fix: utf8 -> utf16 decoding bug on surrogate pairs #1486
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This fixes protobufjs#1473 The custom utf8 -> utf16 decoder appears to be subtly flawed. From my reading it appears the chunking mechanism doesn't account for surrogate pairs at the end of a chunk causing variable size chunks. A larger chunk followed by a smaller chunk leaves behind garbage that'll be included in the latter chunk. It looks like the chunking mechanism was added to prevent stack overflows when calling `formCharCode` with too many args. From some benchmarking it appears putting utf16 code units in an array and spreading that into `fromCharCode` wasn't helping performance much anyway. I simplified it significantly. Here's a repro of the existing encoding bug in a fuzzing suite https://repl.it/@turbio/oh-no-our-strings#decoder.js
turbio
commented
Sep 10, 2020
| chunk[i++] = t; | ||
| else if (t > 191 && t < 224) | ||
| chunk[i++] = (t & 31) << 6 | buffer[start++] & 63; | ||
| else if (t > 239 && t < 365) { |
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My understanding of this function is that the passed in buffer is always an array of uint8s, each byte being a utf8 code unit. The condition t < 365 is a bit odd under this assumption, from profiling it actually forces v8 to deopt when t is compared to a non uint8 value. Alternatively I may have grossly misunderstood how this is expected to work.
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Hi @turbio, Would it be possible to have a test that shows the problem included in the PR? Thank you! |
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@alexander-fenster yup! added a test which fails on the old |
alexander-fenster
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Oct 9, 2020
alexander-fenster
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masad-frost
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Dec 29, 2020
protobufjs/protobuf.js#1486 released in protobufjs 6.10.2 and updated in @replit/protocol in replit/protocol#9 and released in 0.2.15
This was referenced May 20, 2022
Merged
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hi guys, the merged code seems not included in the latest release. 
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This fixes #1473
The custom utf8 -> utf16 decoder appears to be subtly flawed. From my reading it appears the chunking mechanism doesn't account for surrogate pairs at the end of a chunk causing variable size chunks. A larger chunk followed by a smaller chunk leaves behind garbage that'll be included in the latter chunk.
The chunking mechanism was added to prevent stack overflows when calling
formCharCodewith too many args. From some simple benchmarking it doesn't seem like putting utf16 code units in an array and spreading that intofromCharCodeis helping performance much anyway. I've removed the chunking code which simplifies it significantly and fixes the particular issue we've encountered.Here's a repro of the existing encoding bug in a fuzzing suite
https://repl.it/@turbio/oh-no-our-strings#decoder.js