linux-kernel - Re: Question on commit dc7109aaa233 ("futex: Validate waiter correctly in futex_proxy_trylock

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Message-ID: <87a6kyfr4r.ffs@tglx>
Date:   Tue, 31 Aug 2021 01:29:40 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: Question on commit dc7109aaa233 ("futex: Validate waiter
 correctly in futex_proxy_trylock_atomic()")

Lukas,

On Fri, Aug 20 2021 at 13:17, Lukas Bulwahn wrote:
> in commit dc7109aaa233 ("futex: Validate waiter correctly in
> futex_proxy_trylock_atomic()") visible on next-20210819, you add:
>
> +    /*
> +     * Ensure that this is a waiter sitting in futex_wait_requeue_pi()
> +     * and waiting on the 'waitqueue' futex which is always !PI.
> +     */
> +    if (!top_waiter->rt_waiter || top_waiter->pi_state)
> +        ret = -EINVAL;
>
> However, ret is unconditionally reassigned later and erases any
> intended effect of this assignment. This is making that assignment
> above a Dead Store, which clang-analyzer correctly warns about and
> which motivates me to write you an email.
>
> Did you intend to return -EINVAL here? So:
>
> +       if (!top_waiter->rt_waiter || top_waiter->pi_state)
> +               return -EINVAL;

Duh, yes.

> Static analysis tools are as foolish as they are... but every dog has its day...

IOW: Even a blind hen sometimes finds a grain of corn :)

Care to send a patch?

Thanks,

        tglx