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