process_unix.rs

use crate::convert::TryInto;
use crate::fmt;
use crate::io::{self, Error, ErrorKind};
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};

////////////////////////////////////////////////////////////////////////////////
// Command
////////////////////////////////////////////////////////////////////////////////

impl Command {
    pub fn spawn(
        &mut self,
        default: Stdio,
        needs_stdin: bool,
    ) -> io::Result<(Process, StdioPipes)> {
        const CLOEXEC_MSG_FOOTER: [u8; 4] = *b"NOEX";

        let envp = self.capture_env();

        if self.saw_nul() {
            return Err(io::Error::new(ErrorKind::InvalidInput, "nul byte found in provided data"));
        }

        let (ours, theirs) = self.setup_io(default, needs_stdin)?;

        if let Some(ret) = self.posix_spawn(&theirs, envp.as_ref())? {
            return Ok((ret, ours));
        }

        let (input, output) = sys::pipe::anon_pipe()?;

        let (result, pidfd) = self.do_fork()?;

        let pid = unsafe {
            match result {
                0 => {
                    drop(input);
                    let Err(err) = self.do_exec(theirs, envp.as_ref());
                    let errno = err.raw_os_error().unwrap_or(libc::EINVAL) as u32;
                    let errno = errno.to_be_bytes();
                    let bytes = [
                        errno[0],
                        errno[1],
                        errno[2],
                        errno[3],
                        CLOEXEC_MSG_FOOTER[0],
                        CLOEXEC_MSG_FOOTER[1],
                        CLOEXEC_MSG_FOOTER[2],
                        CLOEXEC_MSG_FOOTER[3],
                    ];
                    // pipe I/O up to PIPE_BUF bytes should be atomic, and then
                    // we want to be sure we *don't* run at_exit destructors as
                    // we're being torn down regardless
                    rtassert!(output.write(&bytes).is_ok());
                    libc::_exit(1)
                }
                n => n as pid_t,
            }
        };

        let mut p = Process { pid, status: None, pidfd };
        drop(output);
        let mut bytes = [0; 8];

        // loop to handle EINTR
        loop {
            match input.read(&mut bytes) {
                Ok(0) => return Ok((p, ours)),
                Ok(8) => {
                    let (errno, footer) = bytes.split_at(4);
                    assert_eq!(
                        CLOEXEC_MSG_FOOTER, footer,
                        "Validation on the CLOEXEC pipe failed: {:?}",
                        bytes
                    );
                    let errno = i32::from_be_bytes(errno.try_into().unwrap());
                    assert!(p.wait().is_ok(), "wait() should either return Ok or panic");
                    return Err(Error::from_raw_os_error(errno));
                }
                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                Err(e) => {
                    assert!(p.wait().is_ok(), "wait() should either return Ok or panic");
                    panic!("the CLOEXEC pipe failed: {:?}", e)
                }
                Ok(..) => {
                    // pipe I/O up to PIPE_BUF bytes should be atomic
                    assert!(p.wait().is_ok(), "wait() should either return Ok or panic");
                    panic!("short read on the CLOEXEC pipe")
                }
            }
        }
    }

    // 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();

        if self.saw_nul() {
            return io::Error::new(ErrorKind::InvalidInput, "nul byte found in provided data");
        }

        match self.setup_io(default, true) {
            Ok((_, theirs)) => {
                unsafe {
                    // Similar to when forking, we want to ensure that access to
                    // the environment is synchronized, so make sure to grab the
                    // environment lock before we try to exec.
                    let _lock = sys::os::env_lock();

                    let Err(e) = self.do_exec(theirs, envp.as_ref());
                    e
                }
            }
            Err(e) => e,
        }
    }

