https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/153486
Backport 85cd3d9 Requested by: @zygoloid >From 7c72e1eda56b221386cdaa730cc3ec9511e071b0 Mon Sep 17 00:00:00 2001 From: Richard Smith <[email protected]> Date: Wed, 13 Aug 2025 12:39:25 -0700 Subject: [PATCH] Work around documented Linux mmap bug. (#152595) On Linux, mmap doesn't always zero-fill slack bytes ([man page]), despite being required to do so by POSIX. If the final page of a file is in the page cache and the bytes past the end of the file get overwritten by some process, those bytes then remain non-zero until the page falls out of the cache or another process overwrites them. Stop trusting that mmap behaves properly and instead check whether the buffer was indeed properly terminated. If not, fall back to using `read` to read the file contents. This fixes an obscure clang crash bug that can occur if another program (such as an editor) mmap's a source file and writes past the end of the mmap'd region shortly before clang or clangd attempts to parse the file. [man page]: https://man7.org/linux/man-pages/man2/mmap.2.html#BUGS (cherry picked from commit 85cd3d98686c47d015dbcc17f1f7d0714b00e172) --- llvm/lib/Support/MemoryBuffer.cpp | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp index 601f11f6d23c8..1c4645ad83641 100644 --- a/llvm/lib/Support/MemoryBuffer.cpp +++ b/llvm/lib/Support/MemoryBuffer.cpp @@ -501,8 +501,14 @@ getOpenFileImpl(sys::fs::file_t FD, const Twine &Filename, uint64_t FileSize, std::unique_ptr<MB> Result( new (NamedBufferAlloc(Filename)) MemoryBufferMMapFile<MB>( RequiresNullTerminator, FD, MapSize, Offset, EC)); - if (!EC) - return std::move(Result); + if (!EC) { + // On at least Linux, and possibly on other systems, mmap may return pages + // from the page cache that are not properly filled with trailing zeroes, + // if some prior user of the page wrote non-zero bytes. Detect this and + // don't use mmap in that case. + if (!RequiresNullTerminator || *Result->getBufferEnd() == '\0') + return std::move(Result); + } } #ifdef __MVS__ _______________________________________________ llvm-branch-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
