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

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Date:   Fri, 25 Feb 2022 12:21:56 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
To:     "mcgrof@...nel.org" <mcgrof@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "cl@...ux.com" <cl@...ux.com>, "mbenes@...e.cz" <mbenes@...e.cz>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        "void@...ifault.com" <void@...ifault.com>,
        "atomlin@...mlin.com" <atomlin@...mlin.com>,
        "allen.lkml@...il.com" <allen.lkml@...il.com>,
        "joe@...ches.com" <joe@...ches.com>,
        "msuchanek@...e.de" <msuchanek@...e.de>,
        "oleksandr@...alenko.name" <oleksandr@...alenko.name>
Subject: Re: [PATCH v8 09/13] module: Move kallsyms support into a separate file

On Fri 2022-02-25 10:27 +0000, Aaron Tomlin wrote:
> 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.

Luis,

If I understand correctly, it might be cleaner to resolve the above in two
separate patches for a v9 i.e. a) address the sparse and lockdep feedback
and b) refactor the code, before the latest version [1] is merged into
module-next. I assume the previous iteration will be reverted first?

Please let me know your thoughts

[1]: https://lore.kernel.org/all/20220222141303.1392190-1-atomlin@redhat.com/


Kind regards,

-- 
Aaron Tomlin