wait: introduce EXIT_TRACE to avoid the racy EXIT_DEAD->EXIT_ZOMBIE transition
wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and
drops tasklist_lock. If this task is not the natural child and it is
traced, we change its state back to EXIT_ZOMBIE for ->real_parent.
The last transition is racy, this is even documented in 50b8d25748
"ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE
race". wait_consider_task() tries to detect this transition and clear
->notask_error but we can't rely on ptrace_reparented(), debugger can
exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE.
And there is another problem which were missed before: this transition
can also race with reparent_leader() which doesn't reset >exit_signal if
EXIT_DEAD, assuming that this task must be reaped by someone else. So
the tracee can be re-parented with ->exit_signal != SIGCHLD, and if
/sbin/init doesn't use __WALL it becomes unreapable. This was fixed by
the previous commit, but it was the temporary hack.
1. Add the new exit_state, EXIT_TRACE. It means that the task is the
traced zombie, debugger is going to detach and notify its natural
parent.
This new state is actually EXIT_ZOMBIE | EXIT_DEAD. This way we
can avoid the changes in proc/kgdb code, get_task_state() still
reports "X (dead)" in this case.
Note: with or without this change userspace can see Z -> X -> Z
transition. Not really bad, but probably makes sense to fix.
2. Change wait_task_zombie() to use EXIT_TRACE instead of EXIT_DEAD
if we need to notify the ->real_parent.
3. Revert the previous hack in reparent_leader(), now that EXIT_DEAD
is always the final state we can safely ignore such a task.
4. Change wait_consider_task() to check EXIT_TRACE separately and kill
the racy and no longer needed ptrace_reparented() case.
If ptrace == T an EXIT_TRACE thread should be simply ignored, the
owner of this state is going to ptrace_unlink() this task. We can
pretend that it was already removed from ->ptraced list.
Otherwise we should skip this thread too but clear ->notask_error,
we must be the natural parent and debugger is going to untrace and
notify us. IOW, this doesn't differ from "EXIT_ZOMBIE && p->ptrace"
even if the task was already untraced.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Reported-by: Michal Schmidt <mschmidt@redhat.com>
Tested-by: Michal Schmidt <mschmidt@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
parent
dfccbb5e49
commit
abd50b39e7
2 changed files with 24 additions and 31 deletions
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@ -212,6 +212,7 @@ print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq);
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/* in tsk->exit_state */
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#define EXIT_ZOMBIE 16
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#define EXIT_DEAD 32
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#define EXIT_TRACE (EXIT_ZOMBIE | EXIT_DEAD)
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/* in tsk->state again */
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#define TASK_DEAD 64
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#define TASK_WAKEKILL 128
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@ -560,6 +560,9 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
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struct list_head *dead)
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{
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list_move_tail(&p->sibling, &p->real_parent->children);
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if (p->exit_state == EXIT_DEAD)
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return;
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/*
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* If this is a threaded reparent there is no need to
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* notify anyone anything has happened.
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@ -567,19 +570,9 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p,
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if (same_thread_group(p->real_parent, father))
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return;
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/*
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* We don't want people slaying init.
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*
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* Note: we do this even if it is EXIT_DEAD, wait_task_zombie()
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* can change ->exit_state to EXIT_ZOMBIE. If this is the final
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* state, do_notify_parent() was already called and ->exit_signal
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* doesn't matter.
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*/
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/* We don't want people slaying init. */
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p->exit_signal = SIGCHLD;
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if (p->exit_state == EXIT_DEAD)
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return;
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/* If it has exited notify the new parent about this child's death. */
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if (!p->ptrace &&
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p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
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@ -1043,17 +1036,13 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
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return wait_noreap_copyout(wo, p, pid, uid, why, status);
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}
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/*
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* Try to move the task's state to DEAD
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* only one thread is allowed to do this:
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*/
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state = xchg(&p->exit_state, EXIT_DEAD);
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if (state != EXIT_ZOMBIE) {
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BUG_ON(state != EXIT_DEAD);
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return 0;
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}
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traced = ptrace_reparented(p);
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/*
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* Move the task's state to DEAD/TRACE, only one thread can do this.
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*/
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state = traced ? EXIT_TRACE : EXIT_DEAD;
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if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) != EXIT_ZOMBIE)
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return 0;
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/*
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* It can be ptraced but not reparented, check
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* thread_group_leader() to filter out sub-threads.
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@ -1114,7 +1103,7 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
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/*
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* Now we are sure this task is interesting, and no other
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* thread can reap it because we set its state to EXIT_DEAD.
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* thread can reap it because we its state == DEAD/TRACE.
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*/
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read_unlock(&tasklist_lock);
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@ -1159,14 +1148,14 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
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* If this is not a sub-thread, notify the parent.
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* If parent wants a zombie, don't release it now.
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*/
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state = EXIT_DEAD;
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if (thread_group_leader(p) &&
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!do_notify_parent(p, p->exit_signal)) {
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p->exit_state = EXIT_ZOMBIE;
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p = NULL;
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}
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!do_notify_parent(p, p->exit_signal))
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state = EXIT_ZOMBIE;
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p->exit_state = state;
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write_unlock_irq(&tasklist_lock);
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}
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if (p != NULL)
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if (state == EXIT_DEAD)
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release_task(p);
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return retval;
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@ -1362,12 +1351,15 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
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}
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/* dead body doesn't have much to contribute */
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if (unlikely(p->exit_state == EXIT_DEAD)) {
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if (unlikely(p->exit_state == EXIT_DEAD))
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return 0;
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if (unlikely(p->exit_state == EXIT_TRACE)) {
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/*
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* But do not ignore this task until the tracer does
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* wait_task_zombie()->do_notify_parent().
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* ptrace == 0 means we are the natural parent. In this case
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* we should clear notask_error, debugger will notify us.
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*/
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if (likely(!ptrace) && unlikely(ptrace_reparented(p)))
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if (likely(!ptrace))
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wo->notask_error = 0;
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return 0;
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}
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