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

[Konrad Kleine via llvm-commits]

================
@@ -0,0 +1,105 @@
+//===- 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 1U
+#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 lol-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");
+  if (f) {
+    char *buf = NULL;
+    size_t len;
+
+    while (getline(&buf, &len, f) != -1) {
+      if (strncmp(buf, "PaX:", 4))
+        continue;
+
+      // Look for 'm', indicating PaX MPROTECT is disabled.
+      status = !strchr(buf + 4, 'm');
+      break;
+    }
+
+    fclose(f);
+    free(buf);
+
+    if (status != -1)
+      return status;
+  }
+
+  // Create a temporary writable mapping and try to make it excecutable.  If
+  // this fails, test 'errno' to ensure it failed because we were not allowed
+  // to create such a mapping and not because of some transient error.
+  size_t size = Process::getPageSizeEstimate();
+  void *addr = ::mmap(NULL, size, PROT_READ | PROT_WRITE,
+                      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (addr == MAP_FAILED) {
+    // Must be low on memory or have too many mappings alrady, not much we can
----------------
kwk wrote:

``
```suggestion
    // Must be low on memory or have too many mappings already, not much we can
```

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