Re: [PATCH v8 09/13] module: Move kallsyms support into a separate file

Aaron Tomlin
2022-02-25T10:27:40Z

On Fri 2022-02-25 11:15 +0100, Petr Mladek wrote:
> rcu_dereference_sched() makes sparse happy. But lockdep complains
> because the _rcu pointer is not accessed under:
>
> rcu_read_lock_sched();
> rcu_read_unlock_sched();

Hi Petr,

>
> This is not the case here. Note that module_mutex does not
> disable preemtion.
>
> Now, the code is safe. The RCU access makes sure that "mod"
> can't be freed in the meantime:
>
> + add_kallsyms() is called by the module loaded when the module
> is being loaded. It could not get removed in parallel
> by definition.
>
> + module_kallsyms_on_each_symbol() takes module_mutex.
> It means that the module could not get removed.

Indeed, which is why I did not use rcu_read_lock_sched() and
rcu_read_unlock_sched() with rcu_dereference_sched(). That being said, I
should have mentioned this in the commit message.

> IMHO, we have two possibilities here:
>
> + Make sparse and lockdep happy by using rcu_dereference_sched()
> and calling the code under rcu_read_lock_sched().
>
> + Cast (struct mod_kallsyms *)mod->kallsyms when accessing
> the value.

I prefer the first option.

> I do not have strong preference. I am fine with both.
>
> Anyway, such a fix should be done in a separate patch!

Agreed.

Kind regards,

--
Aaron Tomlin