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Date:   Thu, 3 Nov 2022 17:49:06 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     yury.norov@gmail.com
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Subject: Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning
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On Thu, Nov 03, 2022 at 09:30:54AM -0700, yury.norov@gmail.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