On 09/15/2013 06:59 PM, James Yonan wrote:
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.
Well, the code snippet I've provided from NaCl [1] is not really "fast-path"
as you say, but rather to prevent the compiler from doing such optimizations
by having a transformation of the "accumulated" bits into 0 and 1 as an end
result (likely to prevent a short circuit), plus it has static size, so no
loops applied here that could screw up.
Variable size could be done under arch/ in asm, and if not available, that
just falls back to normal memcmp that is being transformed into a same return
value. By that, all other archs could easily migrate afterwards. What do you
think?
[1] http://www.spinics.net/lists/linux-crypto/msg09558.html
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