[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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@@ -0,0 +1,109 @@
+//===- Unix/MemoryLinux.h - Linux specific Helper Fuctions ------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines Linux specific helper functions for memory management.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_SUPPORT_UNIX_MEMORYLINUX_H
+#define LLVM_LIB_SUPPORT_UNIX_MEMORYLINUX_H
+
+#ifndef __linux__
+#error Linux only support header!
+#endif
+
+#include "llvm/Support/Process.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include <sys/mman.h>
+#include <sys/syscall.h>
+
+#ifndef MFD_CLOEXEC
+#define MFD_CLOEXEC 0x0001U
+#endif
+#ifndef MFD_EXEC
+#define MFD_EXEC 0x0010U
+#endif
+
+namespace llvm {
+namespace sys {
+namespace {
+
+static inline bool isPermissionError(int err) {
+  // PaX uses EPERM, SELinux uses EACCES
+  return err == EPERM || err == EACCES;
+}
+
+// FIXME: Make this either more low-level C'ish or C++'ish
+static inline bool execProtChangeNeedsNewMapping() {
+  static int status = -1;
+
+  if (status != -1)
+    return status;
+
+  // Try to get the status from /proc/self/status, looking for PaX flags.
+  FILE *f = fopen("/proc/self/status", "re");
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DavidSpickett wrote:

> the last line in /proc/self/status.

It is for now, I appreciate testing that would mean rebuilding the kernel but if it's easy to account for without doing that it's worth it I think.

Perhaps split at the first `\n` after `PaX:`? That handles the end of the file, or the entry if there are future additions.

> I already tested this works an does the right thing ;)

Sure, I assume most people do but sometimes the reasons things work are not the reasons we expect them to, then those change and it not longer works. We just had a bug found parsing `/proc/stat` along these lines.

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