    // And at this point we've reached a special time in the life of the
    // child. The child must now be considered hamstrung and unable to
    // do anything other than syscalls really. Consider the following
    // scenario:
    //
    //      1. Thread A of process 1 grabs the malloc() mutex
    //      2. Thread B of process 1 forks(), creating thread C
    //      3. Thread C of process 2 then attempts to malloc()
    //      4. The memory of process 2 is the same as the memory of
    //         process 1, so the mutex is locked.
    //
    // This situation looks a lot like deadlock, right? It turns out
    // that this is what pthread_atfork() takes care of, which is
    // presumably implemented across platforms. The first thing that
    // threads to *before* forking is to do things like grab the malloc
    // mutex, and then after the fork they unlock it.
    //
    // Despite this information, libnative's spawn has been witnessed to
    // deadlock on both macOS and FreeBSD. I'm not entirely sure why, but
    // all collected backtraces point at malloc/free traffic in the
    // child spawned process.
    //
    // For this reason, the block of code below should contain 0
    // invocations of either malloc of free (or their related friends).
    //
    // As an example of not having malloc/free traffic, we don't close
    // this file descriptor by dropping the FileDesc (which contains an
    // allocation). Instead we just close it manually. This will never
    // have the drop glue anyway because this code never returns (the
    // child will either exec() or invoke libc::exit)
    unsafe fn do_exec(
        &mut self,
        stdio: ChildPipes,
        maybe_envp: Option<&CStringArray>,
    ) -> Result<!, io::Error> {
        use crate::sys::{self, cvt_r};

        if let Some(fd) = stdio.stdin.fd() {
            cvt_r(|| libc::dup2(fd, libc::STDIN_FILENO))?;
        }
        if let Some(fd) = stdio.stdout.fd() {
            cvt_r(|| libc::dup2(fd, libc::STDOUT_FILENO))?;
        }
        if let Some(fd) = stdio.stderr.fd() {
            cvt_r(|| libc::dup2(fd, libc::STDERR_FILENO))?;
        }

        #[cfg(not(target_os = "l4re"))]
        {
            if let Some(u) = self.get_gid() {
                cvt(libc::setgid(u as gid_t))?;
            }
            if let Some(u) = self.get_uid() {
                // When dropping privileges from root, the `setgroups` call
                // will remove any extraneous groups. If we don't call this,
                // then even though our uid has dropped, we may still have
                // groups that enable us to do super-user things. This will
                // fail if we aren't root, so don't bother checking the
                // return value, this is just done as an optimistic
                // privilege dropping function.
                //FIXME: Redox kernel does not support setgroups yet
                #[cfg(not(target_os = "redox"))]
                let _ = libc::setgroups(0, ptr::null());
                cvt(libc::setuid(u as uid_t))?;
            }
        }
        if let Some(ref cwd) = *self.get_cwd() {
            cvt(libc::chdir(cwd.as_ptr()))?;
        }

        // emscripten has no signal support.
        #[cfg(not(target_os = "emscripten"))]
        {
            use crate::mem::MaybeUninit;
            // Reset signal handling so the child process starts in a
            // standardized state. libstd ignores SIGPIPE, and signal-handling
            // libraries often set a mask. Child processes inherit ignored
            // signals and the signal mask from their parent, but most
            // UNIX programs do not reset these things on their own, so we
            // need to clean things up now to avoid confusing the program
            // we're about to run.
            let mut set = MaybeUninit::<libc::sigset_t>::uninit();
            cvt(sigemptyset(set.as_mut_ptr()))?;
            cvt(libc::pthread_sigmask(libc::SIG_SETMASK, set.as_ptr(), ptr::null_mut()))?;
            let ret = sys::signal(libc::SIGPIPE, libc::SIG_DFL);
            if ret == libc::SIG_ERR {
                return Err(io::Error::last_os_error());
            }
        }

        for callback in self.get_closures().iter_mut() {
            callback()?;
        }

        // Although we're performing an exec here we may also return with an
        // error from this function (without actually exec'ing) in which case we
        // want to be sure to restore the global environment back to what it
        // once was, ensuring that our temporary override, when free'd, doesn't
        // corrupt our process's environment.
        let mut _reset = None;
        if let Some(envp) = maybe_envp {
            struct Reset(*const *const libc::c_char);

            impl Drop for Reset {
                fn drop(&mut self) {
                    unsafe {
                        *sys::os::environ() = self.0;
                    }
                }
            }

