[Support] Fix thread safety issue in raw_null_ostream #162787
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@llvm/pr-subscribers-llvm-support Author: Scott Pillow (scottp101) ChangesThe global raw_null_ostream singleton returned by llvm::nulls() is For example: raw_ostream::write(const char *Ptr, size_t Size) This can manifest as a heap corruption when multiple threads write to the The fix is to explicitly pass Unbuffered=true to the raw_pwrite_stream For example, this can fix multithreaded applications using MCELFStreamer Full diff: https://github.com/llvm/llvm-project/pull/162787.diff 2 Files Affected:
diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
index f87344e860518..70916d8e4adb0 100644
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -739,7 +739,7 @@ class LLVM_ABI raw_null_ostream : public raw_pwrite_stream {
uint64_t current_pos() const override;
public:
- explicit raw_null_ostream() = default;
+ explicit raw_null_ostream() : raw_pwrite_stream(/*Unbuffered=*/true) {}
~raw_null_ostream() override;
};
diff --git a/llvm/unittests/Support/raw_ostream_test.cpp b/llvm/unittests/Support/raw_ostream_test.cpp
index fbeff37d26a35..a007baa8527b9 100644
--- a/llvm/unittests/Support/raw_ostream_test.cpp
+++ b/llvm/unittests/Support/raw_ostream_test.cpp
@@ -626,6 +626,11 @@ TEST(raw_ostreamTest, writeToDevNull) {
EXPECT_TRUE(DevNullIsUsed);
}
+TEST(raw_ostreamTest, nullStreamZeroBufferSize) {
+ raw_ostream &NullStream = nulls();
+ EXPECT_EQ(NullStream.GetBufferSize(), 0);
+}
+
TEST(raw_ostreamTest, writeToStdOut) {
outs().flush();
testing::internal::CaptureStdout();
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michalpaszkowski
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jurahul,
kuhar,
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jurahul
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LGTM! Let's wait for the builds and checks to finish. Tagging @dwblaikie and @kuhar (maintainers for this part of the project) for visibility/review.
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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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Okay, build succeeded. I don't have merge privileges. Can someone push this through? |
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@scottp101 Congratulations on having your first Pull Request (PR) merged into the LLVM Project! Your changes will be combined with recent changes from other authors, then tested by our build bots. If there is a problem with a build, you may receive a report in an email or a comment on this PR. Please check whether problems have been caused by your change specifically, as the builds can include changes from many authors. It is not uncommon for your change to be included in a build that fails due to someone else's changes, or infrastructure issues. How to do this, and the rest of the post-merge process, is covered in detail here. If your change does cause a problem, it may be reverted, or you can revert it yourself. This is a normal part of LLVM development. You can fix your changes and open a new PR to merge them again. If you don't get any reports, no action is required from you. Your changes are working as expected, well done! |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/154/builds/22604 Here is the relevant piece of the build log for the reference |
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.
5 participants
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.