Add workaround for mmap() with PROT_EXEC on Chrome OS.

components/nacl/loader/nonsfi/irt_memory.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <errno.h>
 #include <sys/mman.h>
 
+#include "base/logging.h"
 #include "components/nacl/loader/nonsfi/irt_interfaces.h"
 #include "components/nacl/loader/nonsfi/irt_util.h"
 #include "native_client/src/trusted/service_runtime/include/machine/_types.h"
 #include "native_client/src/trusted/service_runtime/include/sys/mman.h"
 
 namespace nacl {
 namespace nonsfi {
 namespace {
 
 int NaClProtToProt(int nacl_prot) {
@@ skipping 21 matching lines @@
     flags |= MAP_FIXED;
 
   // Note: NACL_ABI_MAP_ANON is an alias of NACL_ABI_MAP_ANONYMOUS.
   if (nacl_flags & NACL_ABI_MAP_ANONYMOUS)
     flags |= MAP_ANONYMOUS;
   return flags;
 }
 
 int IrtMMap(void** addr, size_t len, int prot, int flags,
             int fd, nacl_abi_off_t off) {
-  void* result =
-      mmap(*addr, len, NaClProtToProt(prot), NaClFlagsToFlags(flags), fd, off);
+  const int host_prot = NaClProtToProt(prot);
+  // On Chrome OS, mmap will fail if PROT_EXEC is set in |host_prot|,

[Mark Seaborn, 2014/04/15 23:28:03]

Nit: "can fail"

[mazda, 2014/04/16 00:33:36]

Done.

+  // but mprotect will allow changing the permissions later.
+  // See the comments for NaClHostDescMap in

[Mark Seaborn, 2014/04/15 23:28:03]

X-refs in comments are a little bit of an anti-pattern, because the code
referenced might move or go away.  Maybe just say 'This is because Chrome OS
mounts writable filesystems with "noexec"' instead?

[mazda, 2014/04/16 00:33:36]

Done.

+  // native_client/src/shared/platform/posix/nacl_host_desc.c for details.
+  void* result = mmap(
+      *addr, len, host_prot & ~PROT_EXEC, NaClFlagsToFlags(flags), fd, off);
   if (result == MAP_FAILED)
     return errno;
+  if (host_prot & PROT_EXEC) {
+    if (mprotect(result, len, host_prot) != 0) {
+      LOG(FATAL) << "IrtMMap: mprotect to turn on PROT_EXEC failed, "

[Mark Seaborn, 2014/04/15 23:28:03]

I was going to suggest doing "return errno" here, but that would leave the new
mapping behind.  Can you add a comment to say that this aborts because it can't
easily undo the mmap() call?

[mazda, 2014/04/16 00:33:36]

Done.

+                 << "errno " << errno;

[Mark Seaborn, 2014/04/15 23:28:03]

It's better to use LOG_ERRNO or PLOG -- they report errno for you, and decode
errno too.

[mazda, 2014/04/16 00:33:36]

Done.

+    }
+  }
 
   *addr = result;
   return 0;
 }
 
 int IrtMUnmap(void* addr, size_t len) {
   return CheckError(munmap(addr, len));
 }
 
 int IrtMProtect(void* addr, size_t len, int prot) {
   return CheckError(mprotect(addr, len, NaClProtToProt(prot)));
 }
 
 }  // namespace
 
 // For mmap, the argument types should be nacl_abi_off_t rather than off_t.
 // However, the definition of nacl_irt_memory uses the host type off_t, so here
 // we need to cast it.
 const nacl_irt_memory kIrtMemory = {
   reinterpret_cast<int(*)(void**, size_t, int, int, int, off_t)>(IrtMMap),
   IrtMUnmap,
   IrtMProtect,
 };
 
 }  // namespace nonsfi
 }  // namespace nacl