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mp_cond_swap: add a brute-force 'volatile'.

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With this change, my new side-channel test system gets a 100% pass
rate when compiled with clang -O3 on Ubuntu 18.10. Previously, it had
three failing tests (namely the three ECC multiply functions), all due
to inconsistent control flow inside mp_cond_swap.

I admit I don't really understand whether this is really necessary or
not, so I'm playing it safe. The problem _seems_ to be that clang has
generated one version of the cond_swap loop using integer arithmetic
and another using MMX vectors, so the obvious suspect is alignment -
probably mp_cond_swap is processing an iteration of the loop up front
until its pointer is 16-byte aligned and then switching over to the
vectorised version. But on the other hand, when I experimentally tried
forcing allocations to be 16- or even 32-byte aligned, it didn't make
a difference. And I don't speak x86 vector instructions very well (in
fact barely at all), so I'm not even completely sure of whether the
code I was reading did what I thought it did; so I'm more comfortable
with simply applying brute force to get some code generation that the
automated test is genuinely happy with.
This commit is contained in:
Simon Tatham 2019-02-10 13:16:12 +00:00
parent 83db341e8a
commit 9cb8c4bcb7
1 changed files with 1 additions and 1 deletions
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2
mpint.c
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@ -115,7 +115,7 @@ void mp_select_into(mp_int *dest, mp_int *src0, mp_int *src1,
void mp_cond_swap(mp_int *x0, mp_int *x1, unsigned swap)
{
assert(x0->nw == x1->nw);
BignumInt mask = -(BignumInt)(1 & swap);
volatile BignumInt mask = -(BignumInt)(1 & swap);
for (size_t i = 0; i < x0->nw; i++) {
BignumInt diff = (x0->w[i] ^ x1->w[i]) & mask;
x0->w[i] ^= diff;