linux-kernel - Re: [PATCH 1/3] x86: fix access_ok() and valid_user_address() using wrong USER_PTR

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Message-ID: <CAGudoHG1P61Nd7gMriCSF=g=gHxESPBPNmhHjtOQvG8HhpW0rg@mail.gmail.com>
Date: Thu, 6 Nov 2025 14:19:06 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 
	"the arch/x86 maintainers" <x86@...nel.org>, brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	tglx@...utronix.de, pfalcato@...e.de
Subject: Re: [PATCH 1/3] x86: fix access_ok() and valid_user_address() using
 wrong USER_PTR_MAX in modules

On Thu, Nov 6, 2025 at 2:10 PM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Nov 06, 2025 at 01:06:06PM +0100, Mateusz Guzik wrote:
> > I don't know what are you trying to say here.
> >
> > Are you protesting the notion that reducing cache footprint of the
> > memory allocator is a good idea, or perhaps are you claiming these
> > vars are too problematic to warrant the effort, or something else?
>
> I'm saying all work which does not change the code in a trivial way should
> have numbers to back it up. As in: "this change X shows this perf improvement
> Y with the benchmark Z."
>
> Because code uglification better have a fair justification.
>
> Not just random "oh yeah, it would be better to have this." If the changes are
> trivial, sure. But the runtime const thing was added for a very narrow case,
> AFAIR, and it wasn't supposed to have a widespread use. And it ain't that
> trivial, codewise.
>
> IOW, no non-trivial changes which become a burden to maintainers without
> a really good reason for them. This has been the guiding principle for
> non-trivial perf optimizations in Linux. AFAIR at least.
>
> But hey, what do I know...

Then, as I pointed out, you should be protesting the patching of
USER_PTR_MAX as it came with no benchmarks and also resulted in a
regression.