Re: [PATCH] crypto: memneq: avoid implicit unaligned accesses

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Jan 19, 2022 at 10:31:09AM +0100, Ard Biesheuvel wrote:
> The C standard does not support dereferencing pointers that are not
> aligned with respect to the pointed-to type, and doing so is technically
> undefined behavior, even if the underlying hardware supports it.
> 
> This means that conditionally dereferencing such pointers based on
> whether CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y is not the right thing
> to do, and actually results in alignment faults on ARM, which are fixed
> up on a slow path. Instead, we should use the unaligned accessors in
> such cases: on architectures that don't care about alignment, they will
> result in identical codegen whereas, e.g., codegen on ARM will avoid
> doubleword loads and stores but use ordinary ones, which are able to
> tolerate misalignment.
> 
> Link: https://lore.kernel.org/linux-crypto/CAHk-=wiKkdYLY0bv+nXrcJz3NH9mAqPAafX7PpW5EwVtxsEu7Q@mail.gmail.com/
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  crypto/memneq.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric