Segmentation fault on 32-bit arm when built with -mthumb

[fornwall]

@michalbednarski In termux/termux-packages#761 it was noticed that proot segfaults when built with -mthumb on 32-bit arm (which Termux&Android does by default to save binary size, as Thumb-2 seems to be recommended for most).

I worked around it by avoiding building proot with -mthumb in termux/termux-packages@28c436b, so it's not really that important, and I guess 32-bit arm is slowly being deprecated as more and more new devices are 64-bit.

So not worth spending a lot of time on, but still interesting to see if you have an idea what could cause this :).

[michalbednarski]

It's caused by r7 register conflict in loader (which is used as syscall number and when thumb mode is in use then also a frame pointer).

Problematic line is:
register word_t number asm("r7") = number_;

According to GCC documentation this is supposed to set register used for this variable in asm block (and that works, however r7 (as frame pointer) value is clobbered).

I'd say that it's clang bug that this compiled but clobbered frame pointer (gcc fails compilation in this case)

Anyway, only loader does need to be compiled without -mthumb.