On 15/09/2013 09:45, Florian Weimer wrote:
* James Yonan:
+ * Constant-time equality testing of memory regions.
+ * Returns 0 when data is equal, non-zero otherwise.
+ * Fast path if size == 16.
+ */
+noinline unsigned long crypto_mem_not_equal(const void *a, const void *b, size_t size)
I think this should really return unsigned or int, to reduce the risk
that the upper bytes are truncated because the caller uses an
inappropriate type, resulting in a bogus zero result. Reducing the
value to 0/1 probably doesn't hurt performance too much. It also
doesn't encode any information about the location of the difference in
the result value, which helps if that ever leaks.
The problem with returning 0/1 within the function body of
crypto_mem_not_equal is that it makes it easier for the compiler to
introduce a short-circuit optimization.
It might be better to move the test where the result is compared against
0 into an inline function:
noinline unsigned long __crypto_mem_not_equal(const void *a, const void
*b, size_t size);
static inline int crypto_mem_not_equal(const void *a, const void *b,
size_t size) {
return __crypto_mem_not_equal(a, b, size) != 0UL ? 1 : 0;
}
This hides the fact that we are only interested in a boolean result from
the compiler when it's compiling crypto_mem_not_equal.c, but also
ensures type safety when users test the return value. It's also likely
to have little or no performance impact.
James
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