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bignum: make mbedtls_mpi_lsb() less leaky

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The path using builtin should be OK, as it should be using dedicated CPU
instructions which are constant time.

This fixes the no-builing path.

GCC gained support for __has_builtin in version 10. We're still testing
with older GCC on the CI, so the non-builtin path is tested on the CI.

https://gcc.gnu.org/gcc-10/changes.html

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard
2025-07-10 21:26:42 +02:00
parent 246d86b941
commit 65b8011f7e
1 changed files with 23 additions and 18 deletions
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41
library/bignum.c
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@@ -430,13 +430,6 @@ cleanup:
return ret;
}
/*
* Return the number of less significant zero-bits
*/
size_t mbedtls_mpi_lsb(const mbedtls_mpi *X)
{
size_t i;
#if defined(__has_builtin)
#if (MBEDTLS_MPI_UINT_MAX == UINT_MAX) && __has_builtin(__builtin_ctz)
#define mbedtls_mpi_uint_ctz __builtin_ctz
@@ -447,22 +440,34 @@ size_t mbedtls_mpi_lsb(const mbedtls_mpi *X)
#endif
#endif
#if defined(mbedtls_mpi_uint_ctz)
#if !defined(mbedtls_mpi_uint_ctz)
static size_t mbedtls_mpi_uint_ctz(mbedtls_mpi_uint x)
{
size_t count = 0;
mbedtls_ct_condition_t done = MBEDTLS_CT_FALSE;
for (size_t i = 0; i < biL; i++) {
mbedtls_ct_condition_t non_zero = mbedtls_ct_bool((x >> i) & 1);
done = mbedtls_ct_bool_or(done, non_zero);
count = mbedtls_ct_size_if(done, count, i + 1);
}
return count;
}
#endif
/*
* Return the number of less significant zero-bits
*/
size_t mbedtls_mpi_lsb(const mbedtls_mpi *X)
{
size_t i;
for (i = 0; i < X->n; i++) {
if (X->p[i] != 0) {
return i * biL + mbedtls_mpi_uint_ctz(X->p[i]);
}
}
#else
size_t count = 0;
for (i = 0; i < X->n; i++) {
for (size_t j = 0; j < biL; j++, count++) {
if (((X->p[i] >> j) & 1) != 0) {
return count;
}
}
}
#endif
return 0;
}