[llvm] Work around documented Linux mmap bug. (PR #152595)

[Richard Smith via llvm-commits]

================
@@ -501,8 +501,15 @@ 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) {
+#ifdef __linux__
+      // On Linux, 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')
+#endif
----------------
zygoloid wrote:

```suggestion
      // 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')
```

https://github.com/llvm/llvm-project/pull/152595