From: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
To: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: Andrew Morton
<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
Aleksa Sarai <asarai-IBi9RG/b67k@public.gmane.org>,
Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>,
Attila Fazekas <afazekas-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
Jann Horn <jann-XZ1E9jl8jIdeoWH0uzbU5w@public.gmane.org>,
Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>,
Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
Ulrich Obergfell
<uobergfe-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped.
Date: Sun, 2 Apr 2017 18:15:18 +0200 [thread overview]
Message-ID: <20170402161518.GC12637@redhat.com> (raw)
In-Reply-To: <8760ir192p.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
On 03/30, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> writes:
>
> > Very nice. So de_thread() returns as soon as all other threads decrement
> > signal->live in do_exit(). Before they do, say, exit_mm(). This is already
> > wrong, for example this breaks OOM. Plus a lot more problems afaics, but
> > lets ignore this.
>
> Which means that we need to keep sig->notify_count.
Yes, although we need to make it less ugly.
> > Note that de_thread() also unshares ->sighand before return. So in the
> > case of mt exec it will likely see oldsighand->count != 1 and alloc the
> > new sighand_struct and this breaks the locking.
> >
> > Because the execing thread will use newsighand->siglock to protect its
> > signal_struct while the zombie threads will use oldsighand->siglock to
> > protect the same signal struct. Yes, tasklist_lock + the fact irq_disable
> > implies rcu_lock mostly save us but not entirely, say, a foreign process
> > doing __send_signal() can take the right or the wrong lock depending on
> > /dev/random.
>
> Which leads to the question how can we get back tot he 2.4 behavior
> of freeing sighand_struct in do_exit?
>
> At which point as soon as we free sighand_struct if we are the last
> to dying thread notify de_thread and everything works.
I was thinking about the similar option, see below, but decided that we
should not do this at least right now.
> For what __ptrace_unlink is doing we should just be able to skip
> acquiring of siglock if PF_EXITING is set.
We can even remove it from release_task() path, this is simple.
> __exit_signal is a little more interesting but half of what it is
> doing looks like it was pulled out of do_exit and just needs to
> be put back.
That is. I think we should actually unhash the exiting sub-thread even
if it is traced. IOW, remove it from thread/pid/parent/etc lists and
nullify its ->sighand. IMO, whatever we do thread_group_empty(current)
should be true after exec.
So the exiting sub-trace should look almost a EXIT_DEAD task except it
still should report to debugger.
But this is dangerous. Say, wait4(upid <= 0) becomes unsafe because
task_pid_type(PIDTYPE_PGID) won't work.
> Which probably adds up to 4 or 5 small carefully written patches to sort
> out that part of the exit path,
Perhaps I am wrong, but I think you underestimate the problems, and it is
not clear to me if we really want this.
=========================================================================
Anyway, Eric, even if we can and want to do this, why we can't do this on
top of my fix?
I simply fail to understand why you dislike it that much. Yes it is not
pretty, I said this many times, but it is safe in that it doesn't really
change the current behaviour.
I am much more worried about 2/2 you didn't argue with, this patch _can_
break something and this is obviously not good even if PTRACE_EVENT_EXIT
was always broken.
Oleg.
next prev parent reply other threads:[~2017-04-02 16:15 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20170213141452.GA30203@redhat.com>
[not found] ` <20170224160354.GA845@redhat.com>
[not found] ` <87shmv6ufl.fsf@xmission.com>
[not found] ` <20170303173326.GA17899@redhat.com>
[not found] ` <20170303173326.GA17899-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-03-03 18:23 ` [PATCH 0/2] fix the traced mt-exec deadlock Eric W. Biederman
2017-03-03 18:59 ` Eric W. Biederman
[not found] ` <87d1dyw5iw.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-03-03 20:06 ` Eric W. Biederman
[not found] ` <87tw7aunuh.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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
[not found] ` <8760ir192p.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-01 5:11 ` [RFC][PATCH 0/2] exec: Fixing ptrace'd mulit-threaded hang Eric W. Biederman
[not found] ` <878tnkpv8h.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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
[not found] ` <87vaqooggs.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-02 15:35 ` Oleg Nesterov
[not found] ` <20170402153517.GA12637-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-04-02 18:53 ` Eric W. Biederman
[not found] ` <877f32k5ew.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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
[not found] ` <874ly6a0h1.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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
[not found] ` <87k2728lrp.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
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
[not found] ` <87zify76z9.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-05 16:18 ` Oleg Nesterov
[not found] ` <20170405161812.GD14536-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-04-05 18:16 ` Eric W. Biederman
[not found] ` <87zifu90to.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-06 15:48 ` Oleg Nesterov
2017-04-02 16:15 ` Oleg Nesterov [this message]
[not found] ` <20170402161518.GC12637-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-04-02 21:07 ` [RFC][PATCH] exec: Don't wait for ptraced threads to be reaped Eric W. Biederman
[not found] ` <87inmmbjsq.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-03 18:37 ` Oleg Nesterov
[not found] ` <20170403183728.GB31390-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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
[not found] ` <87fuhpjeco.fsf_-_-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-04-06 15:55 ` Oleg Nesterov
[not found] ` <20170406155540.GC7444-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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
[not found] ` <87tw7axlr0.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
2017-03-04 16:54 ` [PATCH 0/2] fix the traced mt-exec deadlock Oleg Nesterov
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