linux-kernel - Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

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Message-ID: <87lfp7h422.fsf@x220.int.ebiederm.org>
Date:   Wed, 12 Feb 2020 15:46:29 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Security Module <linux-security-module@...r.kernel.org>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...chiereds.net>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Solar Designer <solar@...nwall.com>
Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Wed, Feb 12, 2020 at 12:41 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>>
>> On Wed, Feb 12, 2020 at 08:38:33PM +0000, Al Viro wrote:
>> >
>> > Wait, I thought the whole point of that had been to allow multiple
>> > procfs instances for the same userns?  Confused...
>>
>> s/userns/pidns/, sorry
>
> Right, but we still hold the ref to it here...
>
> [ Looks more ]
>
> Oooh. No we don't. Exactly because we don't hold the lock, only the
> rcu lifetime, the ref can go away from under us. I see what your
> concern is.
>
> Ouch, this is more painful than I expected - the code flow looked so
> simple. I really wanted to avoid a new lock during process shutdown,
> because that has always been somewhat painful.

The good news is proc_flush_task isn't exactly called from process exit.
proc_flush_task is called during zombie clean up. AKA release_task.

So proc_flush_task isn't called with any locks held, and it is
called in a context where it can sleep.

Further after proc_flush_task does it's thing the code goes
and does "write_lock_irq(&task_list_lock);"

So the code is definitely serialized to one processor already.

What would be downside of having a mutex for a list of proc superblocks?
A mutex that is taken for both reading and writing the list.

Eric