Merge commit 'af18c6f22e43' from llvm.org/release/21.x into stable/21.x

Author: git apple-llvm automerger

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__