Date:   Mon, 13 Feb 2023 16:27:10 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Schspa Shi <schspa@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        syzkaller-bugs@googlegroups.com,
        syzbot <syzbot+6cd18e123583550cf469@syzkaller.appspotmail.com>,
        Hillf Danton <hdanton@sina.com>
Subject: Re: [syzbot] WARNING: locking bug in umh_complete
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On Mon, Feb 06, 2023 at 07:51:16AM -0800, Luis Chamberlain wrote:

> I think this seems to be the same issue that Schspa Shi reported / provided a
> fix sugggestion for [0]. This lead me to ask if:
> 
>   a) incorrect usage of completion on stack could be generic and;
>   b) if we should instead have an API helper for that?
> 
> Although he already implemented a suggestion for b) to answer a) we need
> some SmPL constructs yet to be written by Schspa. The reason I asked for
> b) is that if this is a regular pattern it begs for a) as this sort of
> issue could be prevalent in other places. So the status of Schspa's work
> was that he was going to work on the SmPL grammar to check how frequent
> this incorrect patern could be found.

Do I read correctly, from you above alphabet-soup, that someone is
working on some static analysis for on-stack completions or something?

If so, perhaps the simplest rule would to be ensure there is an
unconditional uninterruptible wait-for-completion() before going out of
scope.

This latter can be spelled like wait_for_completion() or
wait_for_completion_state(TASK_UNINTERRUPTIBLE). More specifically,
TASK_INTERRUPTIBLE and TASK_WAKEKILL must not be set in the state mask
for the wait to be uninterruptible.

If it cannot be proven, raise a warning and audit or somesuch.