            _reset = Some(Reset(*sys::os::environ()));
            *sys::os::environ() = envp.as_ptr();
        }

        libc::execvp(self.get_program_cstr().as_ptr(), self.get_argv().as_ptr());
        Err(io::Error::last_os_error())
    }

    #[cfg(not(any(
        target_os = "macos",
        target_os = "freebsd",
    )))]
    fn posix_spawn(
        &mut self,
        _: &ChildPipes,
        _: Option<&CStringArray>,
    ) -> io::Result<Option<Process>> {
        Ok(None)
    }

    // Only support platforms for which posix_spawn() can return ENOENT
    // directly.
    #[cfg(any(
        target_os = "macos",
        target_os = "freebsd",
    ))]
    fn posix_spawn(
        &mut self,
        stdio: &ChildPipes,
        envp: Option<&CStringArray>,
    ) -> io::Result<Option<Process>> {
        use crate::mem::MaybeUninit;
        use crate::sys;

        if self.get_gid().is_some()
            || self.get_uid().is_some()
            || self.env_saw_path()
            || !self.get_closures().is_empty()
        {
            return Ok(None);
        }

        // Only glibc 2.24+ posix_spawn() supports returning ENOENT directly.
        #[cfg(all(target_os = "linux", target_env = "gnu"))]
        {
            if let Some(version) = sys::os::glibc_version() {
                if version < (2, 24) {
                    return Ok(None);
                }
            } else {
                return Ok(None);
            }
        }

        // Solaris, glibc 2.29+, and musl 1.24+ can set a new working directory,
        // and maybe others will gain this non-POSIX function too. We'll check
        // for this weak symbol as soon as it's needed, so we can return early
        // otherwise to do a manual chdir before exec.
        weak! {
            fn posix_spawn_file_actions_addchdir_np(
                *mut libc::posix_spawn_file_actions_t,
                *const libc::c_char
            ) -> libc::c_int
        }
        let addchdir = match self.get_cwd() {
            Some(cwd) => match posix_spawn_file_actions_addchdir_np.get() {
                Some(f) => Some((f, cwd)),
                None => return Ok(None),
            },
            None => None,
        };

        let mut p = Process { pid: 0, status: None };

        struct PosixSpawnFileActions(MaybeUninit<libc::posix_spawn_file_actions_t>);

        impl Drop for PosixSpawnFileActions {
            fn drop(&mut self) {
                unsafe {
                    libc::posix_spawn_file_actions_destroy(self.0.as_mut_ptr());
                }
            }
        }

        struct PosixSpawnattr(MaybeUninit<libc::posix_spawnattr_t>);

        impl Drop for PosixSpawnattr {
            fn drop(&mut self) {
                unsafe {
                    libc::posix_spawnattr_destroy(self.0.as_mut_ptr());
                }
            }
        }

        unsafe {
            let mut file_actions = PosixSpawnFileActions(MaybeUninit::uninit());
            let mut attrs = PosixSpawnattr(MaybeUninit::uninit());

            libc::posix_spawnattr_init(attrs.0.as_mut_ptr());
            libc::posix_spawn_file_actions_init(file_actions.0.as_mut_ptr());

            if let Some(fd) = stdio.stdin.fd() {
                cvt(libc::posix_spawn_file_actions_adddup2(
                    file_actions.0.as_mut_ptr(),
                    fd,
                    libc::STDIN_FILENO,
                ))?;
            }
            if let Some(fd) = stdio.stdout.fd() {
                cvt(libc::posix_spawn_file_actions_adddup2(
                    file_actions.0.as_mut_ptr(),
                    fd,
                    libc::STDOUT_FILENO,
                ))?;
            }
            if let Some(fd) = stdio.stderr.fd() {
                cvt(libc::posix_spawn_file_actions_adddup2(
                    file_actions.0.as_mut_ptr(),
                    fd,
                    libc::STDERR_FILENO,
                ))?;
            }
            if let Some((f, cwd)) = addchdir {
                cvt(f(file_actions.0.as_mut_ptr(), cwd.as_ptr()))?;
            }

