Re: [PATCH] crypto: siphash - use _unaligned version by default

[Arnd Bergmann <arnd@kernel.org>]

On Fri, Nov 26, 2021 at 4:03 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Arnd,
>
> It looks like Ard's old patch never got picked up so you're dusting it
> off. It looks like you're doing two things here -- moving from an
> ifndef to a much nicer IS_ENABLED, and changing the logic a bit. In
> trying to understand the logic part, I changed this in my buffer:

I actually found the issue independently and came up with this patch
before Ard pointed me to his patch, I mainly took the description of the
problem from him, as his explanation was already well written.

> -#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> -       if (!IS_ALIGNED((unsigned long)data, HSIPHASH_ALIGNMENT))
> +       if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
> +           !IS_ALIGNED((unsigned long)data, HSIPHASH_ALIGNMENT))
>                 return __hsiphash_unaligned(data, len, key);
>         return ___hsiphash_aligned(data, len, key);
>
> into this:
>
> -       if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
> -           !IS_ALIGNED((unsigned long)data, HSIPHASH_ALIGNMENT))
> +       if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
> +           !IS_ALIGNED((unsigned long)data, HSIPHASH_ALIGNMENT))
>                 return __hsiphash_unaligned(data, len, key);
>         return ___hsiphash_aligned(data, len, key);
>
> This way I can actually think about what's happening here.
>
> So with the old one, we use the faster aligned version if *either* the
> CPU has efficient unaligned access OR the bytes are statically known
> to be aligned. This seems sensible.
>
> On the new one, we use the faster aligned version if *both* the bytes
> are statically known to be aligned (ok) AND the CPU doesn't actually
> support efficient unaligned accesses (?). This seems kind of weird.

Yes, this is intentional. The point is that __hsiphash_unaligned() is
the portable version that works with any alignment on any architecture,
while __hsiphash_aligned() is either identical, or may only be called
with aligned data. Passing an unaligned pointer into this function triggers
undefined behavior in C99, which is how it broke on armv7, but in fact
any compiler might optimize this function based on "knowing" that
the lower address bits are zero.

> It also means that CPUs with fast aligned accesses wind up calling the
> slower code path in some cases. Is your supposition that the compiler
> will always optimize the slow codepath to the fast one if the CPU it's
> compiling for supports that? Have you tested this on all platforms?

I have not tested this specific patch on all platforms, but I did
extensive testing of the get_unaligned()/put_unaligned() helpers
in my rewrite earlier this year[1], making sure that these are NOPs
on all the important architectures, and that they prevent the use
of trapping ldrd/ldm instructions on ARMv6/ARMv7.

> Would it make sense to instead just fix clang-13? Or even to just get
> rid of CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for armv6 or undef
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS for armv6 just in this file or
> maybe less messy, split CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS into
> two ifdefs that more sense for our usage?

Clang is actually doing the right thing here, it may be considered a missed
optimization that gcc uses two loads instead of a combined ldm or ldrd ;-)

FWIW, the bug that we saw in the decompressor relying on data alignment on x86
earlier this year only happened on gcc.

      Arnd

[1] https://lkml.org/lkml/2021/5/7/775