Merge pull request #3336 from Sonicadvance1/warn_on_mdwe

FEXCore: Warn if MDWE is set

FEXCore/Source/Interface/Core/CPUBackend.cpp

@@ -5,6 +5,10 @@
 #include "Interface/Core/Dispatcher/Dispatcher.h"
 #include <FEXCore/Core/CPUBackend.h>
 
+#ifndef _WIN32
+#include <sys/prctl.h>
+#endif
+
 namespace FEXCore {
 namespace CPU {
 
@@ -349,6 +353,31 @@ auto CPUBackend::GetEmptyCodeBuffer() -> CodeBuffer * {
 }
 
 auto CPUBackend::AllocateNewCodeBuffer(size_t Size) -> CodeBuffer {
+#ifndef _WIN32
+// MDWE (Memory-Deny-Write-Execute) is a new Linux 6.3 feature.
+// It's equivalent to systemd's `MemoryDenyWriteExecute` but implemented entirely in the kernel.
+//
+// MDWE prevents applications from creating RWX memory mappings.
+// This prevents FEX from doing anything JIT related, as FEX uses RWX for JIT memory mappings.
+//
+// A potential workaround to make FEX work with MDWE is to call mprotect every time we need to write or modify code.
+// Alternatively, FEX could use a memory mirror where one half is mapped as RW and the other is RX.
+//
+// Once MDWE is enabled with the prctl, the feature is sealed and it can /NOT/ be turned off.
+//
+// Status of MDWE is queried through prctl using `PR_GET_MDWE`:
+// -1: The kernel doesn't support MDWE
+// 0: MDWE is supported but disabled
+// >0: MDWE is enabled, hence prohibiting RWX mappings
+#ifndef PR_GET_MDWE
+#define PR_GET_MDWE 66
+#endif
+  int MDWE = ::prctl(PR_GET_MDWE, 0, 0, 0, 0);
+  if (MDWE != -1 && MDWE != 0) {
+    LogMan::Msg::EFmt("MDWE was set to 0x{:x} which means FEX can't allocate executable memory", MDWE);
+  }
+#endif
+
   CodeBuffer Buffer;
   Buffer.Size = Size;
   Buffer.Ptr = static_cast<uint8_t *>(