From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
Alexey Dobriyan <adobriyan@gmail.com>,
Alexey Gladkov <legion@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Alexey Gladkov <gladkov.alexey@gmail.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
"Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify the logic in lookup_task
Date: Mon, 27 Apr 2020 06:51:23 -0500 [thread overview]
Message-ID: <878sihjgec.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <20200426172207.GA30118@redhat.com> (Oleg Nesterov's message of "Sun, 26 Apr 2020 19:22:07 +0200")
Oleg Nesterov <oleg@redhat.com> writes:
> Eric,
>
> I am sick today and can't read the code, but I feel this patch is not
> right ... please correct me.
> So, iiuc when posix_cpu_timer_create() is called and CPUCLOCK_PERTHREAD
> is false we roughly have
>
> task = pid_task(pid, PIDTYPE_TGID); // lookup_task()
>
> /* WINDOW */
>
> timer->it.cpu.pid = = get_task_pid(task, PIDTYPE_TGID) // posix_cpu_timer_create()
>
> Now suppose that we race with mt-exec and this "task" is the old leader;
> it can be release_task()'ed in the WINDOW above and then get_task_pid()
> will return NULL.
Except it is asking for PIDTYPE_TGID.
task->signal even if it is freed (which it won't be in a mt-exec)
is valid until after an rcu window.
release_task()
put_task_struct_rcu_user()
call_rcu(..., delayed_put_task_struct())
... rcu delay ...
delayed_put_task_struct()
put_task_struct()
__put_task_struct()
put_signal_struct()
free_signal_struct()
Which means that task->signal->pids[PIDTYPE_TGID] will remain valid even
across mt-exec.
Further the only change I have introduced is to perform this work under
rcu_read_lock vs taking a reference to task_struct. As the reference to
task_struct does not prevent release_task, the situation with respect
to races in the rest of the code does not change.
Hmm....
If the case instead is:
> timer->it.cpu.pid = get_task_pid(task, PIDTYPE_PID) // posix_cpu_timer_create()
Which can also happen for threads in the same thread group.
I have to agree that we can wind up with a NULL pid.
And that is a brand new bug, because we didn't use to use pids.
Sigh.
> That is why I suggested to change lookup_task() to return "struct pid*"
> to eliminate the pid -> task -> pid transition.
Yes. I have to agree. Getting rid of the pid -> task -> pid transition
looks important to close bugs like that.
> Apart from the same_thread_group() check for the "thread" case we do not
> need task_struct at all, lookup_task() can do
>
> if (thread) {
> p = pid_task(pid, PIDTYPE_PID);
> if (p && !same_thread_group(p, current))
> pid = NULL;
> } else {
> ... gettime check ...
>
> if (!pid_has_task(pid, PIDTYPE_TGID))
> pid = NULL;
> }
>
> return pid;
>
> No?
There is also the posix_cpu_clock_get, where we immediately use the
clock instead of create something we can use later.
I want to say the gettime case is another reason to go through the whole
transition but the code can just as easily say "pid = task_tgid(current)"
as it can "p = current";
Eric
next prev parent reply other threads:[~2020-04-27 11:54 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-19 14:10 [PATCH v12 0/7] proc: modernize proc to support multiple private instances Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 1/7] proc: rename struct proc_fs_info to proc_fs_opts Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 2/7] proc: allow to mount many instances of proc in one pid namespace Alexey Gladkov
2020-04-23 11:28 ` [PATCH v13 " Alexey Gladkov
2020-04-23 12:16 ` Eric W. Biederman
