Make compare_digest more constant-time.

ssbr · web-flow · commit 97f81e6ffd76 · 2020-05-27T00:46:18.000-07:00
The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization. (This is change #1 from https://bugs.python.org/issue40791 .)
diff --git a/Modules/_operator.c b/Modules/_operator.c
@@ -735,7 +735,7 @@ _tscmp(const unsigned char *a, const unsigned char *b,
     volatile const unsigned char *left;
     volatile const unsigned char *right;
     Py_ssize_t i;
-    unsigned char result;
+    volatile unsigned char result;
 
     /* loop count depends on length of b */
     length = len_b;
