Date: Thu, 11 Dec 2014 12:17:33 +0100
From: Petr Mladek <pmladek@suse.cz>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Seth Jennings <sjenning@redhat.com>, Jiri Kosina <jkosina@suse.cz>,
        Vojtech Pavlik <vojtech@suse.cz>, Steven Rostedt <rostedt@goodmis.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Christoph Hellwig <hch@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@amacapital.net>, live-patching@vger.kernel.org,
        x86@kernel.org, kpatch@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/6] livepatch v5: avoid race when checking for state of
 the patch
Message-ID: <20141211111733.GA16567@dhcp128.suse.cz>
References: <1418148307-21434-1-git-send-email-pmladek@suse.cz>
 <1418148307-21434-2-git-send-email-pmladek@suse.cz>
 <20141209183249.GA25239@treble.redhat.com>
 <20141210101147.GB2454@pathway.suse.cz>
 <20141210152506.GA32670@treble.redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org

On Wed 2014-12-10 09:25:06, Josh Poimboeuf wrote:
> On Wed, Dec 10, 2014 at 11:11:47AM +0100, Petr Mladek wrote:
> > On Tue 2014-12-09 12:32:49, Josh Poimboeuf wrote:
> > > On Tue, Dec 09, 2014 at 07:05:02PM +0100, Petr Mladek wrote:
> > > > klp_patch_enable() and klp_patch_disable() should check the current state
> > > > of the patch under the klp_lock. Otherwise, it might detect that the operation
> > > > is valid but the situation might change before it takes the lock.
> > > 
> > > Hi Petr,
> > > 
> > > Thanks for the patches.
> > > 
> > > I don't think this patch is necessary.  klp_is_enabled() doesn't check
> > > the state of the patch.  It checks the initialization state of the core
> > > module (klp_root_kobj), which can only be set in klp_init().  It's not
> > > protected by the lock, so I don't see the point of this patch.
> > 
> > Ah, I have misread the name and expected that it checked whether
> > the patch was enabled or disabled. The original code is OK then.
> > 
> > Well, Jiri Kosina pointed out that the check did not make much sense.
> > klp_is_enabled() could not be called if the livepatch module is not
> > loaded. And the later check for klp_patch_is_registered() is enough
> > to check whether the klp_enable_patch()/klp_disable_patch() calls
> > are allowed or not.
> 
> But livepatch isn't a module, it's part of the kernel.

Ah, I remembered that module_init(klp_init) and created a wrong mental link ;-)

> Even if the init
> function returns an error, that doesn't prevent any of the other
> exported functions from getting called.

Well, it still will be covered by that later klp_patch_is_registered()
check. But I am find with leaving it as is.

Best Regards,
Petr
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