Determinism: Avoid Mapping Window into Reserved Indices during Reduction #2869
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
PR facebook#2850 attempted to fix a determinism bug that was uncovered by OSS-Fuzz. It succeeded in addressing that source of non-determinism, but introduced a new one: it was possible, when index reduction occurred, to map indices in the window to the reserved value, which would cause them to be zeroed, potentially altering parsing of the input. This PR addresses this issue. It makes sure that the bottom of the window is always `>= ZSTD_WINDOW_START_INDEX`. I'm not sure if this makes facebook#2850 redundant. I think it's probably still valuable to have that protection as well. Credit to OSS-Fuzz for discovering this issue.
Cyan4973
approved these changes
Nov 17, 2021
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
PR #2850 attempted to fix a determinism bug that was uncovered by OSS-Fuzz. It
succeeded in addressing that source of non-determinism, but introduced a new
one: it was possible, when index reduction occurred, to map indices in the
window to the reserved value, which would cause them to be zeroed, potentially
altering parsing of the input.
This PR addresses this issue. It makes sure that the bottom of the window is
always
>= ZSTD_WINDOW_START_INDEX.I'm not sure if this makes #2850 redundant. I think it's probably still
valuable to have that protection as well.
Credit to OSS-Fuzz for discovering this issue.