[PATCH v3 10/13] rust: introduce `current`

[Wedson Almeida Filho <wedsonaf@gmail.com>]

From: Wedson Almeida Filho <walmeida@microsoft.com>

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>
---
v1 -> v2: Make `current` a macro to prevent it from escaping the caller
v2 -> v3:
- Mention `current` macro in `Task::current`
- Hide implementation of `TaskRef` inside `Task::current`

 rust/helpers.c         |  6 +++
 rust/kernel/prelude.rs |  2 +
 rust/kernel/task.rs    | 88 +++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/rust/helpers.c b/rust/helpers.c
index 58a194042c86..96441744030e 100644
--- a/rust/helpers.c
+++ b/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);
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index fcdc511d2ce8..c28587d68ebc 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -36,3 +36,5 @@ pub use super::error::{code::*, Error, Result};
 pub use super::{str::CStr, ThisModule};
 
 pub use super::init::{InPlaceInit, Init, PinInit};
+
+pub use super::current;
diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index d70cad131956..5269a562cb1b 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -5,7 +5,17 @@
 //! C header: [`include/linux/sched.h`](../../../../include/linux/sched.h).
 
 use crate::{bindings, types::Opaque};
-use core::ptr;
+use core::{marker::PhantomData, ops::Deref, ptr};
+
+/// Returns the currently running task.
+#[macro_export]
+macro_rules! current {
+    () => {
+        // SAFETY: Deref + addr-of below create a temporary `TaskRef` that cannot outlive the
+        // caller.
+        unsafe { &*$crate::task::Task::current() }
+    };
+}
 
 /// Wraps the kernel's `struct task_struct`.
 ///
@@ -15,6 +25,42 @@ use core::ptr;
 ///
 /// 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:
+///
+/// ```
+/// let pid = current!().pid();
+/// ```
+///
+/// Getting the PID of the current process, also zero additional cost:
+///
+/// ```
+/// let pid = 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, types::ARef};
+///
+/// struct State {
+///     creator: ARef<Task>,
+///     index: u32,
+/// }
+///
+/// impl State {
+///     fn new() -> Self {
+///         Self {
+///             creator: current!().into(),
+///             index: 0,
+///         }
+///     }
+/// }
+/// ```
 #[repr(transparent)]
 pub struct Task(pub(crate) Opaque<bindings::task_struct>);
 
@@ -27,6 +73,46 @@ unsafe impl Sync for Task {}
 type Pid = bindings::pid_t;
 
 impl Task {
+    /// Returns a task reference for the currently executing task/thread.
+    ///
+    /// The recommended way to get the current task/thread is to use the
+    /// [`current`](crate::current) macro because it is safe.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that the returned object doesn't outlive the current task/thread.
+    pub unsafe fn current() -> impl Deref<Target = Task> {
+        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()
+            }
+        }
+
+        // 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 a valid task. Valid tasks always
-- 
2.34.1