add pidfd_open(2)

Author: CobaltCause

changelog/2748.added.md

@@ -0,0 +1 @@
+Added `pidfd_open(2)`

src/sys/mod.rs

@@ -63,6 +63,12 @@ feature! {
     pub mod personality;
 }
 
+#[cfg(target_os = "linux")]
+feature! {
+    #![feature = "process"]
+    pub mod pidfd;
+}
+
 #[cfg(target_os = "linux")]
 feature! {
     #![feature = "process"]

src/sys/pidfd.rs

@@ -0,0 +1,60 @@
+//! PID file descriptor APIs.
+
+use std::{
+    hint::unreachable_unchecked,
+    os::fd::{FromRawFd as _, OwnedFd, RawFd},
+};
+
+use bitflags::bitflags;
+
+use crate::{errno::Errno, unistd::Pid};
+
+bitflags! {
+    /// Flags for [`pidfd_open`].
+    #[derive(Copy, Clone)]
+    pub struct PidfdOpenFlags: libc::c_uint {
+        /// See [`pidfd_open(2)`] for details.
+        ///
+        /// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
+        const PIDFD_NONBLOCK = libc::PIDFD_NONBLOCK;
+
+        /// See [`pidfd_open(2)`] for details.
+        ///
+        /// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
+        const PIDFD_THREAD = libc::PIDFD_THREAD;
+    }
+}
+
+/// Open a PIDFD of the provided PID.
+///
+/// See [`pidfd_open(2)`] for details.
+///
+/// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
+///
+/// # Safety
+///
+/// This can race with the PID in `pid` getting released or recycled, unless the
+/// PID is that of the calling process, in which case the only way a race could
+/// happen is if the current process exits.
+///
+/// This function is [async-signal-safe], although it may modify `errno`.
+///
+/// [async-signal-safe]: https://man7.org/linux/man-pages/man7/signal-safety.7.html
+pub fn pidfd_open(pid: Pid, flags: PidfdOpenFlags) -> Result<OwnedFd, Errno> {
+    // SAFETY:
+    //
+    // * Arguments passed to the syscall have the correct types.
+    // * The kernel should not return a value that cannot fit in `RawFd`.
+    // * The file descriptor returned by the kernel is open, owned, and requires
+    //   only `close` to release its resources.
+    // * The kernel should not return any negative value other than `-1`.
+    unsafe {
+        match libc::syscall(libc::SYS_pidfd_open, pid.as_raw(), flags.bits()) {
+            fd if fd >= 0 => {
+                Ok(OwnedFd::from_raw_fd(RawFd::try_from(fd).unwrap_unchecked()))
+            }
+            -1 => Err(Errno::last()),
+            _ => unreachable_unchecked(),
+        }
+    }
+}

test/sys/mod.rs

@@ -96,3 +96,6 @@ mod test_resource;
 // only enable this for FreeBSD for now.
 #[cfg(target_os = "freebsd")]
 mod test_memfd;
+
+#[cfg(all(target_os = "linux", feature = "process"))]
+mod test_pidfd;

test/sys/test_pidfd.rs

@@ -0,0 +1,10 @@
+use nix::{
+    sys::pidfd::{pidfd_open, PidfdOpenFlags},
+    unistd::getpid,
+};
+
+#[test]
+fn test_pidfd_open() {
+    pidfd_open(getpid(), PidfdOpenFlags::empty())
+        .expect("should be able to get pidfd");
+}