Fix crash in mb_substr with MacJapanese encoding #20959
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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
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Jan 18, 2026
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Indeed. MacJapanese is sometimes multi codepoints.
LGTM
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Thanks to @youkidearitai for code review. Merged. |
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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