Rollup merge of #126827 - the8472:pidfd-spawn, r=workingjubilee

Use pidfd_spawn for faster process spawning when a PidFd is requested

glibc 2.39 added `pidfd_spawnp` and `pidfd_getpid` which makes it possible to get pidfds while staying on the CLONE_VFORK path.

verified that vfork gets used with strace:

```
$  strace -ff -e pidfd_open,clone3,openat,execve,waitid,close ./x test std --no-doc -- pidfd
[...]
[pid 2820532] clone3({flags=CLONE_VM|CLONE_PIDFD|CLONE_VFORK|CLONE_CLEAR_SIGHAND, pidfd=0x7b7f885fec6c, exit_signal=SIGCHLD, stack=0x7b7f88aff000, stack_size=0x9000}strace: Process 2820533 attached
 <unfinished ...>
[pid 2820533] execve("/home/the8472/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/home/the8472/.cargo/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/usr/local/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */) = -1 ENOENT (No such file or directory)
[pid 2820533] execve("/usr/bin/sleep", ["sleep", "1000"], 0x7ffdd0e268d8 /* 107 vars */ <unfinished ...>
[pid 2820532] <... clone3 resumed> => {pidfd=[3]}, 88) = 2820533
[pid 2820533] <... execve resumed>)     = 0
[pid 2820532] openat(AT_FDCWD, "/proc/self/fdinfo/3", O_RDONLY|O_CLOEXEC) = 4
[pid 2820532] close(4)                  = 0
```

Tracking issue: #82971

library/std/src/sys/pal/unix/linux/pidfd/tests.rs

@@ -1,7 +1,7 @@
 use crate::assert_matches::assert_matches;
 use crate::os::fd::{AsRawFd, RawFd};
-use crate::os::linux::process::{ChildExt, CommandExt};
-use crate::os::unix::process::ExitStatusExt;
+use crate::os::linux::process::{ChildExt, CommandExt as _};
+use crate::os::unix::process::{CommandExt as _, ExitStatusExt};
 use crate::process::Command;
 
 #[test]
@@ -21,6 +21,7 @@ fn test_command_pidfd() {
         let flags = super::cvt(unsafe { libc::fcntl(pidfd.as_raw_fd(), libc::F_GETFD) }).unwrap();
         assert!(flags & libc::FD_CLOEXEC != 0);
     }
+    assert!(child.id() > 0 && child.id() < -1i32 as u32);
     let status = child.wait().expect("error waiting on pidfd");
     assert_eq!(status.code(), Some(1));
 
@@ -42,6 +43,17 @@ fn test_command_pidfd() {
         .unwrap()
         .pidfd()
         .expect_err("pidfd should not have been created");
+
+    // exercise the fork/exec path since the earlier attempts may have used pidfd_spawnp()
+    let mut child =
+        unsafe { Command::new("false").pre_exec(|| Ok(())) }.create_pidfd(true).spawn().unwrap();
+
+    assert!(child.id() > 0 && child.id() < -1i32 as u32);
+
+    if pidfd_open_available {
+        assert!(child.pidfd().is_ok())
+    }
+    child.wait().expect("error waiting on child");
 }
 
 #[test]

library/std/src/sys/pal/unix/mod.rs

@@ -305,10 +305,13 @@ fn is_minus_one(&self) -> bool {
 
 impl_is_minus_one! { i8 i16 i32 i64 isize }
 
+/// Convert native return values to Result using the *-1 means error is in `errno`*  convention.
+/// Non-error values are `Ok`-wrapped.
 pub fn cvt<T: IsMinusOne>(t: T) -> crate::io::Result<T> {
     if t.is_minus_one() { Err(crate::io::Error::last_os_error()) } else { Ok(t) }
 }
 
+/// `-1` → look at `errno` → retry on `EINTR`. Otherwise `Ok()`-wrap the closure return value.
 pub fn cvt_r<T, F>(mut f: F) -> crate::io::Result<T>
 where
     T: IsMinusOne,
@@ -323,6 +326,7 @@ pub fn cvt_r<T, F>(mut f: F) -> crate::io::Result<T>
 }
 
 #[allow(dead_code)] // Not used on all platforms.
+/// Zero means `Ok()`, all other values are treated as raw OS errors. Does not look at `errno`.
 pub fn cvt_nz(error: libc::c_int) -> crate::io::Result<()> {
     if error == 0 { Ok(()) } else { Err(crate::io::Error::from_raw_os_error(error)) }
 }

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

@@ -449,17 +449,82 @@ fn posix_spawn(
         use crate::mem::MaybeUninit;
         use crate::sys::weak::weak;
         use crate::sys::{self, cvt_nz, on_broken_pipe_flag_used};
+        #[cfg(target_os = "linux")]
+        use core::sync::atomic::{AtomicU8, Ordering};
 
         if self.get_gid().is_some()
             || self.get_uid().is_some()
             || (self.env_saw_path() && !self.program_is_path())
             || !self.get_closures().is_empty()
             || self.get_groups().is_some()
-            || self.get_create_pidfd()
         {
             return Ok(None);
         }
 
