Re: [RFC] Linux system call builtins

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

The 04/09/2024 23:59, Matheus Afonso Martins Moreira via Gcc wrote:
> > and using raw syscalls outside of the single runtime the
> > application is using is problematic (at least on linux).
> 
> Why do you say they are problematic on Linux though? Please elaborate.

because the portable c api layer and syscall abi layer
has a large enough gap that applications can break
libc internals by doing raw syscalls.

and it's not just the call convention that's target
specific (this makes the c syscall() function hard to
use on linux)

and linux evolves fast enough that raw syscalls have
to be adjusted over time (to support new features)
which is harder when they are all over the place
instead of in the libc only.

> 
> The ABI being stable should mean that I can for example
> strace a program, analyze the system calls and implement
> a new version of it that performs the same functions.

you could do that with syscall() but it is not very
useful as the state of the system is not the same
when you rerun a process so syscalls would likely
fail or do different things than in the first run.

> > clone cannot even be used from c code in general
> > as CLONE_VM is not compatible with c semantics
> > without a new stack (child clobbers the parent stack)
> > so the c builtin would not always work
> > it is also a syscall that only freestanding
> > application can use not something that calls
> > into the libc
> 
> There are major projects out there which do use it regardless.

that does not make it right.

> For example, systemd:
> 
> https://github.com/systemd/systemd/blob/main/src/basic/raw-clone.h
> https://github.com/systemd/systemd/blob/main/src/shared/async.h
> https://github.com/systemd/systemd/blob/main/src/shared/async.c
> https://github.com/systemd/systemd/blob/main/docs/CODING_STYLE.md
> 
> > even in a freestanding application it is tricky to use right
> 
> No argument from me there. It is tricky...
> The compiler should make it possible though.
> 
> > so i don't see why clone is the quintessential example.
> 
> I think it is the best example because attempting to use clone
> is not actually supported by glibc.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=10311
> 
> "If you use clone() you're on your own."

should be

"if you use clone() *or* raw clone syscall then
 you're on your own"

which is roughly what i said in that discussion.

so your proposal does not fix this particular issue,
just provide a simpler footgun.

> > i guess it's ok if it is by default an error
> > outside of -ffreestanding.
> 
> Hosted C programs could also make good use of them.

they should not.

> They could certainly start out exclusive to freestanding C
> and then made available to general code if there's demand.