rust: introduce Task::current

This allows Rust code to get a reference to the current task without
having to increment the refcount, but still guaranteeing memory safety.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

rust/helpers.c

@@ -100,6 +100,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
 }
 EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
 
+struct task_struct *rust_helper_get_current(void)
+{
+	return current;
+}
+EXPORT_SYMBOL_GPL(rust_helper_get_current);
+
 void rust_helper_get_task_struct(struct task_struct *t)
 {
 	get_task_struct(t);

rust/kernel/task.rs

@@ -5,14 +5,54 @@
 //! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
 
 use crate::bindings;
-use core::{cell::UnsafeCell, ptr};
+use core::{cell::UnsafeCell, marker::PhantomData, ops::Deref, ptr};
 
 /// Wraps the kernel's `struct task_struct`.
 ///
 /// # Invariants
 ///
 /// Instances of this type are always ref-counted, that is, a call to `get_task_struct` ensures
 /// that the allocation remains valid at least until the matching call to `put_task_struct`.
+///
+/// # Examples
+///
+/// The following is an example of getting the PID of the current thread with zero additional cost
+/// when compared to the C version:
+///
+/// ```
+/// use kernel::task::Task;
+///
+/// let pid = Task::current().pid();
+/// ```
+///
+/// Getting the PID of the current process, also zero additional cost:
+///
+/// ```
+/// use kernel::task::Task;
+///
+/// let pid = Task::current().group_leader().pid();
+/// ```
+///
+/// Getting the current task and storing it in some struct. The reference count is automatically
+/// incremented when creating `State` and decremented when it is dropped:
+///
+/// ```
+/// use kernel::{task::Task, ARef};
+///
+/// struct State {
+///     creator: ARef<Task>,
+///     index: u32,
+/// }
+///
+/// impl State {
+///     fn new() -> Self {
+///         Self {
+///             creator: Task::current().into(),
+///             index: 0,
+///         }
+///     }
+/// }
+/// ```
 #[repr(transparent)]
 pub struct Task(pub(crate) UnsafeCell<bindings::task_struct>);
 
@@ -25,6 +65,20 @@ unsafe impl Sync for Task {}
 type Pid = bindings::pid_t;
 
 impl Task {
+    /// Returns a task reference for the currently executing task/thread.
+    pub fn current<'a>() -> TaskRef<'a> {
+        // SAFETY: Just an FFI call with no additional safety requirements.
+        let ptr = unsafe { bindings::get_current() };
+
+        TaskRef {
+            // SAFETY: If the current thread is still running, the current task is valid. Given
+            // that `TaskRef` is not `Send`, we know it cannot be transferred to another thread
+            // (where it could potentially outlive the caller).
+            task: unsafe { &*ptr.cast() },
+            _not_send: PhantomData,
+        }
+    }
+
     /// Returns the group leader of the given task.
     pub fn group_leader(&self) -> &Task {
         // SAFETY: By the type invariant, we know that `self.0` is valid.
@@ -69,3 +123,30 @@ unsafe impl crate::types::AlwaysRefCounted for Task {
         unsafe { bindings::put_task_struct(obj.cast().as_ptr()) }
     }
 }
+
+/// A wrapper for a shared reference to [`Task`] that isn't [`Send`].
+///
+/// We make this explicitly not [`Send`] so that we can use it to represent the current thread
+/// without having to increment/decrement the task's reference count.
+///
+/// # Invariants
+///
+/// The wrapped [`Task`] remains valid for the lifetime of the object.
+pub struct TaskRef<'a> {
+    task: &'a Task,
+    _not_send: PhantomData<*mut ()>,
+}
+
+impl Deref for TaskRef<'_> {
+    type Target = Task;
+
+    fn deref(&self) -> &Self::Target {
+        self.task
+    }
+}
+
+impl From<TaskRef<'_>> for crate::types::ARef<Task> {
+    fn from(t: TaskRef<'_>) -> Self {
+        t.deref().into()
+    }
+}