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// Copyright 2021, The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! A library for passing arbitrary file descriptors when spawning child processes.
//!
//! # Example
//!
//! ```rust
//! use command_fds::{CommandFdExt, FdMapping};
//! use std::fs::File;
//! use std::io::stdin;
//! use std::os::fd::AsFd;
//! use std::os::unix::io::AsRawFd;
//! use std::process::Command;
//!
//! // Open a file.
//! let file = File::open("Cargo.toml").unwrap();
//!
//! // Prepare to run `ls -l /proc/self/fd` with some FDs mapped.
//! let mut command = Command::new("ls");
//! command.arg("-l").arg("/proc/self/fd");
//! command
//! .fd_mappings(vec![
//! // Map `file` as FD 3 in the child process.
//! FdMapping {
//! parent_fd: file.into(),
//! child_fd: 3,
//! },
//! // Map this process's stdin as FD 5 in the child process.
//! FdMapping {
//! parent_fd: stdin().as_fd().try_clone_to_owned().unwrap(),
//! child_fd: 5,
//! },
//! ])
//! .unwrap();
//!
//! // Spawn the child process.
//! let mut child = command.spawn().unwrap();
//! child.wait().unwrap();
//! ```
#[cfg(feature = "tokio")]
pub mod tokio;
use nix::fcntl::{fcntl, FcntlArg, FdFlag};
use nix::unistd::dup2;
use std::cmp::max;
use std::io;
use std::os::fd::{AsRawFd, FromRawFd, OwnedFd};
use std::os::unix::io::RawFd;
use std::os::unix::process::CommandExt;
use std::process::Command;
use thiserror::Error;
/// A mapping from a file descriptor in the parent to a file descriptor in the child, to be applied
/// when spawning a child process.
///
/// This takes ownership of the `parent_fd` to ensure that it is kept open until after the child is
/// spawned.
#[derive(Debug)]
pub struct FdMapping {
pub parent_fd: OwnedFd,
pub child_fd: RawFd,
}
/// Error setting up FD mappings, because there were two or more mappings for the same child FD.
#[derive(Copy, Clone, Debug, Eq, Error, PartialEq)]
#[error("Two or more mappings for the same child FD")]
pub struct FdMappingCollision;
/// Extension to add file descriptor mappings to a [`Command`].
pub trait CommandFdExt {
/// Adds the given set of file descriptors to the command.
///
/// Warning: Calling this more than once on the same command may result in unexpected behaviour.
/// In particular, it is not possible to check that two mappings applied separately don't use
/// the same `child_fd`. If there is such a collision then one will apply and the other will be
/// lost.
///
/// Note that the `Command` takes ownership of the file descriptors, which means that they won't
/// be closed in the parent process until the `Command` is dropped.
fn fd_mappings(&mut self, mappings: Vec<FdMapping>) -> Result<&mut Self, FdMappingCollision>;
/// Adds the given set of file descriptors to be passed on to the child process when the command
/// is run.
///
/// Note that the `Command` takes ownership of the file descriptors, which means that they won't
/// be closed in the parent process until the `Command` is dropped.
fn preserved_fds(&mut self, fds: Vec<OwnedFd>) -> &mut Self;
}
impl CommandFdExt for Command {
fn fd_mappings(
&mut self,
mut mappings: Vec<FdMapping>,
) -> Result<&mut Self, FdMappingCollision> {
let child_fds = validate_child_fds(&mappings)?;
// Register the callback to apply the mappings after forking but before execing.
// Safety: `map_fds` will not allocate, so it is safe to call from this hook.
unsafe {
// If the command is run more than once, the closure will be called multiple times but
// in different forked processes, which will have different copies of `mappings`. So
// their changes to it shouldn't be visible to each other.
self.pre_exec(move || map_fds(&mut mappings, &child_fds));
}
Ok(self)
}
fn preserved_fds(&mut self, fds: Vec<OwnedFd>) -> &mut Self {
unsafe {
self.pre_exec(move || preserve_fds(&fds));
}
self
}
}
/// Validates that there are no conflicting mappings to the same child FD.
