[dpdk-dev v5 1/2] eal: introduce rte_timingsafe_memcmp() based on OpenBSD API

[Morten Brørup]

> From: Kai Ji [mailto:kai.ji at intel.com]
> Sent: Wednesday, 1 October 2025 17.33
> 
> Bugzilla ID: 1773
> https://bugs.dpdk.org/show_bug.cgi?id=1773
> 
> Signed-off-by: Kai Ji <kai.ji at intel.com>
> ---
>  lib/eal/include/rte_memory.h | 38 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/lib/eal/include/rte_memory.h
> b/lib/eal/include/rte_memory.h
> index dcc0e69cfe..6939c1caad 100644
> --- a/lib/eal/include/rte_memory.h
> +++ b/lib/eal/include/rte_memory.h
> @@ -746,6 +746,44 @@ __rte_experimental
>  void
>  rte_memzero_explicit(void *dst, size_t sz);
> 
> +/**
> + * @warning
> + * @b EXPERIMENTAL: this API may change without prior notice.
> + *
> + * Constant-time memory comparison.
> + *
> + * This function compares two memory regions in constant time, making
> it
> + * resistant to timing side-channel attacks. The execution time
> depends only
> + * on the length parameter, not on the actual data values being
> compared.
> + *
> + * This is particularly important for cryptographic operations where
> timing
> + * differences could leak information about secret keys, passwords, or
> other
> + * sensitive data.
> + *
> + * @param a
> + *   Pointer to the first memory region to compare
> + * @param b
> + *   Pointer to the second memory region to compare
> + * @param n
> + *   Number of bytes to compare
> + * @return
> + *   0 if the memory regions are identical, non-zero if they differ
> + */
> +__rte_experimental
> +static inline int
> +rte_timingsafe_memcmp(const void *a, const void *b, size_t n)
> +{
> +	const volatile uint8_t *pa = (const volatile uint8_t *)a;
> +	const volatile uint8_t *pb = (const volatile uint8_t *)b;
> +	uint8_t result = 0;
> +	size_t i;
> +
> +	for (i = 0; i < n; i++)
> +		result |= pa[i] ^ pb[i];
> +
> +	return result;
> +}
> +
>  #ifdef __cplusplus
>  }
>  #endif
> --
> 2.34.1

NAK.
This returns (binary) non-equality only. It does not return (tri-state) <0, 0, or >0, so it's not like memcmp or FreeBSD timingsafe_memcmp.

Also, please put the function ("memeq") first in the name, and then the extra property ("timingsafe") last, like rte_memzero_explicit.
Like this:
__rte_experimental
static inline bool
rte_memeq_timingsafe(const void *a, const void *b, size_t n)
{
	const volatile uint8_t *pa = (const volatile uint8_t *)a;
	const volatile uint8_t *pb = (const volatile uint8_t *)b;
	uint8_t result = 0;
	size_t i;

	for (i = 0; i < n; i++)
		result |= pa[i] ^ pb[i];

	return result == UINT8_C(0);
}

Stephen, agree?