From: Oleg Nesterov <oleg@redhat.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Aleksa Sarai <asarai@suse.com>,
Andy Lutomirski <luto@amacapital.net>,
Attila Fazekas <afazekas@redhat.com>, Jann Horn <jann@thejh.net>,
Kees Cook <keescook@chromium.org>,
Michal Hocko <mhocko@kernel.org>,
Ulrich Obergfell <uobergfe@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] fix the traced mt-exec deadlock
Date: Fri, 3 Mar 2017 18:33:26 +0100 [thread overview]
Message-ID: <20170303173326.GA17899@redhat.com> (raw)
In-Reply-To: <87shmv6ufl.fsf@xmission.com>
On 03/02, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > our discussion was a bit confusing, and it seems that we did not
> > fully convince each other. So let me ask what do you finally think
> > about this fix.
> >
> > Let me repeat. Even if I do not agree with some of your objections,
> > I do agree that 1/2 does not look nice and clean. And we seem to
> > agree that either way, with or without this fix, we need more changes
> > in this area.
> >
> > But we need a simple and backportable fix for stable trees, say for
> > rhel7. This bug was reported many times, and this is the simplest
> > solution I was able to find.
>
> I am not 100% convinced that we need a backportable solution,
I think we need, this was already requested,
> but
> regardless my experience is that good clean solutions are almost always
> easier to backport that something we just settle for.
Sure.
> The patch below needs a little more looking and testing but arguably
> it is sufficient.
>
> It implements autoreaping for non-leader threads always. We might want
> to limit this to the case of exec.
I should have mentioned this. Of course, this change was proposed from the
very beginning, when this problem was reported first time. And of course
I like this fix much more than my patch (but yes, we shouldd limit it to
the case of exec).
The only problem is that it is incompatible change, and I have no idea what
can be broken.
Trivial test-case:
void *thread(void *arg)
{
for (;;)
pause();
return NULL;
}
int main(void)
{
pthread_t pt;
pthread_create(&pt, NULL, thread, NULL);
execlp("true", "true", NULL);
}
with your patch applied
$ strace -f ./test
strace: wait4(__WALL): No child processes
Yes, this is just a warning, but still. Now we need to change strace. And we
can't know what else can be confused/broken by this change.
man(ptrace) even documents that all other threads except the thread group leader
report death as if they exited via _exit(2).
Yes, yes, it also says that other threads stop in PTRACE_EVENT_EXIT stop,
so 2/2 (which we need with your change too) is is not compatible too and
I am worried, but:
- this was never really true, an already exiting thread won't stop
if it races with exec
- PTRACE_O_TRACEEXEC is not used that often, it never really worked
- man(ptrace) also mentions that PTRACE_EVENT_EXIT behaviour may be
change in the future.
In short. Of course I considered this change. Looks too risky to me. But.
I will be happy if you send this change and take all the blame ;) I won't
argue (if you limit it to execve case).
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -690,7 +690,9 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> thread_group_empty(tsk) &&
> !ptrace_reparented(tsk) ?
> tsk->exit_signal : SIGCHLD;
> - autoreap = do_notify_parent(tsk, sig);
> + do_notify_parent(tsk, sig);
> + /* Autoreap threads even when ptraced */
> + autoreap = !thread_group_leader(tsk);
> } else if (thread_group_leader(tsk)) {
> autoreap = thread_group_empty(tsk) &&
> do_notify_parent(tsk, tsk->exit_signal);
This is all you need,
> @@ -699,8 +701,6 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> }
>
> tsk->exit_state = autoreap ? EXIT_DEAD : EXIT_ZOMBIE;
> - if (tsk->exit_state == EXIT_DEAD)
> - list_add(&tsk->ptrace_entry, &dead);
>
> /* mt-exec, de_thread() is waiting for group leader */
> if (unlikely(tsk->signal->notify_count < 0))
> @@ -711,6 +711,8 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
> list_del_init(&p->ptrace_entry);
> release_task(p);
> }
> + if (autoreap)
> + release_task(tsk);
These 2 changes are not needed. release_task(tsk) will be called by
list_for_each_entry_safe() above if autoreap == T.
