linux-kernel - Re: [PATCH 8/9] module: Switch to kvfree

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Message-ID: <YafcZIKm9UJqs7c4@pc638.lan>
Date:   Wed, 1 Dec 2021 21:34:44 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Miroslav Benes <mbenes@...e.cz>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Miroslav Benes <mbenes@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Luis Chamberlain <mcgrof@...nel.org>, jeyu@...nel.org
Subject: Re: [PATCH 8/9] module: Switch to kvfree_rcu() API

On Wed, Dec 01, 2021 at 10:24:15AM +0100, Uladzislau Rezki wrote:
> On Tue, Nov 30, 2021 at 12:04:19PM +0100, Sebastian Andrzej Siewior wrote:
> > On 2021-11-30 11:39:09 [+0100], Miroslav Benes wrote:
> > > > --- a/kernel/module.c
> > > > +++ b/kernel/module.c
> > > > @@ -4150,8 +4150,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
> > > >   ddebug_cleanup:
> > > >  	ftrace_release_mod(mod);
> > > >  	dynamic_debug_remove(mod, info->debug);
> > > > -	synchronize_rcu();
> > > > -	kfree(mod->args);
> > > > +	kvfree_rcu(mod->args);
> > > >   free_arch_cleanup:
> > > >  	cfi_cleanup(mod);
> > > >  	module_arch_cleanup(mod);
> > > 
> > > hm, if I am not missing something, synchronize_rcu() is not really 
> > > connected to kfree(mod->args) there. synchronize_rcu() was added a long 
> > > time ago when kernel/module.c removed stop_machine() from the code and 
> > > replaced it with RCU to protect (at least?) mod->list. You can find 
> > > list_del_rcu(&mod->list) a couple of lines below.
> > 
> > so instead synchronize_rcu() + kfree() you could do
> > call_rcu(&mod->args->rcu, kfree()) but since you have no RCU-head around
> > in args you wait for the grace period and then invoke kfree.
> > 
> > kvfree_rcu() is somehow like call_rcu() + kfree() but without the needed
> > RCU-head. 
> > So you avoid waiting for the grace period but mod->args is freed later,
> > after as expected.
> > 
> > > And yes, one could ask how this all works. The error/cleanup sequence in 
> > > load_module() is a giant mess... well, load_module() is a mess too, but 
> > > the error path is really not nice.
> > 
> > Well, spring is coming :)
> > 
> Indeed that error path sequence is terrible. I will double check if that
> synchronize_rcu() + kfree() is related to any RCU protection and freeing.
> 
> If it is not i will drop this patch.
> 
> 
OK, that kfree has nothing to do with RCU protection:

<snip>
commit 6526c534b2677ca601b7b92851437feb041d02a1
Author: Rusty Russell <rusty@...tcorp.com.au>
Date:   Thu Aug 5 12:59:10 2010 -0600

    module: move module args strndup_user to just before use
    
    Instead of copying and allocating the args and storing it in
    load_info, we can just allocate them right before we need them.
    
    Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
<snip>

so, i will drop my patch, since the intention is not to free a ptr
after a grace period.

Thank you for the review!

--
Vlad Rezki