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

On Thu Jan 29, 2026 at 8:41 AM GMT, jongan.kim wrote:
> 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) };

This `as c_int` part is not needed.

> +        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)) }
> +}

I would probably put this as assoc fn, so you write `PidNamespace::init_ns()`
instead of `kernel::pid_namespace::init_pid_ns()`.

Best,
Gary

On Thu Jan 29, 2026 at 2:33 PM GMT, Gary Guo wrote:
> On Thu Jan 29, 2026 at 8:41 AM GMT, jongan.kim wrote:
>> 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) };
>
> This `as c_int` part is not needed.
>
>> +        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)) }
>> +}
>
> I would probably put this as assoc fn, so you write `PidNamespace::init_ns()`
> instead of `kernel::pid_namespace::init_pid_ns()`.
>
> Best,
> Gary

Thanks for review and feedback.
I will update the patch to address both suggestions in the next revision.

Best Regards,
Heesu Kim

On Thu, Jan 29, 2026 at 05:41:18PM +0900, jongan.kim@lge.com wrote:
> 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>

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

On Thu, Jan 29, 2026 at 10:32:26AM +0000, Alice Ryhl wrote:
> On Thu, Jan 29, 2026 at 05:41:18PM +0900, jongan.kim@lge.com wrote:
>> 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>
>
> 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

Thanks for the detailed review and feedback.

I agree with your suggestions. While our immediate use case only requires
RCU-protected access, implementing Send, Sync, and AlwaysRefCounted makes
sense from the broader rust/kernel perspective, as these abstractions
will likely be used in various contexts across the kernel.

I will incorporate all suggestions in the next revision.

Best Regards,
Heesu Kim