Fix invalid pointer casts in arm64 implementation #65
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It is UB in C and C++ to type-pun pointers in this way, for two reasons. First, casting to a pointer is forbidden if the pointer is not aligned. Second, this kind of type-punning is a strict aliasing violation. Instead, the way to read 8 bytes as a time as a uint64_t is to call memcpy. Compilers recognize this pattern and optimize it, lowering it to the same code, but without breaking the language's abstract state machine. This is needed to fix some UBSan warnings in Chromium.
pwnall
requested changes
Sep 6, 2024
src/crc32c_arm64.cc
Outdated
| @@ -24,17 +25,31 @@ | |||
| #define KBYTES 1032 | |||
| #define SEGMENTBYTES 256 | |||
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| static uint64_t LoadU64(const uint8_t in[sizeof(uint64_t)]) { | |||
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Can you use and extend crc32c_read_le.h?
I think that ReadUint32LE() and ReadUint64LE() there work as you need them to, and we checked that they're compiled as efficiently as memcpy on little-endian architectures.
You can add a ReadUint16LE() with tests, and then use it here.
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Ah nice! Done. See commit message of 1b4eb1a for some notes on what I changed there.
In doing so, I fixed up crc32c_read_le.h a bit:
1. The documentation and tests say the buffers need to be aligned, but
they don't.
2. Add a 16-bit version.
3. Remove an unnecessary static_cast<uint8_t>. The pointer is already
uint8_t.
4. Replace the necessary static_casts with uintN_t{x} per the Google
style guide. https://google.github.io/styleguide/cppguide.html#Casting
pwnall
approved these changes
Sep 6, 2024
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It is UB in C and C++ to type-pun pointers in this way, for two reasons. First, casting to a pointer is forbidden if the pointer is not aligned. Second, this kind of type-punning is a strict aliasing violation.
Instead, the way to read 8 bytes as a time as a uint64_t is to call memcpy. Compilers recognize this pattern and optimize it, lowering it to the same code, but without breaking the language's abstract state machine.
This is needed to fix some UBSan warnings in Chromium.