Implement pidfd APIs for Linux

CHANGELOG.md

@@ -30,6 +30,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   ([#1841](https://github.com/nix-rust/nix/pull/1841))
 - Added `eaccess()` on FreeBSD, DragonFly and Linux (glibc and musl).
   ([#1842](https://github.com/nix-rust/nix/pull/1842))
+- Added `pidfd_{open,getfd,send_signal}()` on Linux
+  ([#1859](https://github.com/nix-rust/nix/pull/1859))
 
 ### Changed
 

Cargo.toml

@@ -39,7 +39,7 @@ memoffset = { version = "0.6.3", optional = true }
 default = [
   "acct", "aio", "dir", "env", "event", "feature", "fs",
   "hostname", "inotify", "ioctl", "kmod", "mman", "mount", "mqueue",
-  "net", "personality", "poll", "process", "pthread", "ptrace", "quota",
+  "net", "personality", "pidfd", "poll", "process", "pthread", "ptrace", "quota",
   "reboot", "resource", "sched", "signal", "socket", "term", "time",
   "ucontext", "uio", "user", "zerocopy",
 ]
@@ -60,6 +60,7 @@ mount = ["uio"]
 mqueue = ["fs"]
 net = ["socket"]
 personality = []
+pidfd = ["process"]
 poll = []
 pthread = []
 ptrace = ["process"]

src/lib.rs

@@ -23,6 +23,7 @@
 //! * `mqueue` - POSIX message queues
 //! * `net` - Networking-related functionality
 //! * `personality` - Set the process execution domain
+//! * `pidfd` - Linux's PID file descriptors
 //! * `poll` - APIs like `poll` and `select`
 //! * `process` - Stuff relating to running processes
 //! * `pthread` - POSIX threads

src/sys/mod.rs

@@ -68,6 +68,12 @@ feature! {
     pub mod personality;
 }
 
+#[cfg(target_os = "linux")]
+feature! {
+    #![feature = "pidfd"]
+    pub mod pidfd;
+}
+
 feature! {
     #![feature = "pthread"]
     pub mod pthread;

src/sys/pidfd.rs

@@ -0,0 +1,72 @@
+//! Interfaces for Linux's PID file descriptors.
+
+use std::os::unix::io::RawFd;
+use crate::Result;
+use crate::errno::Errno;
+use crate::unistd::Pid;
+
+libc_bitflags!(
+    /// Options that change the behavior of [`pidfd_open`].
+    pub struct PidFdOpenFlag: libc::c_uint {
+        /// Return a nonblocking file descriptor. (since Linux 5.10)
+        ///
+        /// If the process referred to by the file descriptor has not yet terminated,
+        /// then an attempt to wait on the file descriptor using [`waitid(2)`] will
+        /// immediately return the error `EAGAIN` rather than blocking.
+        ///
+        /// [`waitid(2)`]: https://man7.org/linux/man-pages/man2/waitid.2.html
+        PIDFD_NONBLOCK;
+    }
+);
+
+/// Obtain a file descriptor that refers to a process. (since Linux 5.3)
+///
+/// The `pidfd_open(2)` creates a file descriptor that refers to the process
+/// whose PID is specified in pid. The file descriptor is returned as the function
+/// result; the close-on-exec flag is set on the file descriptor.
+///
+/// For more information, see [`pidfd_open(2)`].
+///
+/// [`pidfd_open(2)`]: https://man7.org/linux/man-pages/man2/pidfd_open.2.html
+pub fn pidfd_open(pid: Pid, flags: PidFdOpenFlag) -> Result<RawFd> {
+    let ret = unsafe {
+        libc::syscall(libc::SYS_pidfd_open, pid, flags)
+    };
+    Errno::result(ret).map(|r| r as RawFd)
+}
+
+/// Obtain a duplicate of another process's file descriptor. (since Linux 5.6)
+///
+/// The `pidfd_getfd(2)` system call allocates a new file descriptor in the calling
+/// process. This new file descriptor is a duplicate of an existing file descriptor,
+/// `target_fd`, in the process referred to by the PID file descriptor pidfd.
+///
+/// For more information, see [`pidfd_getfd(2)`].
+///
+/// [`pidfd_getfd(2)`]: https://man7.org/linux/man-pages/man2/pidfd_getfd.2.html
+pub fn pidfd_getfd(pidfd: RawFd, target_fd: RawFd) -> Result<RawFd> {
+    let ret = unsafe {
+        libc::syscall(libc::SYS_pidfd_getfd, pidfd, target_fd, 0)
+    };
+    Errno::result(ret).map(|r| r as RawFd)
+}
+
+/// Send a signal to a process specified by a file descriptor.
+///
+/// The `pidfd_send_signal(2)` system call sends the signal sig to the target process
+/// referred to by pidfd, a PID file descriptor that refers to a process.
+///
+/// For more information, see [`pidfd_send_signal(2)`].
+///
+/// [`pidfd_send_signal(2)`]: https://man7.org/linux/man-pages/man2/pidfd_send_signal.2.html
+#[cfg(feature = "signal")]
+pub fn pidfd_send_signal<T: Into<Option<crate::sys::signal::Signal>>>(pidfd: RawFd, signal: T, sig_info: Option<&libc::siginfo_t>) -> Result<()> {
+    let signal = match signal.into() {
+        Some(signal) => signal as libc::c_int,
+        _ => 0,
+    };
+    let ret = unsafe {
+        libc::syscall(libc::SYS_pidfd_send_signal, pidfd, signal, sig_info, 0)
+    };
+    Errno::result(ret).map(drop)
+}

