * amount time once all the subthreads are released and
+ * will then return without clearing.
+ *
+ * When @ptrace:
+ *
+ * Stopped state is per-task and thus can't change once the
+ * target task dies. Only continued and exited can happen.
+ * Clear notask_error if WCONTINUED | WEXITED.
+ */
+ if (likely(!ptrace) || (wo->wo_flags & (WCONTINUED | WEXITED)))
+ wo->notask_error = 0;
+ } else {
+ /*
+ * @p is alive and it's gonna stop, continue or exit, so
+ * there always is something to wait for.
+ */
+ wo->notask_error = 0;
+ }
if (task_stopped_code(p, ptrace))
return wait_task_stopped(wo, ptrace, p);