Add Linux-specific pidfd process extensions

library/std/src/os/linux/mod.rs

@@ -3,4 +3,5 @@
 #![stable(feature = "raw_ext", since = "1.1.0")]
 
 pub mod fs;
+pub mod process;
 pub mod raw;

library/std/src/os/linux/process.rs

@@ -0,0 +1,47 @@
+//! Linux-specific extensions to primitives in the `std::process` module.
+
+#![unstable(feature = "linux_pidfd", issue = "none")]
+
+use crate::process;
+use crate::sys_common::AsInnerMut;
+use crate::io::Result;
+
+/// Os-specific extensions to [`process::Child`]
+///
+/// [`process::Child`]: crate::process::Child
+pub trait ChildExt {
+    /// Obtains the pidfd created for this child process, if available.
+    ///
+    /// A pidfd will only ever be available if `create_pidfd(true)` was called
+    /// when the corresponding `Command` was created.
+    ///
+    /// Even if `create_pidfd(true)` is called, a pidfd may not be available
+    /// due to an older version of Linux being in use, or if
+    /// some other error occured.
+    ///
+    /// See `man pidfd_open` for more details about pidfds.
+    fn pidfd(&self) -> Result<i32>;
+}
+
+/// Os-specific extensions to [`process::Command`]
+///
+/// [`process::Command`]: crate::process::Command
+pub trait CommandExt {
+    /// Sets whether or this `Command` will attempt to create a pidfd
+    /// for the child. If this method is never called, a pidfd will
+    /// not be crated.
+    ///
+    /// The pidfd can be retrieved from the child via [`ChildExt::pidfd`]
+    ///
+    /// A pidfd will only be created if it is possible to do so
+    /// in a guaranteed race-free manner (e.g. if the `clone3` system call is
+    /// supported). Otherwise, [`ChildExit::pidfd`] will return an error.
+    fn create_pidfd(&mut self, val: bool) -> &mut process::Command;
+}
+
+impl CommandExt for process::Command {
+    fn create_pidfd(&mut self, val: bool) -> &mut process::Command {
+        self.as_inner_mut().create_pidfd(val);
+        self
+    }
+}

library/std/src/process.rs

@@ -161,7 +161,7 @@ use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 /// [`wait`]: Child::wait
 #[stable(feature = "process", since = "1.0.0")]
 pub struct Child {
-    handle: imp::Process,
+    pub(crate) handle: imp::Process,
 
     /// The handle for writing to the child's standard input (stdin), if it has
     /// been captured. To avoid partially moving

library/std/src/sys/unix/process/process_common.rs

@@ -88,6 +88,7 @@ pub struct Command {
     stdin: Option<Stdio>,
     stdout: Option<Stdio>,
     stderr: Option<Stdio>,
+    pub(crate) make_pidfd: bool,
 }
 
 // Create a new type for argv, so that we can make it `Send` and `Sync`
@@ -149,6 +150,7 @@ impl Command {
             stdin: None,
             stdout: None,
             stderr: None,
+            make_pidfd: false,
         }
     }
 
@@ -181,6 +183,9 @@ impl Command {
     pub fn gid(&mut self, id: gid_t) {
         self.gid = Some(id);
     }
+    pub fn create_pidfd(&mut self, val: bool) {
+        self.make_pidfd = val;
+    }
 
     pub fn saw_nul(&self) -> bool {
         self.saw_nul

library/std/src/sys/unix/process/process_unix.rs

@@ -5,6 +5,7 @@ use crate::ptr;
 use crate::sys;
 use crate::sys::cvt;
 use crate::sys::process::process_common::*;
+use crate::sync::atomic::{AtomicBool, Ordering};
 
 use libc::{c_int, gid_t, pid_t, uid_t};
 
@@ -34,18 +35,7 @@ impl Command {
 
         let (input, output) = sys::pipe::anon_pipe()?;
 
-        // Whatever happens after the fork is almost for sure going to touch or
-        // look at the environment in one way or another (PATH in `execvp` or
-        // accessing the `environ` pointer ourselves). Make sure no other thread
-        // is accessing the environment when we do the fork itself.
-        //
-        // Note that as soon as we're done with the fork there's no need to hold
-        // a lock any more because the parent won't do anything and the child is
-        // in its own process.
-        let result = unsafe {
-            let _env_lock = sys::os::env_lock();
-            cvt(libc::fork())?
-        };
+        let (result, pidfd) = self.do_fork()?;
 
         let pid = unsafe {
             match result {
@@ -70,11 +60,11 @@ impl Command {
                     rtassert!(output.write(&bytes).is_ok());
                     libc::_exit(1)
                 }
-                n => n,
+                n => n as pid_t,
             }
         };
 
-        let mut p = Process { pid, status: None };
+        let mut p = Process { pid, status: None, pidfd };
         drop(output);
         let mut bytes = [0; 8];
 
