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

components/nacl/loader/nonsfi/irt_memory.cc

Index: components/nacl/loader/nonsfi/irt_memory.cc
diff --git a/components/nacl/loader/nonsfi/irt_memory.cc b/components/nacl/loader/nonsfi/irt_memory.cc
index cd7796515db0890b284c218fd4c89d6a0881ff50..16f38e69d030a03c1edaa8d5b619c10b0f0e513e 100644
--- a/components/nacl/loader/nonsfi/irt_memory.cc
+++ b/components/nacl/loader/nonsfi/irt_memory.cc
@@ -5,6 +5,7 @@
 #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"
@@ -46,10 +47,21 @@ int NaClFlagsToFlags(int nacl_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;