+        cfg_if::cfg_if! {
+            if #[cfg(target_os = "linux")] {
+                weak! {
+                    fn pidfd_spawnp(
+                        *mut libc::c_int,
+                        *const libc::c_char,
+                        *const libc::posix_spawn_file_actions_t,
+                        *const libc::posix_spawnattr_t,
+                        *const *mut libc::c_char,
+                        *const *mut libc::c_char
+                    ) -> libc::c_int
+                }
+
+                weak! { fn pidfd_getpid(libc::c_int) -> libc::c_int }
+
+                static PIDFD_SUPPORTED: AtomicU8 = AtomicU8::new(0);
+                const UNKNOWN: u8 = 0;
+                const SPAWN: u8 = 1;
+                // Obtaining a pidfd via the fork+exec path might work
+                const FORK_EXEC: u8 = 2;
+                // Neither pidfd_spawn nor fork/exec will get us a pidfd.
+                // Instead we'll just posix_spawn if the other preconditions are met.
+                const NO: u8 = 3;
+
+                if self.get_create_pidfd() {
+                    let mut support = PIDFD_SUPPORTED.load(Ordering::Relaxed);
+                    if support == FORK_EXEC {
+                        return Ok(None);
+                    }
+                    if support == UNKNOWN {
+                        support = NO;
+                        let our_pid = crate::process::id();
+                        let pidfd = cvt(unsafe { libc::syscall(libc::SYS_pidfd_open, our_pid, 0) } as c_int);
+                        match pidfd {
+                            Ok(pidfd) => {
+                                support = FORK_EXEC;
+                                if let Some(Ok(pid)) = pidfd_getpid.get().map(|f| cvt(unsafe { f(pidfd) } as i32)) {
+                                    if pidfd_spawnp.get().is_some() && pid as u32 == our_pid {
+                                        support = SPAWN
+                                    }
+                                }
+                                unsafe { libc::close(pidfd) };
+                            }
+                            Err(e) if e.raw_os_error() == Some(libc::EMFILE) => {
+                                // We're temporarily(?) out of file descriptors.  In this case obtaining a pidfd would also fail
+                                // Don't update the support flag so we can probe again later.
+                                return Err(e)
+                            }
+                            _ => {}
+                        }
+                        PIDFD_SUPPORTED.store(support, Ordering::Relaxed);
+                        if support == FORK_EXEC {
+                            return Ok(None);
+                        }
+                    }
+                    core::assert_matches::debug_assert_matches!(support, SPAWN | NO);
+                }
+            } else {
+                if self.get_create_pidfd() {
+                    unreachable!("only implemented on linux")
+                }
+            }
+        }
+
         // Only glibc 2.24+ posix_spawn() supports returning ENOENT directly.
         #[cfg(all(target_os = "linux", target_env = "gnu"))]
         {
@@ -543,9 +608,6 @@ fn posix_spawn_file_actions_addchdir_np(
 
         let pgroup = self.get_pgroup();
 
-        // Safety: -1 indicates we don't have a pidfd.
-        let mut p = unsafe { Process::new(0, -1) };
-
         struct PosixSpawnFileActions<'a>(&'a mut MaybeUninit<libc::posix_spawn_file_actions_t>);
 
         impl Drop for PosixSpawnFileActions<'_> {
@@ -640,6 +702,47 @@ fn drop(&mut self) {
             #[cfg(target_os = "nto")]
             let spawn_fn = retrying_libc_posix_spawnp;
 
+            #[cfg(target_os = "linux")]
+            if self.get_create_pidfd() && PIDFD_SUPPORTED.load(Ordering::Relaxed) == SPAWN {
+                let mut pidfd: libc::c_int = -1;
+                let spawn_res = pidfd_spawnp.get().unwrap()(
+                    &mut pidfd,
+                    self.get_program_cstr().as_ptr(),
+                    file_actions.0.as_ptr(),
+                    attrs.0.as_ptr(),
+                    self.get_argv().as_ptr() as *const _,
+                    envp as *const _,
+                );
+
+                let spawn_res = cvt_nz(spawn_res);
+                if let Err(ref e) = spawn_res
+                    && e.raw_os_error() == Some(libc::ENOSYS)
+                {
+                    PIDFD_SUPPORTED.store(FORK_EXEC, Ordering::Relaxed);
+                    return Ok(None);
+                }
+                spawn_res?;
+
+                let pid = match cvt(pidfd_getpid.get().unwrap()(pidfd)) {
+                    Ok(pid) => pid,
+                    Err(e) => {
+                        // The child has been spawned and we are holding its pidfd.
+                        // But we cannot obtain its pid even though pidfd_getpid support was verified earlier.
+                        // This might happen if libc can't open procfs because the file descriptor limit has been reached.
+                        libc::close(pidfd);
+                        return Err(Error::new(
+                            e.kind(),
+                            "pidfd_spawnp succeeded but the child's PID could not be obtained",
+                        ));
+                    }
+                };
+
+                return Ok(Some(Process::new(pid, pidfd)));
+            }
+
+            // Safety: -1 indicates we don't have a pidfd.
+            let mut p = Process::new(0, -1);
+
             let spawn_res = spawn_fn(
                 &mut p.pid,
                 self.get_program_cstr().as_ptr(),
@@ -786,6 +889,12 @@ pub struct Process {
 
 impl Process {
     #[cfg(target_os = "linux")]
+    /// # Safety
+    ///
+    /// `pidfd` must either be -1 (representing no file descriptor) or a valid, exclusively owned file
+    /// descriptor (See [I/O Safety]).
+    ///
+    /// [I/O Safety]: crate::io#io-safety
     unsafe fn new(pid: pid_t, pidfd: pid_t) -> Self {
         use crate::os::unix::io::FromRawFd;
         use crate::sys_common::FromInner;