From 31729366e2bc09632e78f3896dbce0ae64914f28 Mon Sep 17 00:00:00 2001
From: Devin Jeanpierre <jeanpierreda@google.com>
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

