User-space ELF loader

The idea is to emulate exec() as faithfully as possible in user-space.

Lots of things are still unsupported or not working correctly, and even when everything is implemented, kernel-mode exec() is practically atomic while a user-space implementation has many ways to fail with a partially loaded new process or partially unloaded old process.

License

MIT license, see the file LICENSE for details.

Known issues

• ELF interpreters (e.g. PIE and dynamic executables) are not supported
• exec() in a program with running threads will most likely crash as the code for the running threads gets unmapped. This doesn't affect fork/exec though since the forked child does not have any threads.
• The start/end of code/data segment in prctl_mm_map is not correct. Unclear how the kernel actually uses these. Might be visible as incorrect statistics in some /proc/self files?
• /proc/self/exe is not updated. This file cannot be updated without being root in the current namespace.
• Enough checks are not done before the "point of no return", so the previous process can't handle errors.
• Not all resources will be released in error cases.
• One page of executable code will remain mapped in the new process. We'd need to find a way for this code to unmap itself at the same time as it transfers control to the new executable.