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Date:   Thu, 16 Apr 2020 07:20:51 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Miroslav Benes <mbenes@suse.cz>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jessica Yu <jeyu@kernel.org>
Subject: Re: [PATCH 0/7] livepatch,module: Remove .klp.arch and
 module_disable_ro()
Message-ID: <20200416122051.p3dk5i7h6ty4cwuc@treble>
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 <20200414190814.glra2gceqgy34iyx@treble>
 <alpine.LSU.2.21.2004161136340.10475@pobox.suse.cz>
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On Thu, Apr 16, 2020 at 11:45:05AM +0200, Miroslav Benes wrote:
> On Tue, 14 Apr 2020, Josh Poimboeuf wrote:
> 
> > On Tue, Apr 14, 2020 at 08:27:26PM +0200, Peter Zijlstra wrote:
> > > On Tue, Apr 14, 2020 at 11:28:36AM -0500, Josh Poimboeuf wrote:
> > > > Better late than never, these patches add simplifications and
> > > > improvements for some issues Peter found six months ago, as part of his
> > > > non-writable text code (W^X) cleanups.
> > > 
> > > Excellent stuff, thanks!!
> > >
> > > I'll go brush up these two patches then:
> > > 
> > >   https://lkml.kernel.org/r/20191018074634.801435443@infradead.org
> > >   https://lkml.kernel.org/r/20191018074634.858645375@infradead.org
> > 
> > Ah right, I meant to bring that up.  I actually played around with those
> > patches.  While it would be nice to figure out a way to converge the
> > ftrace module init, I didn't really like the first patch.
> > 
> > It bothers me that both the notifiers and the module init() both see the
> > same MODULE_STATE_COMING state, but only in the former case is the text
> > writable.
> > 
> > I think it's cognitively simpler if MODULE_STATE_COMING always means the
> > same thing, like the comments imply, "fully formed" and thus
> > not-writable:
> > 
> > enum module_state {
> > 	MODULE_STATE_LIVE,	/* Normal state. */
> > 	MODULE_STATE_COMING,	/* Full formed, running module_init. */
> > 	MODULE_STATE_GOING,	/* Going away. */
> > 	MODULE_STATE_UNFORMED,	/* Still setting it up. */
> > };
> > 
> > And, it keeps tighter constraints on what a notifier can do, which is a
> > good thing if we can get away with it.
> 
> Agreed.
> 
> On the other hand, the first patch would remove the tiny race window when 
> a module state is still UNFORMED, but the protections are (being) set up. 
> Patches 4/7 and 5/7 allow to use memcpy in that case, because it is early. 
> But it is in fact not already. I haven't checked yet if it really matters 
> somewhere (a race with livepatch running klp_module_coming while another 
> module is being loaded or anything like that).

Maybe I'm missing your point, but I don't see any races here.

apply_relocate_add() only writes to the patch module's text, so there
can't be races with other modules.

-- 
Josh