test/sys/mod.rs

@@ -45,6 +45,8 @@ mod test_wait;
 mod test_epoll;
 #[cfg(target_os = "linux")]
 mod test_inotify;
+#[cfg(target_os = "linux")]
+mod test_pidfd;
 mod test_pthread;
 #[cfg(any(
     target_os = "android",

test/sys/test_pidfd.rs

@@ -0,0 +1,69 @@
+use nix::poll::{poll, PollFd, PollFlags};
+use nix::sys::pidfd::*;
+use nix::sys::signal::*;
+use nix::unistd::*;
+use std::io::{Read, Seek, SeekFrom, Write};
+use std::os::unix::prelude::{AsRawFd, FromRawFd};
+
+#[test]
+fn test_pidfd_open() {
+    let pidfd = pidfd_open(getpid(), PidFdOpenFlag::empty()).unwrap();
+    close(pidfd).unwrap();
+}
+
+#[test]
+fn test_pidfd_getfd() {
+    let pidfd = pidfd_open(getpid(), PidFdOpenFlag::empty()).unwrap();
+
+    let mut tempfile = tempfile::tempfile().unwrap();
+    tempfile.write_all(b"hello").unwrap();
+    tempfile.seek(SeekFrom::Start(0)).unwrap();
+
+    let tempfile2 = pidfd_getfd(pidfd, tempfile.as_raw_fd()).unwrap();
+    let mut tempfile2 = unsafe { std::fs::File::from_raw_fd(tempfile2) };
+
+    // Drop the original file. Since `tempfile2` should hold the same file, it would not be deleted.
+    drop(tempfile);
+    let mut buf = String::new();
+    tempfile2.read_to_string(&mut buf).unwrap();
+    assert_eq!(buf, "hello");
+
+    close(pidfd).unwrap();
+}
+
+#[test]
+fn test_pidfd_poll_send_signal() {
+    let me_pidfd = pidfd_open(getpid(), PidFdOpenFlag::empty()).unwrap();
+
+    let child = match unsafe { fork() }.expect("Error: Fork Failed") {
+        ForkResult::Child => {
+            sleep(1);
+            unsafe { libc::_exit(42) }
+        }
+        ForkResult::Parent { child } => child,
+    };
+
+    let child_pidfd = pidfd_open(child, PidFdOpenFlag::empty()).unwrap();
+    let mut poll_fds = [
+        PollFd::new(me_pidfd, PollFlags::POLLIN),
+        PollFd::new(child_pidfd, PollFlags::POLLIN),
+    ];
+
+    // Timeout.
+    assert_eq!(poll(&mut poll_fds, 100).unwrap(), 0);
+    // Both parent and child are running.
+    assert!(!poll_fds[0].revents().unwrap().contains(PollFlags::POLLIN));
+    assert!(!poll_fds[1].revents().unwrap().contains(PollFlags::POLLIN));
+
+    pidfd_send_signal(child_pidfd, Signal::SIGINT, None).unwrap();
+
+    // Child pidfd is ready.
+    assert_eq!(poll(&mut poll_fds, 100).unwrap(), 1);
+    // Parent is still running.
+    assert!(!poll_fds[0].revents().unwrap().contains(PollFlags::POLLIN));
+    // Child is dead.
+    assert!(poll_fds[1].revents().unwrap().contains(PollFlags::POLLIN));
+
+    close(me_pidfd).unwrap();
+    close(child_pidfd).unwrap();
+}