Add a rustix::fs::ABS constant.

Add a `rustix::fs::ABS` constant, which corresponds to the undocumented
but commonly used convention of using `-EBADF` as the file descriptor in
`openat` and similar calls. This makes them fail if the path is not
absolute.

Fixes #1187.

src/backend/libc/fs/syscalls.rs

@@ -596,14 +596,9 @@ pub(crate) fn stat(path: &CStr) -> io::Result<Stat> {
         )
     ))]
     {
-        match crate::fs::statx(
-            crate::fs::CWD,
-            path,
-            AtFlags::empty(),
-            StatxFlags::BASIC_STATS,
-        ) {
+        match crate::fs::statx(CWD, path, AtFlags::empty(), StatxFlags::BASIC_STATS) {
             Ok(x) => statx_to_stat(x),
-            Err(io::Errno::NOSYS) => statat_old(crate::fs::CWD, path, AtFlags::empty()),
+            Err(io::Errno::NOSYS) => statat_old(CWD, path, AtFlags::empty()),
             Err(err) => Err(err),
         }
     }
@@ -637,13 +632,13 @@ pub(crate) fn lstat(path: &CStr) -> io::Result<Stat> {
     ))]
     {
         match crate::fs::statx(
-            crate::fs::CWD,
+            CWD,
             path,
             AtFlags::SYMLINK_NOFOLLOW,
             StatxFlags::BASIC_STATS,
         ) {
             Ok(x) => statx_to_stat(x),
-            Err(io::Errno::NOSYS) => statat_old(crate::fs::CWD, path, AtFlags::SYMLINK_NOFOLLOW),
+            Err(io::Errno::NOSYS) => statat_old(CWD, path, AtFlags::SYMLINK_NOFOLLOW),
             Err(err) => Err(err),
         }
     }

src/backend/linux_raw/c.rs

@@ -8,7 +8,7 @@
 
 pub(crate) type size_t = usize;
 pub(crate) use linux_raw_sys::ctypes::*;
-pub(crate) use linux_raw_sys::errno::EINVAL;
+pub(crate) use linux_raw_sys::errno::{EBADF, EINVAL};
 pub(crate) use linux_raw_sys::general::{__kernel_fd_set as fd_set, __FD_SETSIZE as FD_SETSIZE};
 pub(crate) use linux_raw_sys::ioctl::{FIONBIO, FIONREAD};
 // Import the kernel's `uid_t` and `gid_t` if they're 32-bit.

src/backend/linux_raw/conv.rs

@@ -162,7 +162,7 @@ impl<'a, Num: ArgNumber> From<BorrowedFd<'a>> for ArgReg<'a, Num> {
 pub(super) unsafe fn raw_fd<'a, Num: ArgNumber>(fd: RawFd) -> ArgReg<'a, Num> {
     // Use `no_fd` when passing `-1` is intended.
     #[cfg(feature = "fs")]
-    debug_assert!(fd == crate::fs::CWD.as_raw_fd() || fd >= 0);
+    debug_assert!(fd == crate::fs::CWD.as_raw_fd() || fd == crate::fs::ABS.as_raw_fd() || fd >= 0);
 
     // Don't pass the `io_uring_register_files_skip` sentry value this way.
     #[cfg(feature = "io_uring")]

src/fs/at.rs

@@ -1,9 +1,10 @@
 //! POSIX-style `*at` functions.
 //!
 //! The `dirfd` argument to these functions may be a file descriptor for a
-//! directory, or the special value [`CWD`].
+//! directory, the special value [`CWD`], or the special value [`ABS`].
 //!
-//! [`cwd`]: crate::fs::CWD
+//! [`CWD`]: crate::fs::CWD
+//! [`ABS`]: crate::fs::ABS
 
 use crate::fd::OwnedFd;
 use crate::ffi::CStr;

src/fs/mod.rs

@@ -6,10 +6,6 @@ mod at;
 mod constants;
 #[cfg(linux_kernel)]
 mod copy_file_range;
-#[cfg(not(any(target_os = "espidf", target_os = "redox")))]
-#[cfg(not(target_os = "haiku"))]
-// Haiku needs <https://git.haiku-os.org/haiku/commit/?id=b8caef69155fbe87def579305622b9718d7779dc>
-mod cwd;
 #[cfg(all(feature = "alloc", not(any(target_os = "espidf", target_os = "redox"))))]
 mod dir;
 #[cfg(not(any(
@@ -54,6 +50,8 @@ mod raw_dir;
 mod seek_from;
 #[cfg(target_os = "linux")]
 mod sendfile;
+#[cfg(not(any(target_os = "espidf", target_os = "redox")))]
+mod special;
 #[cfg(linux_kernel)]
 mod statx;
 #[cfg(not(any(
@@ -72,10 +70,6 @@ pub use at::*;
 pub use constants::*;
 #[cfg(linux_kernel)]
 pub use copy_file_range::copy_file_range;
-#[cfg(not(any(target_os = "espidf", target_os = "redox")))]
-#[cfg(not(target_os = "haiku"))]
-// Haiku needs <https://git.haiku-os.org/haiku/commit/?id=b8caef69155fbe87def579305622b9718d7779dc>
-pub use cwd::*;
 #[cfg(all(feature = "alloc", not(any(target_os = "espidf", target_os = "redox"))))]
 pub use dir::{Dir, DirEntry};
 #[cfg(not(any(
@@ -118,6 +112,8 @@ pub use raw_dir::{RawDir, RawDirEntry};
 pub use seek_from::SeekFrom;
 #[cfg(target_os = "linux")]
 pub use sendfile::sendfile;
+#[cfg(not(any(target_os = "espidf", target_os = "redox")))]
+pub use special::*;
 #[cfg(linux_kernel)]
 pub use statx::statx;
 #[cfg(not(any(

