Re: [PATCH] rust: task: restrict Task::group_leader() to current

[Gary Guo <gary@garyguo.net>]

On Thu, 18 Dec 2025 09:41:00 +0000
Alice Ryhl <aliceryhl@google.com> wrote:

> The Task::group_leader() method currently allows you to access the
> group_leader() of any task, for example one you hold a refcount to. But
> this is not safe in general since the group leader could change when a
> task exits. See for example commit a15f37a40145c ("kernel/sys.c: fix the
> racy usage of task_lock(tsk->group_leader) in sys_prlimit64() paths").
> 
> All existing users of Task::group_leader() call this method on current,
> which is guaranteed running, so there's not an actual issue in Rust code
> today. 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.
> 
> Reported-by: Oleg Nesterov <oleg@redhat.com>
> Closes: https://lore.kernel.org/all/aTLnV-5jlgfk1aRK@redhat.com/
> Fixes: 313c4281bc9d ("rust: add basic `Task`")
> Cc: stable@vger.kernel.org
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/task.rs | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)