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

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Date:   Fri, 25 Feb 2022 10:27:18 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "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 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