Comment by mebeim

2 years ago

> for the case where you want to work with semi-structured IR (GIMPLE) rather than machine code

Most of the code for syscall handlers is carefully hand-crafted assembly, so probably not GIMPLE. Maybe something like Valgrind's VEX IR. I see what you mean though.

> How does that even work, libc-wise?

It works as you say, the "return an error like a stub" logic is the same as the "this syscall isn't implemented logic". AFAIK libc will provide the wrappers regardless (if there are wrappers, not all syscalls have them) and the kernel will just return -ENOSYS, like it would do for any invalid syscall number.

> If so, maybe consider that a bug? Submit a patch to have an arch's stubbed optional syscalls return a different error than for syscalls that don't exist for that arch

I am 99.9% sure that'd be impossible. The "stubbed optional syscalls" return -ENOSYS (as if they did not exist) by design. Although annoying, it's not really a bug, it's the way it's intended to work. I doubt such a patch would such an API-breaking change would be accepted, as a lot of existing code relies on this behavior. I don't think there even is an appropriate errno number to return in such case. It's unfortunate, but it is what it is.

> To be clear, I wasn't talking about compile-time code inclusion; I was talking about runtime

Yeah, it was clear that you meant runtime but less clear what you exactly meant with "all kernel services were already initialized". Now I see what you mean. Yes, what you describe definitely seems doable from a theoretical point of view for some architectures, but I struggle to think about such a solution given its complexity. It would still require manual recognition of interesting source code files and syscall handler code, plus a significant amount of scripting/patching/compiling to get it to work. Not to talk about emulation since this would need to be done for different archs. That's why even though it'd be nice in theory, it practically seems like a borderline unapproachable problem to me, from multiple sides.