            let mut set = MaybeUninit::<libc::sigset_t>::uninit();
            cvt(sigemptyset(set.as_mut_ptr()))?;
            cvt(libc::posix_spawnattr_setsigmask(attrs.0.as_mut_ptr(), set.as_ptr()))?;
            cvt(sigaddset(set.as_mut_ptr(), libc::SIGPIPE))?;
            cvt(libc::posix_spawnattr_setsigdefault(attrs.0.as_mut_ptr(), set.as_ptr()))?;

            let flags = libc::POSIX_SPAWN_SETSIGDEF | libc::POSIX_SPAWN_SETSIGMASK;
            cvt(libc::posix_spawnattr_setflags(attrs.0.as_mut_ptr(), flags as _))?;

            // Make sure we synchronize access to the global `environ` resource
            let _env_lock = sys::os::env_lock();
            let envp = envp.map(|c| c.as_ptr()).unwrap_or_else(|| *sys::os::environ() as *const _);
            let ret = libc::posix_spawnp(
                &mut p.pid,
                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 _,
            );
            if ret == 0 { Ok(Some(p)) } else { Err(io::Error::from_raw_os_error(ret)) }
        }
    }
}

////////////////////////////////////////////////////////////////////////////////
// Processes
////////////////////////////////////////////////////////////////////////////////

/// The unique ID of the process (this should never be negative).
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 {
    pub fn id(&self) -> u32 {
        self.pid as u32
    }

    pub fn kill(&mut self) -> io::Result<()> {
        // If we've already waited on this process then the pid can be recycled
        // and used for another process, and we probably shouldn't be killing
        // random processes, so just return an error.
        if self.status.is_some() {
            Err(Error::new(
                ErrorKind::InvalidInput,
                "invalid argument: can't kill an exited process",
            ))
        } else {
            cvt(unsafe { libc::kill(self.pid, libc::SIGKILL) }).map(drop)
        }
    }

    pub fn wait(&mut self) -> io::Result<ExitStatus> {
        use crate::sys::cvt_r;
        if let Some(status) = self.status {
            return Ok(status);
        }
        let mut status = 0 as c_int;
        cvt_r(|| unsafe { libc::waitpid(self.pid, &mut status, 0) })?;
        self.status = Some(ExitStatus::new(status));
        Ok(ExitStatus::new(status))
    }

    pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
        if let Some(status) = self.status {
            return Ok(Some(status));
        }
        let mut status = 0 as c_int;
        let pid = cvt(unsafe { libc::waitpid(self.pid, &mut status, libc::WNOHANG) })?;
        if pid == 0 {
            Ok(None)
        } else {
            self.status = Some(ExitStatus::new(status));
            Ok(Some(ExitStatus::new(status)))
        }
    }
}

/// Unix exit statuses
#[derive(PartialEq, Eq, Clone, Copy, Debug)]
pub struct ExitStatus(c_int);

impl ExitStatus {
    pub fn new(status: c_int) -> ExitStatus {
        ExitStatus(status)
    }

    fn exited(&self) -> bool {
        libc::WIFEXITED(self.0)
    }

    pub fn success(&self) -> bool {
        self.code() == Some(0)
    }

    pub fn code(&self) -> Option<i32> {
        if self.exited() { Some(libc::WEXITSTATUS(self.0)) } else { None }
    }

    pub fn signal(&self) -> Option<i32> {
        if !self.exited() { Some(libc::WTERMSIG(self.0)) } else { None }
    }
}

/// Converts a raw `c_int` to a type-safe `ExitStatus` by wrapping it without copying.
impl From<c_int> for ExitStatus {
    fn from(a: c_int) -> ExitStatus {
        ExitStatus(a)
    }
}

impl fmt::Display for ExitStatus {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        if let Some(code) = self.code() {
            write!(f, "exit code: {}", code)
        } else {
            let signal = self.signal().unwrap();
            write!(f, "signal: {}", signal)
        }
    }
}

#[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))
        }
    }
}