Improve safety comments in Pidfd::open

Also remove invocation of one `unsafe` function `errno_location`.

Signed-off-by: Jiahao XU <[email protected]>

tokio/src/process/unix/pidfd_reaper.rs

@@ -7,7 +7,7 @@ use crate::{
  util::error::RUNTIME_SHUTTING_DOWN_ERROR,
 };
 
-use libc::{syscall, SYS_pidfd_open, __errno_location, ENOSYS, PIDFD_NONBLOCK};
+use libc::{syscall, SYS_pidfd_open, ENOSYS, PIDFD_NONBLOCK};
 use mio::{event::Source, unix::SourceFd};
 use std::{
  fs::File,
@@ -36,21 +36,22 @@ impl Pidfd {
  return None;
  }
 
- unsafe {
- let fd = syscall(SYS_pidfd_open, pid, PIDFD_NONBLOCK);
- if fd == -1 {
- let errno = *__errno_location();
+ // Safety: The following function calls invovkes syscall pidfd_open,
+ // which takes two parameter: pidfd_open(fd: c_int, flag: c_int)
+ let fd = unsafe { syscall(SYS_pidfd_open, pid, PIDFD_NONBLOCK) };
+ if fd == -1 {
+ let errno = io::Error::last_os_error().raw_os_error().unwrap();
 
- if errno == ENOSYS {
- NO_PIDFD_SUPPORT.store(true, Relaxed)
- }
-
- None
- } else {
- Some(Pidfd {
- fd: File::from_raw_fd(fd as i32),
- })
+ if errno == ENOSYS {
+ NO_PIDFD_SUPPORT.store(true, Relaxed)
  }
+
+ None
+ } else {
+ // Safety: pidfd_open returns -1 on error or a valid fd with ownership.
+ Some(Pidfd {
+ fd: unsafe { File::from_raw_fd(fd as i32) },
+ })
  }
  }
 }