@@ -107,6 +97,95 @@ impl Command {
         }
     }
 
+    // Attempts to fork the process. If successful, returns
+    // Ok((0, -1)) in the child, and Ok((child_pid, child_pidfd)) in the parent.
+    fn do_fork(&mut self) -> Result<(libc::c_long, libc::pid_t), io::Error> {
+        // Whatever happens after the fork is almost for sure going to touch or
+        // look at thbe environment in one way or another (PATH in `execvp` or
+        // accessing the `environ` pointer ourselves). Make sure no other thread
+        // is accessing the environment when we do the fork itself.
+        //
+        // Note that as soon as we're done with the fork there's no need to hold
+        // a lock any more because the parent won't do anything and the child is
+        // in its own process.
+        let _env_lock = unsafe { sys::os::env_lock() };
+
+        // If we fail to create a pidfd for any reason, this will
+        // stay as -1, which indicates an error
+        let mut pidfd: libc::pid_t = -1;
+
+        // On Linux, attempt to use the `clone3` syscall, which
+        // supports more argument (in particular, the ability to create a pidfd).
+        // If this fails, we will fall through this block to a call to `fork()`
+        cfg_if::cfg_if! {
+            if #[cfg(target_os = "linux")] {
+                static HAS_CLONE3: AtomicBool = AtomicBool::new(true);
+
+                const CLONE_PIDFD: u64 = 0x00001000;
+
+                #[repr(C)]
+                struct clone_args {
+                    flags: u64,
+                    pidfd: u64,
+                    child_tid: u64,
+                    parent_tid: u64,
+                    exit_signal: u64,
+                    stack: u64,
+                    stack_size: u64,
+                    tls: u64,
+                    set_tid: u64,
+                    set_tid_size: u64,
+                    cgroup: u64,
+                }
+
+                syscall! {
+                    fn clone3(cl_args: *mut clone_args, len: libc::size_t) -> libc::c_long
+                }
+
+                if HAS_CLONE3.load(Ordering::Relaxed) {
+                    let mut flags = 0;
+                    if self.make_pidfd {
+                        flags |= CLONE_PIDFD;
+                    }
+
+                    let mut args = clone_args {
+                        flags,
+                        pidfd: &mut pidfd as *mut libc::pid_t as u64,
+                        child_tid: 0,
+                        parent_tid: 0,
+                        exit_signal: libc::SIGCHLD as u64,
+                        stack: 0,
+                        stack_size: 0,
+                        tls: 0,
+                        set_tid: 0,
+                        set_tid_size: 0,
+                        cgroup: 0
+                    };
+
+                    let args_ptr = &mut args as *mut clone_args;
+                    let args_size = crate::mem::size_of::<clone_args>();
+
+                    let res = cvt(unsafe { clone3(args_ptr, args_size) });
+                    match res {
+                        Ok(n) => return Ok((n, pidfd)),
+                        Err(e) => match e.raw_os_error() {
+                            // Multiple threads can race to execute this store,
+                            // but that's fine - that just means that multiple threads
+                            // will have tried and failed to execute the same syscall,
+                            // with no other side effects.
+                            Some(libc::ENOSYS) => HAS_CLONE3.store(false, Ordering::Relaxed),
+                            _ => return Err(e)
+                        }
+                    }
+                }
+            }
+        }
+        // If we get here, we are either not on Linux,
+        // or we are on Linux and the 'clone3' syscall does not exist
+        cvt(unsafe { libc::fork() }.into()).map(|res| (res, pidfd))
+    }
+
+
     pub fn exec(&mut self, default: Stdio) -> io::Error {
         let envp = self.capture_env();
 
@@ -252,8 +331,6 @@ impl Command {
     #[cfg(not(any(
         target_os = "macos",
         target_os = "freebsd",
-        all(target_os = "linux", target_env = "gnu"),
-        all(target_os = "linux", target_env = "musl"),
     )))]
     fn posix_spawn(
         &mut self,
@@ -268,8 +345,6 @@ impl Command {
     #[cfg(any(
         target_os = "macos",
         target_os = "freebsd",
-        all(target_os = "linux", target_env = "gnu"),
-        all(target_os = "linux", target_env = "musl"),
     ))]
     fn posix_spawn(
         &mut self,
@@ -404,6 +479,12 @@ impl Command {
 pub struct Process {
     pid: pid_t,
     status: Option<ExitStatus>,
+    // On Linux, stores the pidfd created for this child.
+    // This is -1 if the user did not request pidfd creation,
+    // or if the pidfd could not be created for some reason
+    // (e.g. the `clone3` syscall was not available).
+    #[cfg(target_os = "linux")]
+    pidfd: libc::c_int,
 }
 
 impl Process {
@@ -494,3 +575,15 @@ impl fmt::Display for ExitStatus {
         }
     }
 }
+
+#[cfg(target_os = "linux")]
+#[unstable(feature = "linux_pidfd", issue = "none")]
+impl crate::os::linux::process::ChildExt for crate::process::Child {
+    fn pidfd(&self) -> crate::io::Result<i32> {
+        if self.handle.pidfd > 0 {
+            Ok(self.handle.pidfd)
+        } else {
+            Err(crate::io::Error::from(crate::io::ErrorKind::Other))
+        }
+    }
+}

src/test/ui/command/command-create-pidfd.rs

@@ -0,0 +1,27 @@
+// run-pass
+// linux-only - pidfds are a linux-specific concept
+
+#![feature(linux_pidfd)]
+use std::os::linux::process::{CommandExt, ChildExt};
+use std::process::Command;
+
+fn main() {
+    // We don't assert the precise value, since the standard libarary
+    // may be opened other file descriptors before our code ran.
+    let _ = Command::new("echo")
+        .create_pidfd(true)
+        .spawn()
+        .unwrap()
+        .pidfd().expect("failed to obtain pidfd");
+
+    let _ = Command::new("echo")
+        .create_pidfd(false)
+        .spawn()
+        .unwrap()
+        .pidfd().expect_err("pidfd should not have been created when create_pid(false) is set");
+
+    let _ = Command::new("echo")
+        .spawn()
+        .unwrap()
+        .pidfd().expect_err("pidfd should not have been created");
+}