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

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Message-ID: <Y2PxAiJ1yNzcUSgs@zn.tnic>
Date:   Thu, 3 Nov 2022 17:49:06 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     yury.norov@...il.com
Cc:     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 Thu, Nov 03, 2022 at 09:30:54AM -0700, yury.norov@...il.com wrote:a
> Callers should pass sane arguments into internal functions if they
> expect sane output.

What internal function? It's in a global header.

> The API not exported to userspace shouldn't sanity-check all inputs
> arguments.

That doesn't have anything to do with userspace at all.

APIs exported to the rest of the kernel should very well check their
inputs. Otherwise they're not APIs - just some random functions which
are visible to the compiler.

> So, the portable code shouldn't expect from cpumasks more than
> documentation said: for a _valid_ offset cpumask_next() returns next
> set bit or >= nr_cpu_ids.

Lemme quote from my previous mail:

"First make sure cpumask_next()'s valid accepted range has been settled
upon, has been explicitly documented"

So where is that valid range documented?

> cpumask_check() has been broken for years. Attempting to fix it faced
> so much resistance, that I had to revert the patch.

The suggestion on that thread made sense: you first fix the callers and
then the interface. Just like any other "broken" kernel API.

Nothing's stopping you from fixing it properly - it'll just take a while
and if it is such a widely used interface, you probably should come up
with a strategy first how to fix it without impacting current use.

Interfaces and their in-kernel users get refactored constantly.

Thx.

-- 
Regards/Gruss,
    Boris.

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