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Address potential perf regression

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Ensure platforms that don't have an assembly implementation for
mbedtls_get_unaligned_volatile_uint32() don't experience a performance
regression.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2022-12-30 21:25:35 +00:00
parent 36dfc5a237
commit 051225d07a
1 changed files with 20 additions and 18 deletions
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38
library/constant_time.c
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@ -50,18 +50,29 @@
/*
* Define MBEDTLS_EFFICIENT_UNALIGNED_ACCESS for architectures where unaligned memory
* accesses are known to be safe and efficient.
*/
#if defined(__ARM_FEATURE_UNALIGNED)
/* __ARM_FEATURE_UNALIGNED is defined by armcc, gcc 7, clang 9 and later versions */
#define MBEDTLS_EFFICIENT_UNALIGNED_ACCESS
#endif
/*
* Define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS where assembly is present to
* perform fast unaligned access to volatile data.
*
* This is needed because mbedtls_get_unaligned_uintXX etc don't support volatile
* memory accesses.
*
* This macro could be moved into alignment.h but for now it's only used here.
* Some of these definitions could be moved into alignment.h but for now they are
* only used here.
*/
#if defined(__ARM_FEATURE_UNALIGNED)
/* __ARM_FEATURE_UNALIGNED is defined by armcc, gcc 7, clang 9 and later versions. */
#define MBEDTLS_EFFICIENT_UNALIGNED_ACCESS
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) && defined(MBEDTLS_HAVE_ASM)
#if defined(__arm__) || defined(__thumb__) || defined(__thumb2__) || defined(__aarch64__)
#define MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS
#endif
#endif
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS) && defined(MBEDTLS_HAVE_ASM)
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
static inline uint32_t mbedtls_get_unaligned_volatile_uint32(volatile const unsigned char *p)
{
/* This is UB, even where it's safe:
@ -71,21 +82,12 @@ static inline uint32_t mbedtls_get_unaligned_volatile_uint32(volatile const unsi
uint32_t r;
#if defined(__arm__) || defined(__thumb__) || defined(__thumb2__)
asm ("ldr %0, [%1]" : "=r" (r) : "r" (p) :);
return r;
#endif
#if defined(__aarch64__)
#elif defined(__aarch64__)
asm ("ldr %w0, [%1]" : "=r" (r) : "r" (p) :);
return r;
#endif
/* Always safe, but inefficient, fall-back */
if (MBEDTLS_IS_BIG_ENDIAN) {
return (p[0] << 24) | (p[1] << 16) | (p[2] << 8) | p[3];
} else {
return p[0] | (p[1] << 8) | (p[2] << 16) | (p[3] << 24);
}
return r;
}
#endif /* MBEDTLS_EFFICIENT_UNALIGNED_ACCESS */
#endif /* MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS */
int mbedtls_ct_memcmp(const void *a,
const void *b,
@ -96,7 +98,7 @@ int mbedtls_ct_memcmp(const void *a,
volatile const unsigned char *B = (volatile const unsigned char *) b;
volatile uint32_t diff = 0;
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
#if defined(MBEDTLS_EFFICIENT_UNALIGNED_VOLATILE_ACCESS)
for (; (i + 4) <= n; i += 4) {
uint32_t x = mbedtls_get_unaligned_volatile_uint32(A + i);
uint32_t y = mbedtls_get_unaligned_volatile_uint32(B + i);