Tiny ELF binaries on ARM

Looks like breadbox didn't want to go down this rabbithole. So we'll have to do it instead.

• e_machine seems to be the only checked header field (e_entry alignment checks are normal, because if it wouldn't be aligned, the code would segfault on entry.)
• phdr parsing etc. is done architecture-independent, so the same tricks should be usable here as well.
  • However, on x86, we were using the way page mapping works to only have to specify a few flags, this probably can't be ported over.
• Each ARM opcode is 4 bytes long, and needs to be aligned. This kinda sucks for all the overlapping tricks. Also, arbitrary constants can't be loaded into registers easily.
  • Do we want to depend on Thumb-mode?
• Apparently the kernel doesn't look at the immediate field of swi instructions if it's configured as EABI-only.
• Dynamic linking stuff

A few questions on the target platform

• How many ARM Linux machines have...
  • Thumb support? (thumb in /proc/cpuinfo)
  • Halfword load/store support? (ldrh/strh, half in /proc/cpuinfo)
  • A fast multiplier? (fastmult in /proc/cpuinfo)
• How many kernels are OABI, EABI with OABI support, or EABI-only?