fn validate_child_fds(mappings: &[FdMapping]) -> Result<Vec<RawFd>, FdMappingCollision> {
let mut child_fds: Vec<RawFd> = mappings.iter().map(|mapping| mapping.child_fd).collect();
child_fds.sort_unstable();
child_fds.dedup();
if child_fds.len() != mappings.len() {
return Err(FdMappingCollision);
}
Ok(child_fds)
}
// This function must not do any allocation, as it is called from the pre_exec hook.
fn map_fds(mappings: &mut [FdMapping], child_fds: &[RawFd]) -> io::Result<()> {
if mappings.is_empty() {
// No need to do anything, and finding first_unused_fd would fail.
return Ok(());
}
// Find the first FD which is higher than any parent or child FD in the mapping, so we can
// safely use it and higher FDs as temporary FDs. There may be other files open with these FDs,
// so we still need to ensure we don't conflict with them.
let first_safe_fd = mappings
.iter()
.map(|mapping| max(mapping.parent_fd.as_raw_fd(), mapping.child_fd))
.max()
.unwrap()
+ 1;
// If any parent FDs conflict with child FDs, then first duplicate them to a temporary FD which
// is clear of either range. Mappings to the same FD are fine though, we can handle them by just
// removing the FD_CLOEXEC flag from the existing (parent) FD.
for mapping in mappings.iter_mut() {
if child_fds.contains(&mapping.parent_fd.as_raw_fd())
&& mapping.parent_fd.as_raw_fd() != mapping.child_fd
{
let parent_fd = fcntl(
mapping.parent_fd.as_raw_fd(),
FcntlArg::F_DUPFD_CLOEXEC(first_safe_fd),
)?;
// SAFETY: We just created `parent_fd` so we can take ownership of it.
unsafe {
mapping.parent_fd = OwnedFd::from_raw_fd(parent_fd);
}
}
}
// Now we can actually duplicate FDs to the desired child FDs.
for mapping in mappings {
if mapping.child_fd == mapping.parent_fd.as_raw_fd() {
// Remove the FD_CLOEXEC flag, so the FD will be kept open when exec is called for the
// child.
fcntl(
mapping.parent_fd.as_raw_fd(),
FcntlArg::F_SETFD(FdFlag::empty()),
)?;
} else {
// This closes child_fd if it is already open as something else, and clears the
// FD_CLOEXEC flag on child_fd.
dup2(mapping.parent_fd.as_raw_fd(), mapping.child_fd)?;
}
}
Ok(())
}
fn preserve_fds(fds: &[OwnedFd]) -> io::Result<()> {
for fd in fds {
// Remove the FD_CLOEXEC flag, so the FD will be kept open when exec is called for the
// child.
fcntl(fd.as_raw_fd(), FcntlArg::F_SETFD(FdFlag::empty()))?;
}
Ok(())
}
#[cfg(test)]
mod tests {
use super::*;
use nix::unistd::close;
use std::collections::HashSet;
use std::fs::{read_dir, File};
use std::os::unix::io::AsRawFd;
use std::process::Output;
use std::str;
use std::sync::Once;
static SETUP: Once = Once::new();
#[test]
fn conflicting_mappings() {
setup();
let mut command = Command::new("ls");
let file1 = File::open("testdata/file1.txt").unwrap();
let file2 = File::open("testdata/file2.txt").unwrap();
// Mapping two different FDs to the same FD isn't allowed.
assert!(command
.fd_mappings(vec![
FdMapping {
child_fd: 4,
parent_fd: file1.into(),
},
FdMapping {
child_fd: 4,
parent_fd: file2.into(),
},
])
.is_err());
}
#[test]
fn no_mappings() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
assert!(command.fd_mappings(vec![]).is_ok());
let output = command.output().unwrap();
expect_fds(&output, &[0, 1, 2, 3], 0);
}
#[test]
fn none_preserved() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
command.preserved_fds(vec![]);
let output = command.output().unwrap();
expect_fds(&output, &[0, 1, 2, 3], 0);
}
#[test]
fn one_mapping() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
let file = File::open("testdata/file1.txt").unwrap();
// Map the file an otherwise unused FD.
