[all-commits] [llvm/llvm-project] cb4fdc: [Support] Fix thread safety issue in raw_null_ostr...

[Scott Pillow via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cb4fdc00102f351606c57afc497d939103cac026
      https://github.com/llvm/llvm-project/commit/cb4fdc00102f351606c57afc497d939103cac026
  Author: Scott Pillow <scott.pillow at intel.com>
  Date:   2025-10-10 (Fri, 10 Oct 2025)

  Changed paths:
    M llvm/include/llvm/Support/raw_ostream.h
    M llvm/unittests/Support/raw_ostream_test.cpp

  Log Message:
  -----------
  [Support] Fix thread safety issue in raw_null_ostream (#162787)

The global raw_null_ostream singleton returned by llvm::nulls() is
marked as InternalBuffer rather than Unbuffered, causing it to
allocate a buffer when first written to. In multithreaded environments,
multiple threads can simultaneously trigger buffer allocation via
SetBuffered(), leading to race conditions on the buffer pointer
fields (OutBufCur, OutBufEnd).

For example:

raw_ostream::write(const char *Ptr, size_t Size)
->
raw_ostream::SetBuffered()
->
raw_ostream::SetBufferSize(size_t Size)
->
raw_ostream::SetBufferAndMode(char *BufferStart, size_t Size,
                              BufferKind Mode)

This can manifest as a heap corruption when multiple threads write to
the
null stream concurrently, as the buffer pointers will become corrupted
during the race.

The fix is to explicitly pass Unbuffered=true to the raw_pwrite_stream
constructor, ensuring the null stream never allocates a buffer and
all writes go directly to the no-op write_impl().

For example, this can fix multithreaded applications using MCELFStreamer
where getCommentOS() returns the shared nulls() singleton.



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