2020-04-23 20:01 ` Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 3/7] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 4/7] proc: add option to mount only a pids subset Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 5/7] docs: proc: add documentation for "hidepid=4" and "subset=pid" options and new mount behavior Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 6/7] proc: use human-readable values for hidepid Alexey Gladkov
2020-04-19 14:10 ` [PATCH v12 7/7] proc: use named enums for better readability Alexey Gladkov
[not found] ` <87ftcv1nqe.fsf@x220.int.ebiederm.org>
2020-04-23 17:54 ` [PATCH v2 0/2] proc: Calling proc_flush_task exactly once per task Oleg Nesterov
2020-04-23 19:38 ` Eric W. Biederman
2020-04-23 19:39 ` [PATCH v2 1/2] proc: Use PIDTYPE_TGID in next_tgid Eric W. Biederman
2020-04-24 17:29 ` Oleg Nesterov
2020-04-23 19:39 ` [PATCH v2 2/2] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
2020-04-23 20:28 ` Linus Torvalds
2020-04-24 3:33 ` Eric W. Biederman
2020-04-24 18:02 ` Linus Torvalds
2020-04-24 18:46 ` Linus Torvalds
2020-04-24 19:51 ` Eric W. Biederman
2020-04-24 20:10 ` Linus Torvalds
2020-04-24 17:39 ` Oleg Nesterov
2020-04-24 18:10 ` Eric W. Biederman
2020-04-24 20:50 ` [PATCH] proc: Put thread_pid in release_task not proc_flush_pid Eric W. Biederman
[not found] ` <87mu6ymkea.fsf_-_@x220.int.ebiederm.org>
[not found] ` <87blnemj5t.fsf_-_@x220.int.ebiederm.org>
2020-04-26 17:22 ` [PATCH v3 2/6] posix-cpu-timers: Use PIDTYPE_TGID to simplify the logic in lookup_task Oleg Nesterov
2020-04-27 11:51 ` Eric W. Biederman [this message]
2020-04-28 18:03 ` Oleg Nesterov
2020-04-27 10:32 ` Thomas Gleixner
2020-04-27 19:46 ` Eric W. Biederman
[not found] ` <875zdmmj4y.fsf_-_@x220.int.ebiederm.org>
2020-04-26 17:40 ` [PATCH v3 3/6] rculist: Add hlist_swap_before_rcu Linus Torvalds
2020-04-27 14:28 ` Eric W. Biederman
2020-04-27 20:27 ` Linus Torvalds
2020-04-28 12:16 ` [PATCH v4 0/2] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
2020-04-28 12:18 ` [PATCH v4 1/2] rculist: Add hlists_swap_heads_rcu Eric W. Biederman
2020-04-28 12:19 ` [PATCH v4 2/2] proc: Ensure we see the exit of each process tid exactly once Eric W. Biederman
2020-04-28 16:53 ` [PATCH v4 0/2] proc: Ensure we see the exit of each process tid exactly Linus Torvalds
2020-04-28 17:55 ` Eric W. Biederman
2020-04-28 18:55 ` Eric W. Biederman
2020-04-28 19:36 ` Linus Torvalds
2020-04-28 18:05 ` Oleg Nesterov
2020-04-28 18:54 ` Eric W. Biederman
2020-04-28 21:39 ` [PATCH v1 0/4] signal: Removing has_group_leader_pid Eric W. Biederman
2020-04-28 21:45 ` [PATCH v1 1/4] posix-cpu-timer: Tidy up group_leader logic in lookup_task Eric W. Biederman
2020-04-28 21:48 ` [PATCH 2/4] posix-cpu-timer: Unify the now redundant code " Eric W. Biederman
2020-04-28 21:53 ` [PATCH v1 3/4] exec: Remove BUG_ON(has_group_leader_pid) Eric W. Biederman
2020-04-28 21:56 ` [PATCH v4 4/4] signal: Remove has_group_leader_pid Eric W. Biederman
2020-04-30 11:54 ` [PATCH v1 0/3] posix-cpu-timers: Use pids not tasks in lookup Eric W. Biederman
2020-04-30 11:55 ` [PATCH v1 1/3] posix-cpu-timers: Extend rcu_read_lock removing task_struct references Eric W. Biederman
2020-04-30 11:56 ` [PATCH v1 2/3] posix-cpu-timers: Replace cpu_timer_pid_type with clock_pid_type Eric W. Biederman
2020-04-30 11:56 ` [PATCH v1 3/3] posix-cpu-timers: Replace __get_task_for_clock with pid_for_clock Eric W. Biederman
[not found] ` <87h7x6mj6h.fsf_-_@x220.int.ebiederm.org>
2020-04-27 9:43 ` [PATCH v3 1/6] posix-cpu-timers: Always call __get_task_for_clock holding rcu_read_lock Thomas Gleixner
2020-04-27 11:53 ` Eric W. Biederman
[not found] ` <87r1w8ete7.fsf@x220.int.ebiederm.org>
2020-04-27 20:23 ` [PATCH v3] proc: Ensure we see the exit of each process tid exactly Eric W. Biederman
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