feat: pidfd_getfd

Author: JonathanWoollett-Light

CHANGELOG.md

@@ -11,6 +11,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1662](https://github.com/nix-rust/nix/pull/1662))
 - Added `CanRaw` to `SockProtocol` and `CanBcm` as a separate `SocProtocol` constant.
   ([#1912](https://github.com/nix-rust/nix/pull/1912))
+- Added `pidfd_getfd` on Linux.
+  ([#1868](https://github.com/nix-rust/nix/pull/1868))
 
 ### Changed
 

src/sys/mod.rs

@@ -226,3 +226,7 @@ feature! {
     #![feature = "time"]
     pub mod timer;
 }
+
+#[cfg(target_os = "linux")]
+/// pidfd related functionality
+pub mod pidfd;

src/sys/pidfd.rs

@@ -0,0 +1,43 @@
+use crate::errno::Errno;
+use crate::Result;
+use std::convert::TryFrom;
+use std::os::unix::io::{AsRawFd, FromRawFd, OwnedFd};
+
+/// Allocates a new file descriptor in the calling process. This new file descriptor is a duplicate
+/// of an existing file descriptor, `target`, in the process referred to by the PID file descriptor
+/// `pid`.
+///
+/// The duplicate file descriptor refers to the same open file description (see
+/// [open(2)](https://man7.org/linux/man-pages/man2/open.2.html)) as the original file descriptor in
+/// the process referred to by `pid`.  The two file descriptors thus share file status flags and
+/// file offset.  Furthermore, operations on the underlying file object (for example, assigning an
+/// address to a socket object using [bind(2)](https://man7.org/linux/man-pages/man2/bind.2.html))
+/// can equally be performed via the duplicate file descriptor.
+///
+/// The close-on-exec flag ([`libc::FD_CLOEXEC`]; see
+/// [fcntl(2)](https://man7.org/linux/man-pages/man2/fcntl.2.html)) is set on the returned file
+/// descriptor.
+///
+/// Permission to duplicate another process's file descriptor is governed by a ptrace access mode
+/// PTRACE_MODE_ATTACH_REALCREDS check (see
+/// [ptrace(2)](https://man7.org/linux/man-pages/man2/ptrace.2.html)).
+pub fn pidfd_getfd<PFd: AsRawFd, TFd: AsRawFd>(
+    pid: PFd,
+    target: TFd,
+) -> Result<OwnedFd> {
+    #[allow(clippy::useless_conversion)] // Not useless on all OSes
+    match unsafe {
+        libc::syscall(
+            libc::SYS_pidfd_getfd,
+            pid.as_raw_fd(),
+            target.as_raw_fd(),
+            0,
+        )
+    } {
+        -1 => Err(Errno::last()),
+        fd @ 0.. => {
+            Ok(unsafe { OwnedFd::from_raw_fd(i32::try_from(fd).unwrap()) })
+        }
+        _ => unreachable!(),
+    }
+}