Messages in this thread | | | Date | Mon, 25 Nov 2013 08:59:03 -0700 | From | James Yonan <> | Subject | Re: [PATCH] crypto: more robust crypto_memneq |
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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
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