Mark `process_vm_writev` as `unsafe`

[Kamila Borowska]

This function can be used to corrupt memory of the process itself when pid is Pid::this(). I think it would make sense to mark it as unsafe in a major release of this crate, similar to how nix::sys::ptrace::write is marked unsafe.

Example usage of this function:

use nix::sys::uio::{process_vm_writev, RemoteIoVec};
use nix::unistd::Pid;
use std::io::IoSlice;

fn main() -> std::io::Result<()> {
    let memory = [0u8; 5];
    process_vm_writev(
        Pid::this(),
        &[IoSlice::new(&[1, 2, 3, 4, 5])],
        &[RemoteIoVec {
            base: memory.as_ptr() as usize,
            len: 5,
        }],
    )?;
    println!("{memory:?}");
    Ok(())
}

This is problematic as it edits immutable memory.

[Alan Somers]

The Rust consensus is that "special operating system stuff" that can crash a process needn't be unsafe. Instead, these features are considered to be beyond the boundary of language design. Otherwise, even std::io::Write::write would need to be unsafe , because you could crash your process by opening /dev/mem and writing to it.
sys::ptrace::write is actually unsafe for a different reason. See 856f841 .