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| author | Devin Jeanpierre <jeanpierreda@google.com> | 2020-11-21 08:55:23 (GMT) |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2020-11-21 08:55:23 (GMT) |
| commit | 31729366e2bc09632e78f3896dbce0ae64914f28 (patch) | |
| tree | 0596a25a57302928a432ff81662d42141bfbc391 /Modules | |
| parent | aa3a3521cef3998d4f9e7f7ff721163b6e3e5f39 (diff) | |
| download | cpython-31729366e2bc09632e78f3896dbce0ae64914f28.zip cpython-31729366e2bc09632e78f3896dbce0ae64914f28.tar.gz cpython-31729366e2bc09632e78f3896dbce0ae64914f28.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 #1 from https://bugs.python.org/issue40791 .)
Diffstat (limited to 'Modules')
| -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 c9d38aa..da1e431 100644 --- a/Modules/_operator.c +++ b/Modules/_operator.c @@ -749,7 +749,7 @@ _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; |