Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()

From: Yury Norov <yury.norov@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Alexey Klimov <klimov.linux@gmail.com>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Andy Whitcroft <apw@canonical.com>,
	Dennis Zhou <dennis@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Guenter Roeck <linux@roeck-us.net>,
	Kees Cook <keescook@chromium.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()
Date: Tue, 25 Oct 2022 10:11:51 -0700	[thread overview]
Message-ID: <Y1gY18JGz4zKw+gW@yury-laptop> (raw)
In-Reply-To: <CAHk-=whij5pwOkG5HhMRzrvc4BTzhgj9h=wHo=tMVGasKE+p1w@mail.gmail.com>

On Sun, Oct 23, 2022 at 03:19:24PM -0700, Linus Torvalds wrote:
> On Wed, Oct 19, 2022 at 8:24 PM Yury Norov <yury.norov@gmail.com> wrote:
> >
> > This patch adds runtime check for the pointers to be aligned.
> 
> No. Don't add pointless things like this. It only adds code, with no advantage.

Sure. Patch #1 is mostly for Russell to address his concern about
unaligned bitmaps on ARM32. And it looks like it found nothing.

> The bitmap ops all operate on 'unsigned long', and if a bitmap isn't
> aligned, we'll take a fault on the architectures that don't do
> unaligned accesses natively.

ARMv6 may or may not support unaligned access depending on SCTLR.U
bit. This is what Russell was concerned about in the other email.
As far as I understand, linux enables that feature.

ARMv7 deprecates that bit and supports unaligned dereference
unconditionally, with few exceptions like exclusive access.

https://developer.arm.com/documentation/ddi0406/b/Appendices/ARMv6-Differences/Application-level-memory-support/Alignment?lang=en
Thanks,
Yury

> And the find-bit functions simply aren't special enough to have this
> kind of random testing, when the *basic* bitmap functions like
> "set_bit()" and friends all do the accesses without any alignment
> checks.
> 
> The fact that filesystem code often uses bitmap functions with a cast
> from 'char *' is immaterial. Those things are already aligned
> (typically they are a whole disk block). They just weren't an array of
> 'unsigned long'.
>
>                 Linus

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