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>
1 file changed