diff options
| author | Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> | 2020-11-21 09:12:21 (GMT) |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2020-11-21 09:12:21 (GMT) |
| commit | 97136d71a78a4b6b816f7e14acc52be426efcb6f (patch) | |
| tree | 15f723a869c7c40d2f01e97e755b59f7ccc3acc6 /Modules/_operator.c | |
| parent | 0762e09eb14269b38f60e1b58d2c7f36056a4694 (diff) | |
| download | cpython-97136d71a78a4b6b816f7e14acc52be426efcb6f.zip cpython-97136d71a78a4b6b816f7e14acc52be426efcb6f.tar.gz cpython-97136d71a78a4b6b816f7e14acc52be426efcb6f.tar.bz2 | |
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>
Diffstat (limited to 'Modules/_operator.c')
| -rw-r--r-- | Modules/_operator.c | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/Modules/_operator.c b/Modules/_operator.c index 5aa229f..a816548 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; |