Fix syscall_info

[cbarrete]

What does this PR do

It fixes ptrace::syscall_info, which is currently broken (at least on Linux 6.12.31).

See the commit message for more details.

Reproduction

The following program panics when built against master and behaves correctly (i.e. it does not panic and prints valid syscall info) when built against this branch:

fn main() {
    let pid = unsafe { nix::unistd::fork().unwrap() };

    match pid {
        nix::unistd::ForkResult::Child => {
            nix::sys::ptrace::traceme().unwrap();
            nix::sys::signal::raise(nix::sys::signal::Signal::SIGCONT).unwrap();
            println!("I'm issuing a syscall!");
        }
        nix::unistd::ForkResult::Parent { child } => {
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            nix::sys::ptrace::setoptions(child, nix::sys::ptrace::Options::PTRACE_O_TRACESYSGOOD)
                .unwrap();

            nix::sys::ptrace::syscall(child, None).unwrap();
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
            println!("{syscall_info:?}");
            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_ENTRY);

            nix::sys::ptrace::syscall(child, None).unwrap();
            nix::sys::wait::waitpid(Some(child), None).unwrap();
            let syscall_info = nix::sys::ptrace::syscall_info(child).unwrap();
            println!("{syscall_info:?}");
            assert!(syscall_info.op == libc::PTRACE_SYSCALL_INFO_EXIT);
        }
    }
}

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[cbarrete]

Hello @SteveLauC, this is my first contribution here, I'm not sure who to ask for a review. Would you mind having a look? Thanks!

[cbarrete]

@asomers instead maybe? Feel free to redirect me to someone else!
Thanks in advance.

[cbarrete]

Sorry for bumping this again, but could you please take a look @asomers?
If this PR is not welcome, I can close this and maintain a fork, but this fix seems desirable to have upstream to me, as ptrace::syscall_info is currently completely broken.

[asomers]

Approved. Also, since you wrote such a nice example program, you might consider adding it to Nix's examples directory.

[cbarrete]

I've rebased the branch and added the example. Sorry for the mess, it took me a while to figure out the right cfg annotations.

If everything looks fine to you, could you merge this after approving it again? I couldn't merge the branch even after you had approved it.

Thanks!