From 31729366e2bc09632e78f3896dbce0ae64914f28 Mon Sep 17 00:00:00 2001 From: Devin Jeanpierre Date: Sat, 21 Nov 2020 01:55:23 -0700 Subject: [PATCH] bpo-40791: Make compare_digest more constant-time. (GH-20444) * bpo-40791: Make compare_digest more constant-time. 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 .) --- Modules/_operator.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/Modules/_operator.c b/Modules/_operator.c index 51daa1f..7fff654 100644 --- 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; -- 2.33.0