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[BitstreamReader] Fix 32-bit overflow #117363

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Nov 27, 2024
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[BitstreamReader] Fix 32-bit overflow #117363

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3 changes: 2 additions & 1 deletion llvm/lib/Bitstream/Reader/BitstreamReader.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -334,7 +334,8 @@ Expected<unsigned> BitstreamCursor::readRecord(unsigned AbbrevID,

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To me this raises a question of whether we want to support writing 4GB+ blobs in bitcode files. We could use ReadVBR64(6) above and EmitVBR64(6) on the writer side, and I think that would be backwards compatible, it just widens the maximum encodable value.

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@mingmingl-llvm mingmingl-llvm Nov 25, 2024 •
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The change to {Read,Emit}VBR64(6) looks backward compatible to me as well, and https://llvm.org/docs/BitCodeFormat.html#define-abbrev-encoding needs an update around Blob (code 5): This field is emitted as a vbr6, followed by padding to a 32-bit boundary (for alignment) and an array of 8-bit objects It's not clear to me (from the doc and code comment around) how 32-bit alignment is related with value width (32 vs 64) and whether alignment needs an update for 4GB+ blobs.

If we decide not to support 4GB blob soon, I think emitting a release-build visible warning (e.g., by LLVMContext's diagnostic handler) when blob size is larger than 4GB around

llvm-project/llvm/include/llvm/Bitstream/BitstreamWriter.h

Line 550 in 76f0ff8

EmitVBR(static_cast<uint32_t>(Bytes.size()), 6);
is generally a good change.

// Figure out where the end of this blob will be including tail padding.
size_t CurBitPos = GetCurrentBitNo();
const size_t NewEnd = CurBitPos + alignTo(NumElts, 4) * 8;
const size_t NewEnd =
CurBitPos + static_cast<uint64_t>(alignTo(NumElts, 4)) * 8;

// Make sure the bitstream is large enough to contain the blob.
if (!canSkipToPos(NewEnd/8))
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