Fix crash in mb_substr with MacJapanese encoding #20959
Open
+39
−1
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
Thanks to the GitHub user vi3tL0u1s (Viet Hoang Luu) for reporting this issue. The MacJapanese legacy text encoding has a very unusual property; it is possible for a string to encode more codepoints than it has bytes. In some corner cases, this resulted in a situation where the implementation code for mb_substr() would allocate a buffer of size -1. As you can probably imagine, that doesn't end well. Fixes phpGH-20832.
youkidearitai
approved these changes
Jan 18, 2026
Contributor
youkidearitai
left a comment
youkidearitai
left a comment
There was a problem hiding this comment.
Indeed. MacJapanese is sometimes multi codepoints.
LGTM
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
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Outdated suggestions cannot be applied.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.
Thanks to the GitHub user @vi3tL0u1s (Viet Hoang Luu) for reporting this issue.
The MacJapanese legacy text encoding has a very unusual property; it is possible for a string to encode more codepoints than it has bytes. In some corner cases, this resulted in a situation where the implementation code for mb_substr() would allocate a buffer of size -1. As you can probably imagine, that doesn't end well.
Fixes GH-20832.
@youkidearitai @ndossche @cmb69