[llvm] [llvm][Support][Memory] Add memfd based fallback for strict W^X Linux systems (PR #98538)

[David Spickett via llvm-commits]

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@@ -177,6 +181,78 @@ std::error_code Memory::protectMappedMemory(const MemoryBlock &M,
       alignAddr((const uint8_t *)M.Address + M.AllocatedSize, PageSize);
 
   bool InvalidateCache = (Flags & MF_EXEC);
+  bool SkipMprotect = false;
+
+#if defined(__linux__)
+  // Check for cases where the EXEC protection flag changes and a possible
+  // strict W^X policy cannot be bypassed via mprotect() alone, e.g. under
+  // PaX's MPROTECT or SELinux's deny_execmem.
+  //
+  // To support such systems, we need to create a fresh mapping with the
+  // target protection flags.
+  if ((M.Flags ^ Flags) & MF_EXEC && execProtChangeNeedsNewMapping()) {
+    static unsigned int flags = MFD_CLOEXEC | MFD_EXEC;
+    class FDWrapper {
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DavidSpickett wrote:

Weirdly, there is a wrapper like this for locking in `llvm/include/llvm/Support/FileSystem.h` (FileLocker), but none for closing.

https://github.com/llvm/llvm-project/pull/98538