[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.

0 files changed, 0 insertions, 0 deletions