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Avoid signed right shift UB

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-08-21 17:06:24 +01:00
parent 0ce0fbc32a
commit 0c99a9083e
1 changed files with 8 additions and 2 deletions
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10
library/constant_time_impl.h
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@ -183,8 +183,14 @@ static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x)
#pragma warning( push )
#pragma warning( disable : 4146 )
#endif
return (mbedtls_ct_condition_t) (((mbedtls_ct_int_t) ((-xo) | -(xo >> 1))) >>
(MBEDTLS_CT_SIZE - 1));
// y is negative (i.e., top bit set) iff x is non-zero
mbedtls_ct_int_t y = (-xo) | -(xo >> 1);
// extract only the sign bit of y so that y == 1 (if x is non-zero) or 0 (if x is zero)
y = (((mbedtls_ct_uint_t) y) >> (MBEDTLS_CT_SIZE - 1));
// -y has all bits set (if x is non-zero), or all bits clear (if x is zero)
return (mbedtls_ct_condition_t) (-y);
#if defined(_MSC_VER)
#pragma warning( pop )
#endif