Monday, 4 May 2009

Progress on Native Client

Back in January I wrote that I was porting glibc to Native Client. I have made some good progress since then.

The port is at the stage where it can run simple statically-linked and dynamically-linked executables, from the command line and from the web browser.

In particular, Python works. I have put together a simple interactive top-level that demonstrates running Python from the web browser:

Upstream NaCl doesn't support any filename-based calls such as open(), but we do. In this setup, open() does not of course access the real filesystem on the host system. open() in NaCl-glibc sends a message to the Javascript code on the web page. The Javascript code can fetch the requested file, create a file descriptor for the file using the plugin's API, and send the file descriptor to the NaCl subprocess in reply.

This has not involved modifying Python at all, although I have added an extension module to wrap a couple of NaCl-specific system calls (imc_sendmsg() and imc_recvmsg()). The Python build runs to completion, including the parts where it runs the executable it builds. A simple instruction-rewriting trick means that dynamically-linked NaCl executables can run outside of NaCl, directly under Linux, linked against the regular Linux glibc. This means we can avoid some of the problems associated with cross-compilation when building for NaCl.

This work has involved extending Native Client:

  • Adding support for dynamic loading of code. Initially I have focused on just making it work. Now I'll focus on ensuring this is secure.
  • Writing a custom browser plugin for NaCl which allows Javascript code to handle asynchronous callbacks from NaCl subprocesses.
  • Making various changes to the NaCl versions of gcc and binutils to support dynamically linked code.
Hopefully these changes can get merged upstream eventually. Some of the toolchain changes have already gone in.

See the web page for instructions on how to get the code and try it out.

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