Dir::open returns EBADF in new glibc

[Patrick Reader]

I tested this with nix 0.27.1:

use nix::dir::Dir;
use nix::fcntl::OFlag;
use nix::sys::stat::Mode;

fn main() {
    let dir = Dir::open("/etc", OFlag::O_DIRECTORY | OFlag::O_PATH, Mode::empty()).unwrap();
    println!("ok: {dir:?}");
}

Under glibc 2.39 (Arch Linux, 2.39-1), this code errors with

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: EBADF', src/main.rs:6:84
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

seemingly for no good reason!

No error is returned by the underlying system call:

...
openat(AT_FDCWD, "/etc", O_RDONLY|O_PATH|O_DIRECTORY) = 3
fstat(3, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
fcntl(3, F_GETFL)                       = 0x210000 (flags O_RDONLY|O_PATH|O_DIRECTORY)
close(3)                                = 0
...

and it doesn't happen when calling openat directly in C:

#define _GNU_SOURCE
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main() {
    int fd = open("/etc", O_RDONLY | O_PATH | O_DIRECTORY);
    if (fd < 0) return perror("open: "), 1;
    printf("%s: %d\n", "ok", fd);
    close(fd);
}

This prints

ok: 3

It also works fine with nix::fcntl::open instead of nix::dir::Dir::open.

This bug doesn't happen with Arch Linux's glibc 2.36-6; there it works fine and prints:

ok: Dir(0x6507c4e8daa0)

I haven't had time to bisect glibc versions any further, to establish what change has caused this bug, and whether it is nix's fault or glibc's.

[SteveLauC]

I am on glibc 0.38, and that code works with no issue:

$ ldd --version
ldd (GNU libc) 2.38
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

$ cat src/main.rs
use nix::dir::Dir;
use nix::fcntl::OFlag;
use nix::sys::stat::Mode;

fn main() {
    let dir = Dir::open("/etc", OFlag::O_DIRECTORY | OFlag::O_PATH, Mode::empty()).unwrap();
    println!("ok: {dir:?}");
}

$ rg nix Cargo.toml
11:nix = { version = "0.27.1" , features = ["mman", "term", "fs", "process", "dir"]}

$ cargo r -q
ok: Dir(0x56529d85dba0)

I tried to run an Arch Linux container with podman, but the glibc is also 2.38:

$ podman pull archlinux

$ podman run -it -d archlinux
98eec04dc5d6f2de27f817ffec2c78600351d2c2adebce3faeba7404dd84e7ba

$ podman exec -it 98 bash

[root@98eec04dc5d6 /]# cat /etc/os-release
NAME="Arch Linux"
PRETTY_NAME="Arch Linux"
ID=arch
BUILD_ID=rolling
VERSION_ID=20240101.0.204074
ANSI_COLOR="38;2;23;147;209"
HOME_URL="https://archlinux.org/"
DOCUMENTATION_URL="https://wiki.archlinux.org/"
SUPPORT_URL="https://bbs.archlinux.org/"
BUG_REPORT_URL="https://bugs.archlinux.org/"
PRIVACY_POLICY_URL="https://terms.archlinux.org/docs/privacy-policy/"
LOGO=archlinux-logo

[root@98eec04dc5d6 /]# ldd --version
ldd (GNU libc) 2.38
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

[SteveLauC]

Since I do not have an environment with glibc 2.39, can I trouble you to investigate where that EBADF origins from, the implementation of Dir::open() is quite simple:

    pub fn open<P: ?Sized + NixPath>(
        path: &P,
        oflag: OFlag,
        mode: sys::stat::Mode,
    ) -> Result<Self> {
        let fd = fcntl::open(path, oflag, mode)?;  // openat(2)
        Dir::from_fd(fd)                           // fdopendir(3)
    }

That errno can come from either openat(2) or fdopendir(3), after we know where it comes from, we can take a look at the code diff between glibc 2.38 and 2.39, then we could possibly solve the issue.

Though from your strace output, openat(2) returns successfully...

[Patrick Reader]

It indeed comes from fopendir, and that error occurs when using that in C as well:

#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main() {
    int fd = open("/etc", O_RDONLY | O_PATH | O_DIRECTORY);
    if (fd < 0) return perror("open"), 1;
    printf("%s: %d\n", "ok", fd);
    DIR *d = fdopendir(fd);
    if (d == 0) return perror("fdopendir"), 1;
    printf("%s: %d\n", "ok", fd);
    closedir(d);
}

fdopendir: Bad file descriptor

So I guess it's just a bug in glibc.

(FYI you can get glibc 2.39 in the container by running pacman -Syu)

[Patrick Reader]

In fact it's a bug fix it glibc, which means it was my code that's buggy - fdopendir is not designed to be used with O_PATH. Sorry for the needless report!