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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 25 Oct 2022 11:38:31 -0700
Message-ID: <CAHk-=wht2pdAoG97kbFqCyOzi1C714x-BY92pvtMcmtGCfUasw@mail.gmail.com>
Subject: Re: [PATCH 1/2] bitmap: add sanity check function for find_bit()
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Yury Norov <yury.norov@gmail.com>,
        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>
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On Tue, Oct 25, 2022 at 11:26 AM Russell King (Oracle)
<linux@armlinux.org.uk> wrote:
>
> However, we still support ARMv5 and ARMv4, both of which _trap_ every
> unaligned access, which will make a findbit call with an unaligned
> pointer using word loads painfully expensive. This is the main reason
> we haven't used word loads in the findbit ops.

The findbit ops really shouldn't be a special case, and bitmaps can
never be unaligned.

Just look at what 'test_bit()' does: the non-constant non-instrumented
version ends up as generic_test_bit(), which uses a "const volatile
unsigned long *" access to do the bitmap load.

So there is absolutely no way that bitmaps can ever be unaligned,
because that would trap.

And test_bit() is a lot more fundamental than one of the "find bits" functions.

Have we had bugs in this area before? Sure. People have used "unsigned
int" for flags and mised the bitmap ops on it, and it has worked on
x86.

But then it fails *miserably* on big-endian machines and on machines
that require more alignment (and even on x86 we have KASAN failures
etc these days and obviously without casts it will warn), so we've
hopefully fixed all those cases up long long ago.

So I really think it's pointless to worry about alignment for
"find_bit()" and friends, when much more fundamental bitop functions
don't worry about it.

               Linus