Oleg.
next prev parent reply other threads:[~2017-03-03 17:33 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-13 14:14 [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
2017-02-13 14:15 ` [PATCH 1/2] exec: don't wait for zombie threads with cred_guard_mutex held Oleg Nesterov
2017-02-13 16:12 ` kbuild test robot
2017-02-13 16:47 ` Oleg Nesterov
2017-02-13 16:39 ` kbuild test robot
2017-02-13 17:27 ` Mika Penttilä
2017-02-13 18:01 ` Oleg Nesterov
2017-02-13 18:04 ` [PATCH V2 " Oleg Nesterov
2017-02-16 11:42 ` Eric W. Biederman
2017-02-20 15:22 ` Oleg Nesterov
2017-02-20 15:36 ` Oleg Nesterov
2017-02-20 22:30 ` Eric W. Biederman
2017-02-21 17:53 ` Oleg Nesterov
2017-02-21 20:20 ` Eric W. Biederman
2017-02-22 17:41 ` Oleg Nesterov
2017-02-17 4:42 ` Eric W. Biederman
2017-02-20 15:50 ` Oleg Nesterov
2017-02-13 14:15 ` [PATCH 2/2] ptrace: ensure PTRACE_EVENT_EXIT won't stop if the tracee is killed by exec Oleg Nesterov
2017-02-24 16:03 ` [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
2017-03-03 1:05 ` Eric W. Biederman
2017-03-03 17:33 ` Oleg Nesterov [this message]
2017-03-03 18:23 ` Eric W. Biederman
2017-03-03 18:59 ` Eric W. Biederman
2017-03-03 20:06 ` Eric W. Biederman
2017-03-03 20:11 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Eric W. Biederman
2017-03-04 17:03 ` Oleg Nesterov
2017-03-30 8:07 ` Eric W. Biederman
2017-04-01 5:11 ` [RFC][PATCH 0/2] exec: Fixing ptrace'd mulit-threaded hang Eric W. Biederman
2017-04-01 5:14 ` [RFC][PATCH 1/2] sighand: Count each thread group once in sighand_struct Eric W. Biederman
2017-04-01 5:16 ` [RFC][PATCH 2/2] exec: If possible don't wait for ptraced threads to be reaped Eric W. Biederman
2017-04-02 15:35 ` Oleg Nesterov
2017-04-02 18:53 ` Eric W. Biederman
2017-04-03 18:12 ` Oleg Nesterov
2017-04-03 21:04 ` Eric W. Biederman
2017-04-05 16:44 ` Oleg Nesterov
2017-04-02 15:38 ` [RFC][PATCH 0/2] exec: Fixing ptrace'd mulit-threaded hang Oleg Nesterov
2017-04-02 22:50 ` [RFC][PATCH v2 0/5] " Eric W. Biederman
2017-04-02 22:51 ` [RFC][PATCH v2 1/5] ptrace: Don't wait in PTRACE_O_TRACEEXIT for exec or coredump Eric W. Biederman
2017-04-05 16:19 ` Oleg Nesterov
2017-04-02 22:51 ` [RFC][PATCH v2 2/5] sighand: Count each thread group once in sighand_struct Eric W. Biederman
2017-04-02 22:52 ` [RFC][PATCH v2 3/5] clone: Disallown CLONE_THREAD with a shared sighand_struct Eric W. Biederman
2017-04-05 16:24 ` Oleg Nesterov
2017-04-05 17:34 ` Eric W. Biederman
2017-04-05 18:11 ` Oleg Nesterov
2017-04-02 22:53 ` [RFC][PATCH v2 4/5] exec: If possible don't wait for ptraced threads to be reaped Eric W. Biederman
2017-04-05 16:15 ` Oleg Nesterov
2017-04-02 22:57 ` [RFC][PATCH v2 5/5] signal: Don't allow accessing signal_struct by old threads after exec Eric W. Biederman
2017-04-05 16:18 ` Oleg Nesterov
2017-04-05 18:16 ` Eric W. Biederman
2017-04-06 15:48 ` Oleg Nesterov
2017-04-02 16:15 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Oleg Nesterov
2017-04-02 21:07 ` Eric W. Biederman
2017-04-03 18:37 ` Oleg Nesterov
2017-04-03 22:49 ` Eric W. Biederman
2017-04-03 22:49 ` scope of cred_guard_mutex Eric W. Biederman
2017-04-05 16:08 ` Oleg Nesterov
2017-04-05 16:11 ` Kees Cook
2017-04-05 17:53 ` Eric W. Biederman
2017-04-05 18:15 ` Oleg Nesterov
2017-04-06 15:55 ` Oleg Nesterov
2017-04-07 22:07 ` Kees Cook
2017-09-04 3:19 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Robert O'Callahan
2017-03-04 16:54 ` [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
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