Re: [EXTERNAL] [dpdk-dev v1] cryptodev: introduce constant-time memory comparison

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Sep 26, 2025 at 08:55:55AM +0100, Bruce Richardson wrote:
> On Thu, Sep 25, 2025 at 10:47:42PM +0200, Thomas Monjalon wrote:
> > 25/09/2025 12:33, Akhil Goyal:
> > > > +/**
> > > > + * Constant-time memory comparison for cryptographic use.
> > > > + * Returns 0 if the memory regions are equal, nonzero otherwise.
> > > > + * Runs in constant time with respect to the length to prevent timing attacks.
> > > > + *
> > > > + * @param a
> > > > + *   Pointer to the first memory region.
> > > > + * @param b
> > > > + *   Pointer to the second memory region.
> > > > + * @param n
> > > > + *   Number of bytes to compare.
> > > > + * @return
> > > > + *   0 if memory regions are equal, nonzero otherwise.
> > > > + */
> > > > +#define rte_consttime_memcmp(a, b, n) __extension__ ({ \
> > > > +	const volatile uint8_t *__pa = (const volatile uint8_t *)(a); \
> > > > +	const volatile uint8_t *__pb = (const volatile uint8_t *)(b); \
> > > > +	uint8_t __result = 0; \
> > > > +	for (size_t __i = 0; __i < (n); __i++) \
> > > > +		__result |= __pa[__i] ^ __pb[__i]; \
> > > > +	__result; \
> > > > +})
> > > 
> > > I believe this is not the right place to add this define.
> > > It should be somewhere in common eal if it is already not there.
> > 
> > Yes indeed.
> > cryptodev is the API for managing crypto devices.
> > A new memcmp function would be better hosted in libc,
> > and in EAL for compatibility with all supported libc.
> > 
> > I mean please add it in EAL, and propose it to glibc as well.
> > 
> 
> Just for reference, there is a good discussion of such functions and
> reference code under MIT license at [1]. After reading that, I note that
> the proposed macro above it not strictly a memcmp function because it just
> returns a zero/non-zero value, rather than a value indicating which array
> value is greater. Therefore some feedback on this code:
> * Use an inline function returning bool rather than a macro
> * A more accurate name might be rte_consttime_memneq, since the code
>   returns 0 (false) if equal.
> 

Or better, change the return value from result to !result to make it a
"memeq" function, which also has the benefit of clamping return values to
just 0 and 1.

> Regars,
> /Bruce
> 
> [1] https://github.com/chmike/cst_time_memcmp