[RFC] Linux system call builtins

[Matheus Afonso Martins Moreira <matheus.a.m.moreira@gmail.com>]

> Now the question comes is the argument long or some other type?

I believe so. Every library I've seen and the kernel itself uses long.
Other types just get typecasted to long. I think it's just supposed to
mean "register type" since all the arguments must be in registers.

> E.g. for some 32bit ABIs built on top of 64bit ISA might always
> just pass 32bits or they might allow passing the full 64bits.
> (x32 might fall under this and MIPS n32).

The registers used are documented here:

https://www.man7.org/linux/man-pages/man2/syscall.2.html

x32 uses the exact same registers as x86_64, all 64 bit:

> x86-64    rdi rsi rdx r10 r8 r9
> x32       rdi rsi rdx r10 r8 r9

MIPS calling conventions work like this:

> mips/n32,64 a0 a1 a2 a3 a4 a5
> mips/o32    a0 a1 a2 a3 ...
> mips/o32    args5-8 are passed on the stack

> Maybe you should warn/error out
> if not passed the correct sized argument.

Yes. It should be an error if the inputs are not register sized.
As far as I know all system call inputs are either register sized
values or pointers to C strings, buffers or much larger structures.

> Also do you sign or zero extend a 32bit argument for LP64 targets?

I'm not sure. System call stubs just declare these register values as
signed long so whatever happens in that case seems to be appropriate.