Adventuring with a.out

29 Feb 2008

The first step in the Glendix project was to write a binary loader for the Plan 9 a.out format. Linux has a clean interface for registering new binary format handlers from a module. Basically, you define a structure of type linux_binfmt and call register_binfmt during initialization of the module. Now all that’s left to do is implement the three functions that you pointed to in your structure: load_binary, load_shlib and core_dump.

Luckily for me, all Plan 9 executables are statically linked so I can just leave load_shlib as NULL. core_dump is also not that important during the development stages, although the final product must definitely implement it. To get a feel of what I needed to do in load_binary, I decided to take a peek into some of the other binary format handlers. I tried to comprehend the code for ELF with not much luck. I then turned to UTLK, which helped me understand what was going on. I highly recommend the book for anyone interested in kernel programming.

Anyway, here is when I found out that all ELF executables have sections that are actually page aligned! That means every ELF executable contains a bunch of zeroes after the TEXT section, so that the DATA section starts at the next page address. That’s how the executable is supposed to be laid out in memory, but I had no idea someone would actually think of doing it in the file. I guess they have their reasons, all the binary format loader does is mmap the file. Maybe for ELF2 they could put in zeros for the BSS section in the file too ;)

Plan 9 executables on the other hand, are just normal files with no padding. This gives me a headache because I can no longer use mmap. Recall that all addressees passed to mmap have to be page-aligned. But the DATA section in Plan 9’s a.out will start at a non-page-aligned address most of the time.

One of the first things I tried to do was to mmap the file into a high address, copy portions into the appropriate locations and then free the mapping. That didn’t work so well because:

I ended up writing a userspace program called pad that page-aligns a Plan 9 a.out executable. The loader just mmap’s the file, like all other loaders. The solution is suboptimal, if someone knows a clean way of doing all of this in kernel-space, I’ll be grateful for the help. The ultimate goal is to run Plan 9 executables on Linux, unmodified.

The code for the loader and the pad program can be found on git here.