Enabling 64-bit mojo shell to launch 32-bit child to handle nonsfi content.

shell/out_of_process_native_runner.cc

Index: shell/out_of_process_native_runner.cc
diff --git a/shell/out_of_process_native_runner.cc b/shell/out_of_process_native_runner.cc
index 39cf1f7003abfef747ed59f21afdebf9d08b04ab..fe1bdfa4b61e90fe51ce39817df694860368dd64 100644
--- a/shell/out_of_process_native_runner.cc
+++ b/shell/out_of_process_native_runner.cc
@@ -4,10 +4,13 @@
 
 #include "shell/out_of_process_native_runner.h"
 
+#include <string>
+
 #include "base/bind.h"
 #include "base/callback_helpers.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
+#include "base/strings/string_util.h"
 #include "shell/child_controller.mojom.h"
 #include "shell/child_process_host.h"
 #include "shell/in_process_native_runner.h"
@@ -37,7 +40,10 @@ void OutOfProcessNativeRunner::Start(
   app_completed_callback_ = app_completed_callback;
 
   child_process_host_.reset(new ChildProcessHost(context_));
-  child_process_host_->Start();
+  bool require_32_bit = false;
+  if (EndsWith(app_path.value(), "nacl_content_handler_nonsfi.mojo", true))
+    require_32_bit = true;

[Sean Klein, 2015/09/14 18:10:34]

I am fully aware this is kind of hacky -- mojo people suggested reading ELF
headers to identify if 32-bit child shell is required. That being said, this
version does currently work.

I would be happy to write the ELF-reading code, but I would rather not start
from scratch, since reusing good code is probably better than rolling my own.
Any pointers on which elf util I should be using?

[Mark Seaborn, 2015/09/14 18:38:59]

I think it's up to an owner of this code whether this hack is acceptable.

If you're adding a hack it should at least have a comment to explain what it's
for. :-)
Reading the header is trivial enough that it's not worth adding a dependency.

Have a look at /usr/include/elf.h.

The first 16 bytes of an ELF file are the e_ident header from
Elf32_Ehdr/Elf64_Ehdr.  You could look at the field e_ident[EI_CLASS] to see
whether the file is ELFCLASS32 or ELFCLASS64.

You could start here:
http://sco.com/developers/devspecs/gabi41.pdf
-- See Section 4-4, "ELF Header".
However, this is an old version of the ELF docs which predates ELFCLASS64.

[Sean Klein, 2015/09/15 18:37:50]

Modified to check ELF header of content handler. Alternatively, I could be
checking the ELF header of the content itself, but this is a bit trickier (and
not necessarily and ELF file).

My error handling isn't great at this point, but I'm not sure what else can be
done (it's a void function, but is an assertion the best I can do in case of
error?). Also, should I be checking for the magic ELF header here?

[Mark Seaborn, 2015/09/15 19:42:58]

Yes, checking for "\x7fELF" would be good, to ensure that when you read EI_CLASS
it's meaningful.

+  child_process_host_->Start(require_32_bit);
 
   // TODO(vtl): |app_path.AsUTF8Unsafe()| is unsafe.
   child_process_host_->StartApp(