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

[jongan.kim@lge.com]

From: HeeSu Kim <heesu0025.kim@lge.com>

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@lge.com>
---
 rust/kernel/lib.rs           |   1 +
 rust/kernel/pid.rs           | 100 +++++++++++++++++++++++++++++++++++
 rust/kernel/pid_namespace.rs |   9 ++++
 3 files changed, 110 insertions(+)
 create mode 100644 rust/kernel/pid.rs

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f812cf120042..60a518d65d0e 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -122,6 +122,7 @@
 pub mod page;
 #[cfg(CONFIG_PCI)]
 pub mod pci;
+pub mod pid;
 pub mod pid_namespace;
 pub mod platform;
 pub mod prelude;
diff --git a/rust/kernel/pid.rs b/rust/kernel/pid.rs
new file mode 100644
index 000000000000..00989b953c23
--- /dev/null
+++ b/rust/kernel/pid.rs
@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! 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};
+
+/// 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>,
+}
+
+impl Pid {
+    /// Returns a raw pointer to the inner C struct.
+    #[inline]
+    pub fn as_ptr(&self) -> *mut bindings::pid {
+        self.inner.get()
+    }
+
+    /// Finds a `struct pid` by its pid number within the current task's PID namespace.
+    ///
+    /// Returns `None` if no such pid exists.
+    ///
+    /// The returned reference is only valid for the duration of the RCU read-side
+    /// critical section represented by the `rcu::Guard`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::pid::Pid;
+    /// use kernel::sync::rcu;
+    ///
+    /// let guard = rcu::read_lock();
+    /// if let Some(pid) = Pid::find_vpid_with_guard(1, &guard) {
+    ///     pr_info!("Found pid 1\n");
+    /// }
+    /// ```
+    ///
+    /// Returns `None` for non-existent PIDs:
+    ///
+    /// ```
+    /// use kernel::pid::Pid;
+    /// use kernel::sync::rcu;
+    ///
+    /// let guard = rcu::read_lock();
+    /// // PID 0 (swapper/idle) is not visible via find_vpid.
+    /// assert!(Pid::find_vpid_with_guard(0, &guard).is_none());
+    /// ```
+    #[inline]
+    pub fn find_vpid_with_guard<'a>(nr: i32, _rcu_guard: &'a rcu::Guard) -> Option<&'a Self> {
+        // SAFETY: Called under RCU protection as guaranteed by the Guard reference.
+        let ptr = unsafe { bindings::find_vpid(nr as c_int) };
+        if ptr.is_null() {
+            None
+        } else {
+            // SAFETY: `find_vpid` returns a valid pointer under RCU protection,
+            // and `Pid` is `#[repr(transparent)]` over `bindings::pid`.
+            Some(unsafe { &*(ptr as *const Self) })
+        }
+    }
+
+    /// Gets the task associated with this PID.
+    ///
+    /// Returns `None` if no task is associated with this PID.
+    ///
+    /// The returned reference is only valid for the duration of the RCU read-side
+    /// critical section represented by the `rcu::Guard`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use kernel::pid::Pid;
+    /// use kernel::sync::rcu;
+    ///
+    /// let guard = rcu::read_lock();
+    /// if let Some(pid) = Pid::find_vpid_with_guard(1, &guard) {
+    ///     if let Some(task) = pid.pid_task_with_guard(&guard) {
+    ///         pr_info!("Found task for pid 1\n");
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    pub fn pid_task_with_guard<'a>(&'a self, _rcu_guard: &'a rcu::Guard) -> Option<&'a Task> {
+        // SAFETY: Called under RCU protection as guaranteed by the Guard reference.
+        let task_ptr = unsafe { bindings::pid_task(self.as_ptr(), bindings::pid_type_PIDTYPE_PID) };
+        if task_ptr.is_null() {
+            None
+        } else {
+            // SAFETY: `pid_task` returns a valid pointer under RCU protection,
+            // and `Task` is `#[repr(transparent)]` over `bindings::task_struct`.
+            Some(unsafe { &*task_ptr.cast() })
+        }
+    }
+}
diff --git a/rust/kernel/pid_namespace.rs b/rust/kernel/pid_namespace.rs
index 979a9718f153..6029e3e120d0 100644
--- a/rust/kernel/pid_namespace.rs
+++ b/rust/kernel/pid_namespace.rs
@@ -63,3 +63,12 @@ unsafe impl Send for PidNamespace {}
 // SAFETY: It's OK to access `PidNamespace` through shared references from other threads because
 // we're either accessing properties that don't change or that are properly synchronised by C code.
 unsafe impl Sync for PidNamespace {}
+
+/// 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)) }
+}
-- 
2.25.1