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

[tylersaunders]

Hello, while moving some cpp code over to rust inside of AOSP, I encounted openat2 not being useable on android linux due to some configuration, and a minor issue with the open_how struct not being available outside of linux_raw_sys

rustix::fs::openat2 is currently gated on #[cfg(linux_raw_dep)] in src/fs/mod.rs, src/backend/libc/fs/syscalls.rs, and tests/fs/main.rs, making it unavailable on Linux and Android when building with --cfg=rustix_no_linux_raw (such as in AOSP, which builds rustix with the libc backend and without linux-raw-sys).

Background Context

In #1478 (fda7cbe6), --cfg=rustix_no_linux_raw was introduced so projects like AOSP and Mesa3D can build rustix on Linux and Android using the libc backend without depending on linux-raw-sys.

In that change, openat2 was switched from #[cfg(linux_kernel)] to #[cfg(linux_raw_dep)] in:

• src/fs/mod.rs (mod openat2; and pub use openat2::openat2;)
• src/backend/libc/fs/syscalls.rs (pub(crate) fn openat2)

However, all other openat2 support in the libc backend is already independent of linux-raw-sys and gated on #[cfg(linux_kernel)]:

• rustix::fs::ResolveFlags in src/backend/libc/fs/types.rs is gated on #[cfg(linux_kernel)] (and its rustdoc links to [openat2]: crate::fs::openat2, which is broken when not(linux_raw_dep)).
• use crate::fs::ResolveFlags and const SYS_OPENAT2 = 437 in src/backend/libc/fs/syscalls.rs are already gated on #[cfg(linux_kernel)].

The only dependency on linux-raw-sys inside src/backend/libc/fs/syscalls.rs::openat2 is use linux_raw_sys::general::open_how; for the 24-byte kernel ABI struct (struct open_how { flags: u64, mode: u64, resolve: u64 }). Note that libc::open_how cannot be used directly because it is not defined for target_os = "android" in libc and is marked #[non_exhaustive] on target_os = "linux".

Proposed Solution (see attached commit)

1. In src/backend/libc/fs/syscalls.rs::openat2, define #[repr(C)] struct open_how { flags: u64, mode: u64, resolve: u64 } when #[cfg(not(linux_raw_dep))] (retaining use linux_raw_sys::general::open_how; when #[cfg(linux_raw_dep)]).
2. Change the cfg gate on openat2 in src/fs/mod.rs, src/backend/libc/fs/syscalls.rs, and tests/fs/main.rs from #[cfg(linux_raw_dep)] back to #[cfg(linux_kernel)].

I've tested & verified the behavior of this patch in one of our cpp -> rust migrations inside of AOSP. Ideally I'd like to land this upstream rather than maintaining our own AOSP patch downstream.