Re: [PATCH] crypto: more robust crypto_memneq

[James Yonan <james@openvpn.net>]

On 24/11/2013 14:12, Cesar Eduardo Barros wrote:
> Disabling compiler optimizations can be fragile, since a new
> optimization could be added to -O0 or -Os that breaks the assumptions
> the code is making.
>
> Instead of disabling compiler optimizations, use a dummy inline assembly
> (based on RELOC_HIDE) to block the problematic kinds of optimization,
> while still allowing other optimizations to be applied to the code.
>
> The dummy inline assembly is added after every OR, and has the
> accumulator variable as its input and output. The compiler is forced to
> assume that the dummy inline assembly could both depend on the
> accumulator variable and change the accumulator variable, so it is
> forced to compute the value correctly before the inline assembly, and
> cannot assume anything about its value after the inline assembly.
>
> This change should be enough to make crypto_memneq work correctly (with
> data-independent timing) even if it is inlined at its call sites. That
> can be done later in a followup patch.
>
> Compile-tested on x86_64.
>
> Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.eti.br>

This approach using __asm__ ("" : "=r" (var) : "0" (var)) to try to 
prevent compiler optimizations of var is interesting.

I like the fact that it's finer-grained than -Os and doesn't preclude 
inlining.

One concern would be that __asm__ could be optimized out unless 
__volatile__ is present.

James