Re: [PATCH v3] crypto: more robust crypto_memneq

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On 11/26/2013 01:00 AM, 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.

Actually with yet another version, I hoped that the "compile-tested"-only
statement would eventually disappear, ohh well. ;)

Signed-off-by: Cesar Eduardo Barros <cesarb@xxxxxxxxxxxxx>

Resolving the OPTIMIZER_HIDE_VAR() macro for others than GCC jnto a
barrier() seems a bit suboptimal, but assuming 99% of people will use
GCC anyway, then for the minority of the remaining, they will worst case
have a clever compiler and eventually mimic memcmp() in some situations,
or have a not-so-clever compiler and execute the full code as is.

Anyway, I think still better than the rather ugly Makefile workaround
imho, so I'm generally fine with this.
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