Re: [PATCH v2 28/28] cfi: Use RCU while invoking __module_address().

[Sami Tolvanen]

Hi Elliot,

On Mon, Dec 30, 2024 at 7:33 PM Elliot Berman
<elliot.berman@xxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Dec 20, 2024 at 06:41:42PM +0100, Sebastian Andrzej Siewior wrote:
> > __module_address() can be invoked within a RCU section, there is no
> > requirement to have preemption disabled.
> >
> > I'm not sure if using rcu_read_lock() will introduce the regression that
> > has been fixed in commit 14c4c8e41511a ("cfi: Use
> > rcu_read_{un}lock_sched_notrace").
> >
>
> You can replace the rcu_read_lock_sched_notrace() with guard(rcu)().
> Regular rcu lock doesn't generate function traces, so the recursive loop
> isn't possible.
>
> I've tested:
>  - the current kernel (no recursive loop)
>  - Revert back to rcu_read_lock_sched() (fails)

Which kernel version did you test? I assume something pre-KCFI as
arm64 doesn't use this code since v6.1.

>  - Your series as-is (no recurisve loop)

Note that this patch only adds a comment to is_module_cfi_trap(), so I
wouldn't expect a functional change.

Sami