src/fs/special.rs

@@ -0,0 +1,68 @@
+//! The `CWD` and `ABS` constants, representing the current working directory
+//! and absolute-only paths, respectively.
+//!
+//! # Safety
+//!
+//! This file uses `AT_FDCWD`, which is a raw file descriptor, but which is
+//! always valid, and `-EBADF`, which is an undocumented by commonly used
+//! convention for passing a value which will always fail if the accompanying
+//! path isn't absolute.
+
+#![allow(unsafe_code)]
+
+use crate::backend;
+use backend::c;
+use backend::fd::{BorrowedFd, RawFd};
+
+/// `AT_FDCWD`—A handle representing the current working directory.
+///
+/// This is a file descriptor which refers to the process current directory
+/// which can be used as the directory argument in `*at` functions such as
+/// [`openat`].
+///
+/// # References
+///  - [POSIX]
+///
+/// [`openat`]: crate::fs::openat
+/// [POSIX]: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/fcntl.h.html
+// SAFETY: `AT_FDCWD` is a reserved value that is never dynamically
+// allocated, so it'll remain valid for the duration of `'static`.
+#[doc(alias = "AT_FDCWD")]
+pub const CWD: BorrowedFd<'static> =
+    unsafe { BorrowedFd::<'static>::borrow_raw(c::AT_FDCWD as RawFd) };
+
+/// `-EBADF`—A handle that requires paths to be absolute.
+///
+/// This is a file descriptor which refers to no directory, which can be used
+/// as the directory argument in `*at` functions such as [`openat`], which
+/// causes them to fail if the accompanying path is not absolute.
+///
+/// [`openat`]: crate::fs::openat
+// SAFETY: This `-EBADF` convention is commonly used, such as in lxc, so OS's
+// aren't going to break it.
+pub const ABS: BorrowedFd<'static> =
+    unsafe { BorrowedFd::<'static>::borrow_raw(c::EBADF.wrapping_neg() as RawFd) };
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::fd::AsRawFd;
+
+    #[test]
+    fn test_cwd() {
+        assert!(CWD.as_raw_fd() != -1);
+        #[cfg(linux_kernel)]
+        #[cfg(feature = "io_uring")]
+        assert!(CWD.as_raw_fd() != linux_raw_sys::io_uring::IORING_REGISTER_FILES_SKIP);
+    }
+
+    #[test]
+    fn test_abs() {
+        assert!(ABS.as_raw_fd() < 0);
+        assert!(ABS.as_raw_fd() != -1);
+        assert!(ABS.as_raw_fd() != c::AT_FDCWD);
+        #[cfg(linux_kernel)]
+        #[cfg(feature = "io_uring")]
+        assert!(ABS.as_raw_fd() != linux_raw_sys::io_uring::IORING_REGISTER_FILES_SKIP);
+    }
+}

src/process/chdir.rs

@@ -40,7 +40,7 @@ pub fn fchdir<Fd: AsFd>(fd: Fd) -> io::Result<()> {
     backend::process::syscalls::fchdir(fd.as_fd())
 }
 
-/// `getCWD`—Return the current working directory.
+/// `getcwd`—Return the current working directory.
 ///
 /// If `reuse` already has available capacity, reuse it if possible.
 ///

tests/fs/main.rs

@@ -5,7 +5,6 @@
 #![cfg_attr(core_c_str, feature(core_c_str))]
 
 mod chmodat;
-mod cwd;
 #[cfg(not(target_os = "redox"))]
 mod dir;
 mod fcntl;
@@ -42,6 +41,7 @@ mod renameat;
 #[cfg(any(linux_kernel, target_os = "freebsd"))]
 mod seals;
 mod seek;
+mod special;
 #[cfg(not(any(target_os = "haiku", target_os = "redox", target_os = "wasi")))]
 mod statfs;
 #[cfg(linux_kernel)]