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[3.6] bpo-40791: Make compare_digest more constant-time. (GH-23438) #23767

Merged
merged 1 commit into from
Dec 14, 2020
Merged

[3.6] bpo-40791: Make compare_digest more constant-time. (GH-23438) #23767

merged 1 commit into from
Dec 14, 2020

Commits on Dec 14, 2020

  1. bpo-40791: Make compare_digest more constant-time. (pythonGH-23438) 

    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 3172936)

Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>
(cherry picked from commit db95802)

Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
    @miss-islington
    miss-islington committed Dec 14, 2020
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