assert!(command
.fd_mappings(vec![FdMapping {
parent_fd: file.into(),
child_fd: 5,
},])
.is_ok());
let output = command.output().unwrap();
expect_fds(&output, &[0, 1, 2, 3, 5], 0);
}
#[test]
#[ignore = "flaky on GitHub"]
fn one_preserved() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
let file = File::open("testdata/file1.txt").unwrap();
let file_fd: OwnedFd = file.into();
let raw_file_fd = file_fd.as_raw_fd();
assert!(raw_file_fd > 3);
command.preserved_fds(vec![file_fd]);
let output = command.output().unwrap();
expect_fds(&output, &[0, 1, 2, 3, raw_file_fd], 0);
}
#[test]
fn swap_mappings() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
let file1 = File::open("testdata/file1.txt").unwrap();
let file2 = File::open("testdata/file2.txt").unwrap();
let fd1: OwnedFd = file1.into();
let fd2: OwnedFd = file2.into();
let fd1_raw = fd1.as_raw_fd();
let fd2_raw = fd2.as_raw_fd();
// Map files to each other's FDs, to ensure that the temporary FD logic works.
assert!(command
.fd_mappings(vec![
FdMapping {
parent_fd: fd1,
child_fd: fd2_raw,
},
FdMapping {
parent_fd: fd2,
child_fd: fd1_raw,
},
])
.is_ok(),);
let output = command.output().unwrap();
// Expect one more Fd for the /proc/self/fd directory. We can't predict what number it will
// be assigned, because 3 might or might not be taken already by fd1 or fd2.
expect_fds(&output, &[0, 1, 2, fd1_raw, fd2_raw], 1);
}
#[test]
fn one_to_one_mapping() {
setup();
let mut command = Command::new("ls");
command.arg("/proc/self/fd");
let file1 = File::open("testdata/file1.txt").unwrap();
let file2 = File::open("testdata/file2.txt").unwrap();
let fd1: OwnedFd = file1.into();
let fd1_raw = fd1.as_raw_fd();
// Map file1 to the same FD it currently has, to ensure the special case for that works.
assert!(command
.fd_mappings(vec![FdMapping {
parent_fd: fd1,
child_fd: fd1_raw,
}])
.is_ok());
let output = command.output().unwrap();
// Expect one more Fd for the /proc/self/fd directory. We can't predict what number it will
// be assigned, because 3 might or might not be taken already by fd1 or fd2.
expect_fds(&output, &[0, 1, 2, fd1_raw], 1);
// Keep file2 open until the end, to ensure that it's not passed to the child.
drop(file2);
}
#[test]
fn map_stdin() {
setup();
let mut command = Command::new("cat");
let file = File::open("testdata/file1.txt").unwrap();
// Map the file to stdin.
assert!(command
.fd_mappings(vec![FdMapping {
parent_fd: file.into(),
child_fd: 0,
},])
.is_ok());
let output = command.output().unwrap();
assert!(output.status.success());
assert_eq!(output.stdout, b"test 1");
}
/// Parse the output of ls into a set of filenames
fn parse_ls_output(output: &[u8]) -> HashSet<String> {
str::from_utf8(output)
.unwrap()
.split_terminator("\n")
.map(str::to_owned)
.collect()
}
/// Check that the output of `ls /proc/self/fd` contains the expected set of FDs, plus exactly
/// `extra` extra FDs.
fn expect_fds(output: &Output, expected_fds: &[RawFd], extra: usize) {
assert!(output.status.success());
let expected_fds: HashSet<String> = expected_fds.iter().map(RawFd::to_string).collect();
let fds = parse_ls_output(&output.stdout);
if extra == 0 {
assert_eq!(fds, expected_fds);
} else {
assert!(expected_fds.is_subset(&fds));
assert_eq!(fds.len(), expected_fds.len() + extra);
}
}
fn setup() {
SETUP.call_once(close_excess_fds);
}
/// Close all file descriptors apart from stdin, stdout and stderr.
///
/// This is necessary because GitHub Actions opens a bunch of others for some reason.
fn close_excess_fds() {
let dir = read_dir("/proc/self/fd").unwrap();
for entry in dir {
let entry = entry.unwrap();
let fd: RawFd = entry.file_name().to_str().unwrap().parse().unwrap();
if fd > 3 {
close(fd).unwrap();
}
}
}
}