Re: [PATCH v2 2/3] rust: pid: add Pid abstraction and init_pid_ns helper

[Alice Ryhl]

On Thu, Jan 29, 2026 at 05:41:18PM +0900, jongan.kim@xxxxxxx wrote:
> From: HeeSu Kim <heesu0025.kim@xxxxxxx>
> 
> Add a new Pid abstraction in rust/kernel/pid.rs that wraps the
> kernel's struct pid and provides safe Rust interfaces for:
> - find_vpid_with_guard: Find a pid by number under RCU protection
> - pid_task_with_guard: Get the task associated with a pid under RCU
>   protection
> 
> Also add init_pid_ns() helper function to pid_namespace.rs to get
> a reference to the init PID namespace.
> 
> These abstractions use lifetime-bounded references tied to RCU guards
> to ensure memory safety when accessing RCU-protected data structures.
> 
> Signed-off-by: HeeSu Kim <heesu0025.kim@xxxxxxx>

This looks really nice, thanks!

> +//! Process identifiers (PIDs).
> +//!
> +//! C header: [`include/linux/pid.h`](srctree/include/linux/pid.h)
> +
> +use crate::{bindings, ffi::c_int, sync::rcu, task::Task, types::Opaque};

Currently we use this formatting for imports:

use crate::{
    bindings,
    ffi::c_int,
    sync::rcu,
    task::Task,
    types::Opaque, //
};

> +/// Wraps the kernel's `struct pid`.
> +///
> +/// This structure represents the Rust abstraction for a C `struct pid`.
> +/// A `Pid` represents a process identifier that can be looked up in different
> +/// PID namespaces.
> +#[repr(transparent)]
> +pub struct Pid {
> +    inner: Opaque<bindings::pid>,
> +}

I would implement Send, Sync, and AlwaysRefCounted for Pid too.

> +            // SAFETY: `find_vpid` returns a valid pointer under RCU protection,
> +            // and `Pid` is `#[repr(transparent)]` over `bindings::pid`.
> +            Some(unsafe { &*(ptr as *const Self) })

It would be nice to extract this cast into a Pid::from_raw().

> +            // SAFETY: `pid_task` returns a valid pointer under RCU protection,
> +            // and `Task` is `#[repr(transparent)]` over `bindings::task_struct`.
> +            Some(unsafe { &*task_ptr.cast() })

I think it would be nice to add a Task::from_raw() to avoid the cast
here.

	Some(unsafe { Task::from_raw(task_ptr) })

> +    pub fn pid_task_with_guard<'a>(&'a self, _rcu_guard: &'a rcu::Guard) -> Option<&'a Task> {
> +    pub fn find_vpid_with_guard<'a>(nr: i32, _rcu_guard: &'a rcu::Guard) -> Option<&'a Self> {

I think we can drop the 'with_guard' suffixes of these.

> +/// Returns a reference to the init PID namespace.
> +///
> +/// This is the root PID namespace that exists throughout the lifetime of the kernel.
> +#[inline]
> +pub fn init_pid_ns() -> &'static PidNamespace {
> +    // SAFETY: `init_pid_ns` is a global static that is valid for the lifetime of the kernel.
> +    unsafe { PidNamespace::from_ptr(core::ptr::addr_of!(bindings::init_pid_ns)) }

Simplifies to:

	PidNamespace::from_ptr(&raw const bindings::init_pid_ns)

Alice