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author | Tejun Heo <tj@kernel.org> | 2011-03-23 10:37:01 +0100 |
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committer | Tejun Heo <tj@kernel.org> | 2011-03-23 10:37:01 +0100 |
commit | 9b84cca2564b9a5b2d064fb44d2a55a5b44473a0 (patch) | |
tree | 376808993d708f23830ff069fdf157e99bfb7516 /kernel | |
parent | 823b018e5b1196d810790559357447948f644548 (diff) | |
download | linux-stable-9b84cca2564b9a5b2d064fb44d2a55a5b44473a0.tar.gz linux-stable-9b84cca2564b9a5b2d064fb44d2a55a5b44473a0.tar.bz2 linux-stable-9b84cca2564b9a5b2d064fb44d2a55a5b44473a0.zip |
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')
-rw-r--r-- | kernel/exit.c | 44 |
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); |