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 GH-1 from https://bugs.python.org/issue40791 .)
(cherry picked from commit 31729366e2bc09632e78f3896dbce0ae64914f28)
Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
diff --git a/Modules/_operator.c b/Modules/_operator.c
index 8a54829..6f8f68f 100644
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -735,7 +735,7 @@
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;