linux-kernel - Re: [PATCH 1/2] bitmap: add sanity check function for find

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Message-ID: <Y1gY18JGz4zKw+gW@yury-laptop>
Date:   Tue, 25 Oct 2022 10:11:51 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Klimov <klimov.linux@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Andy Whitcroft <apw@...onical.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Kees Cook <keescook@...omium.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()

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@...il.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