linux-kernel - Re: [GIT PULL] timer fixes

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Message-ID: <YvjtkidVZg2sBY0R@zn.tnic>
Date:   Sun, 14 Aug 2022 14:41:54 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, x86-ml <x86@...nel.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [GIT PULL] timer fixes

On Sat, Aug 13, 2022 at 01:27:40PM -0700, Linus Torvalds wrote:
> On Sat, Aug 13, 2022 at 9:25 AM Borislav Petkov <bp@...en8.de> wrote:
> >
> > That task_struct.sighand is marked __rcu and thus noderef and sparse
> > complains:
> 
> I think that RCU marking is misleading.
> 
> Doing a
> 
>         git grep -e '->sighand'
> 
> shows that we basically never treat that as some kind of RCU pointer.
> 
> Adding a
> 
>         grep -i rcu
> 
> to the above shows that we have a couple of places that do this
> carefully, but they are the exception rather than the rule.
> 
> I think the issue is that "current->sighand" is always safe (and that
> "me->sighand" is the same thing), and that sighand has RCU-delayed
> freeing so that __lock_task_sighand() can safely try to take the lock
> of another process' sighand.
> 
> And we have no real way to explain to sparse that *some* cases are
> fine, others are not and need the sighand lock (after that careful
> __lock_task_sighand thing).

Sounds to me like that sparse check was not such a good idea in the
first place. Especially since the 0day bot is probably warning about all
those cases where we try to lock ->sighand.

It was added by

913292c97d75 ("sched.h: Annotate sighand_struct with __rcu")

Lemme add the involved parties to Cc.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette