linux-kernel - Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

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Message-ID: <Y2K6clNJBn0SbWU+@zn.tnic>
Date:   Wed, 2 Nov 2022 19:44:02 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Jones <ajones@...tanamicro.com>
Cc:     Yury Norov <yury.norov@...il.com>, x86@...nel.org,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        openrisc@...ts.librecores.org,
        Michael Ellerman <mpe@...erman.id.au>,
        "open list:LINUX FOR POWERPC PA SEMI PWRFICIENT" 
        <linuxppc-dev@...ts.ozlabs.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning

On Mon, Oct 31, 2022 at 11:03:27AM +0100, Andrew Jones wrote:
> Currently (after the revert of 78e5a3399421)

After the revert?

That commit is still in the latest Linus tree.

> with DEBUG_PER_CPU_MAPS we'll get a warning splat when the cpu is
> outside the range [-1, nr_cpu_ids)

Yah, that range makes sense.

> and cpumask_next() will call find_next_bit() with the input plus one anyway.
> find_next_bit() doesn't explicity document what happens when an input is
> outside the range, but it currently returns the bitmap size without any
> side effects, which means cpumask_next() will return nr_cpu_ids.

That is good to have in the commit message.

> show_cpuinfo() doesn't try to show anything in that case and stops its
> loop, or, IOW, things work fine now with an input of nr_cpu_ids - 1. But,
> show_cpuinfo() is just getting away with a violated cpumask_next()
> contract, which 78e5a3399421 exposed. How about a new commit message like
> this

You're making it sound more complex than it is. All you wanna say is:

"Filter out invalid cpumask_next() inputs by checking its first argument
against nr_cpu_ids because cpumask_next() will call find_next_bit() with
the input plus one but the valid range for n is [-1, nr_cpu_ids)."

But that thing with the revert above needs to be clarified first.

Thx.

-- 
Regards/Gruss,
    Boris.

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