References: <20230127014137.4906-1-hdanton@sina.com>
 <9d9b9652-c1ac-58e9-2eab-9256c17b1da2@I-love.SAKURA.ne.jp>
 <7d1021f1-c88e-5a03-3b92-087f9be37491@I-love.SAKURA.ne.jp>
 <Y9z76ZLe4On96xIN@hirez.programming.kicks-ass.net>
 <Y9z+SerR8mlZYo16@hirez.programming.kicks-ass.net>
 <6c47dbcf-2a17-6bb2-719b-841d851c603b@I-love.SAKURA.ne.jp>
 <Y90ar35uKQoUrLEK@hirez.programming.kicks-ass.net>
 <caa13441-5f95-b7d6-dd5d-1cf49e709714@I-love.SAKURA.ne.jp>
 <Y+Eh9CFdTwwv+hot@bombadil.infradead.org>
 <Y+pWzult7UDgoilC@hirez.programming.kicks-ass.net>
User-agent: mu4e 1.8.10; emacs 29.0.60
From:   Schspa Shi <schspa@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        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
Date:   Tue, 14 Feb 2023 10:31:58 +0800
In-reply-to: <Y+pWzult7UDgoilC@hirez.programming.kicks-ass.net>
Message-ID: <m21qmkahoj.fsf@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk


Peter Zijlstra <peterz@infradead.org> writes:

> 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?
>

Yes, I was trying to do this.

> 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.

This is a good suggestion. I have written a SmPL patch to complete this
check, and now I need to rule out the situation that the driver has
added an additional lock to protect it.

And I have found a lot of bad usage, should we consider adding a new
helper API to simplify the fix this?

Such as:

+
+void complete_on_stack(struct completion **x)
+{
+       struct completion *comp = xchg(*x, NULL);
+
+       if (comp)
+               complete(comp);
+}
+EXPORT_SYMBOL(complete_on_stack);
+
+int __sched wait_for_completion_state_on_stack(struct completion **x,
+                                       unsigned int state)
+{
+       struct completion *comp = *x;
+       int retval;
+
+       retval = wait_for_completion_state(comp, state);
+       if (retval) {
+               if (xchg(*x, NULL))
+                       return retval;
+
+               /*
+                * complete_on_stack will call complete shortly.
+                */
+               wait_for_completion(comp);
+       }
+
+       return retval;
+}
+EXPORT_SYMBOL(wait_for_completion_state_on_stack);

Link: https://lore.kernel.org/all/20221115140233.21981-1-schspa@gmail.com/T/#mf6a41a7009bb47af1b15adf2b7b355e495f609c4

-- 
BRs
Schspa Shi