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Remove volatile from diff; add explanatory comment

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-01-11 17:39:33 +00:00
parent fa96026a0e
commit 7658b63390
1 changed files with 7 additions and 1 deletions
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8
library/constant_time.c
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@ -85,9 +85,15 @@ int mbedtls_ct_memcmp(const void *a,
size_t n) size_t n)
{ {
size_t i = 0; size_t i = 0;
/*
* `A` and `B` are cast to volatile to ensure that the compiler
* generates code that always fully reads both buffers.
* Otherwise it could generate a test to exit early if `diff` has all
* bits set early in the loop.
*/
volatile const unsigned char *A = (volatile const unsigned char *) a; volatile const unsigned char *A = (volatile const unsigned char *) a;
volatile const unsigned char *B = (volatile const unsigned char *) b; volatile const unsigned char *B = (volatile const unsigned char *) b;
volatile uint32_t diff = 0; uint32_t diff = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS) #if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
for (; (i + 4) <= n; i += 4) { for (; (i + 4) <= n; i += 4) {