.  But to prevent code in the future from making this mistake,
restrict Task::group_leader() so that it can only be called on current.

There are some other cases where accessing task->group_leader is okay.
For example it can be safe if you hold tasklist_lock or rcu_read_lock().
However, only supporting current->group_leader is sufficient for all
in-tree Rust users of group_leader right now.  Safe Rust functionality for
accessing it under rcu or while holding tasklist_lock may be added in the
future if required by any future Rust module.

This patch is a bugfix in that it prevents users of this API from writing
incorrect code.  It doesn't change behavior of correct code.

Link: https://lkml.kernel.org/r/20260107-task-group-leader-v2-1-8fbf816f2a2f@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Fixes: 313c4281bc9d ("rust: add basic `Task`")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Closes: https://lore.kernel.org/all/aTLnV-5jlgfk1aRK@redhat.com/
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Cc: Andreas Hindborg <a.hindborg@kernel.org>
Cc: Benno Lossin <lossin@kernel.org>
Cc: "Björn Roy Baron" <bjorn3_gh@protonmail.com>
Cc: Björn Roy Baron <bjorn3_gh@protonmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Panagiotis Foliadis <pfoliadis@posteo.net>
Cc: Shankari Anand <shankari.ak0208@gmail.com>
Cc: Trevor Gross <tmgross@umich.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 rust/kernel/task.rs | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
index 49fad6de06740..cc907fb531bce 100644
--- a/rust/kernel/task.rs
+++ b/rust/kernel/task.rs
@@ -204,18 +204,6 @@ pub fn as_ptr(&self) -> *mut bindings::task_struct {
         self.0.get()
     }
 
-    /// Returns the group leader of the given task.
-    pub fn group_leader(&self) -> &Task {
-        // SAFETY: The group leader of a task never changes after initialization, so reading this
-        // field is not a data race.
-        let ptr = unsafe { *ptr::addr_of!((*self.as_ptr()).group_leader) };
-
-        // SAFETY: The lifetime of the returned task reference is tied to the lifetime of `self`,
-        // and given that a task has a reference to its group leader, we know it must be valid for
-        // the lifetime of the returned task reference.
-        unsafe { &*ptr.cast() }
-    }
-
     /// Returns the PID of the given task.
     pub fn pid(&self) -> Pid {
         // SAFETY: The pid of a task never changes after initialization, so reading this field is
@@ -345,6 +333,18 @@ pub fn active_pid_ns(&self) -> Option<&PidNamespace> {
         // `release_task()` call.
         Some(unsafe { PidNamespace::from_ptr(active_ns) })
     }
+
+    /// Returns the group leader of the current task.
+    pub fn group_leader(&self) -> &Task {
+        // SAFETY: The group leader of a task never changes while the task is running, and `self`
+        // is the current task, which is guaranteed running.
+        let ptr = unsafe { (*self.as_ptr()).group_leader };
+
+        // SAFETY: `current->group_leader` stays valid for at least the duration in which `current`
+        // is running, and the signature of this function ensures that the returned `&Task` can
+        // only be used while `current` is still valid, thus still running.
+        unsafe { &*ptr.cast() }
+    }
 }
 
 // SAFETY: The type invariants guarantee that `Task` is always refcounted.
-- 
2.51.0[PATCH 6.18 276/641] rust: task: restrict Task::group_leader() to currentGreg Kroah-Hartman <gregkh@linuxfoundation.org> undefinedstable@vger.kernel.org undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined undefined³