[PATCH] rust: cred: add safe abstractions for capable() and ns_capable()

[Alice Ryhl]

On Thu, May 07, 2026 at 02:19:13AM +0530, Arnav Sharma wrote:
> The capable() function is the primary privilege gate in the Linux kernel,
> used to check if the current task possesses a specific POSIX capability.
> While bindings for capable() and ns_capable() exist, there are currently
> no safe Rust abstractions for them.
> 
> Introduce safe Rust wrappers for capable() and ns_capable() in the
> kernel::cred module. These functions validate that the requested
> capability is within the valid [0, CAP_LAST_CAP] bounds before calling
> into the C side, ensuring that safe Rust code cannot inadvertently
> trigger a kernel BUG() on invalid inputs.
> 
> The abstractions take a `u32` parameter to ergonomically match the
> generated `bindings::CAP_*` constants without requiring explicit caller
> casts.
> 
> Signed-off-by: Arnav Sharma <arnav4324 at gmail.com>

I have the same question about what the use-case for this is.

> +/// # Safety
> +///
> +/// The caller must ensure that:
> +///
> +/// - `ns` is a non-null pointer to a fully initialized `struct user_namespace`.
> +/// - The `user_namespace` pointed to by `ns` remains valid and is not freed for
> +///   the duration of this call.
> +#[inline]
> +pub unsafe fn ns_capable(ns: *mut bindings::user_namespace, cap: u32) -> bool {

I would add a UserNamespace struct so that this raw pointer could be
avoided, before I add this method.

Alice