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fs::openat2 is unavailable when building with --cfg=rustix_no_linux_raw (e.g., in AOSP) #1713

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fs::openat2 is unavailable when building with --cfg=rustix_no_linux_raw (e.g., in AOSP) #1713

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12 changes: 11 additions & 1 deletion src/backend/libc/fs/syscalls.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1868,16 +1868,26 @@ pub(crate) fn memfd_create(name: &CStr, flags: MemfdFlags) -> io::Result<OwnedFd
unsafe { ret_owned_fd(memfd_create(c_str(name), bitflags_bits!(flags))) }
}

#[cfg(linux_raw_dep)]
#[cfg(linux_kernel)]
pub(crate) fn openat2(
dirfd: BorrowedFd<'_>,
path: &CStr,
oflags: OFlags,
mode: Mode,
resolve: ResolveFlags,
) -> io::Result<OwnedFd> {
#[cfg(linux_raw_dep)]
use linux_raw_sys::general::open_how;

#[cfg(not(linux_raw_dep))]
#[repr(C)]
#[derive(Debug, Copy, Clone)]
struct open_how {
flags: u64,
mode: u64,
resolve: u64,
}

@sunfishcode sunfishcode Oct 27, 2026

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libc (now?) has an open_how declaration. Should we use that instead of declaring this type manually?

@xtqqczze xtqqczze Oct 27, 2026

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It has been there since v0.2.108 in fact.

@tylersaunders tylersaunders Oct 27, 2026 •
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Unfortunately we can't use libc::open_how here for two reasons (I'd love to be wrong here, but this is my read after looking into this):

  1. Not exposed on target_os = "android": In libc, open_how (along with the RESOLVE_* constants) is defined in src/unix/linux_like/linux/mod.rs (#[cfg(target_os = "linux")]) rather than the shared linux_like or android modules, so it is absent on *-linux-android targets (compare x86_64-unknown-linux-gnu docs https://docs.rs/libc/latest/libc/struct.open_how.html vs. aarch64-linux-android docs https://docs.rs/libc/latest/aarch64-linux-android/libc/?search=open_how).

This is the same reason ResolveFlags and SYS_OPENAT2 in rustix's libc backend are defined inline under #[cfg(linux_kernel)] rather than imported from libc.

  1. #[non_exhaustive]: Even on target_os = "linux", libc::open_how is marked #[non_exhaustive], which disallows constructing it via the open_how { flags, mode, resolve } struct literal (E0639) and would require unsafe {core::mem::zeroed() } + field assignment.

Since the kernel ABI for openat2(2) explicitly versions struct open_how by passing size_of::<open_how>() (24 bytes for v0) as the size syscall argument, defining the 3-field #[repr(C)] struct open_how locally when not(linux_raw_dep) works uniformly across both target_os = "linux" and target_os = "android" while keeping the existing struct literal initialization intact.

@xtqqczze xtqqczze Oct 27, 2026

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It seems that Android does declare open_how in linux/openat2.h, so an issue should be opened in the libc crate to expose it on Android targets.

Also, open_how should probably be non-exhaustive, per openat2(2):

Any future extensions to openat2() will be implemented as new fields appended to the above structure, with a zero value in a new field resulting in the kernel behaving as though that extension field was not present. Therefore, the caller must zero-fill this structure on initialization.

Context: rust-lang/libc#2477

@xtqqczze xtqqczze Oct 27, 2026

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Issue opened: rust-lang/libc#5673.


syscall! {
fn openat2(
base_dirfd: c::c_int,
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4 changes: 2 additions & 2 deletions src/fs/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -46,7 +46,7 @@ mod ioctl;
mod makedev;
#[cfg(any(linux_kernel, target_os = "freebsd"))]
mod memfd_create;
#[cfg(linux_raw_dep)]
#[cfg(linux_kernel)]
mod openat2;
#[cfg(linux_kernel)]
mod raw_dir;
Expand Down Expand Up @@ -112,7 +112,7 @@ pub use ioctl::*;
pub use makedev::*;
#[cfg(any(linux_kernel, target_os = "freebsd"))]
pub use memfd_create::memfd_create;
#[cfg(linux_raw_dep)]
#[cfg(linux_kernel)]
pub use openat2::openat2;
#[cfg(linux_kernel)]
pub use raw_dir::{RawDir, RawDirEntry};
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2 changes: 1 addition & 1 deletion tests/fs/main.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -34,7 +34,7 @@ mod mknodat;
mod negative_timestamp;
#[cfg(linux_kernel)]
mod openat;
#[cfg(linux_raw_dep)]
#[cfg(linux_kernel)]
mod openat2;
#[cfg(not(target_os = "redox"))]
mod readdir;
Expand Down
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