mirrors/cpython
1
0
Fork 0
mirror of https://github.com/python/cpython.git synced 2025-08-03 00:23:06 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 6

bpo-40791: Make compare_digest more constant-time. (GH-20444)

Browse source 
* 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 31729366e2)

Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
This commit is contained in:
Miss Islington (bot) 2020-11-21 01:18:41 -08:00 • committed by GitHub
parent ed0201805c
commit c1bbca5b00
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
2 changed files with 2 additions and 1 deletions
Download patch file Download diff file Expand all files Collapse all files

2
Modules/_operator.c
View file

@ -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;