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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Petr Mladek <pmladek@suse.com>
Cc: jikos@kernel.org, Joe Lawrence <joe.lawrence@redhat.com>,
	Miroslav Benes <mbenes@suse.cz>,
	linux-kernel@vger.kernel.org, live-patching@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal
Date: Thu, 5 Sep 2019 08:15:02 -0500
Message-ID: <20190905131502.mgiaplb3grlxsahp@treble> (raw)
In-Reply-To: <20190905130832.dznviqrrg6lfrxvx@treble>

On Thu, Sep 05, 2019 at 08:08:32AM -0500, Josh Poimboeuf wrote:
> On Thu, Sep 05, 2019 at 01:09:55PM +0200, Petr Mladek wrote:
> > > I don't have a number, but it's very common to patch a function which
> > > uses jump labels or alternatives.
> > 
> > Really? My impression is that both alternatives and jump_labels
> > are used in hot paths. I would expect them mostly in core code
> > that is always loaded.
> > 
> > Alternatives are often used in assembly that we are not able
> > to livepatch anyway.
> > 
> > Or are they spread widely via some macros or inlined functions?
> 
> Jump labels are used everywhere.  Looking at vmlinux.o in my kernel:
> 
>   Relocation section [19621] '.rela__jump_table' for section [19620] '__jump_table' at offset 0x197873c8 contains 11913 entries:
> 
> Each jump label entry has 3 entries, so 11913/3 = 3971 jump labels.
> 
> $ readelf -s vmlinux.o |grep FUNC |wc -l
> 46902
> 
> 3971/46902 = ~8.5%
> 
> ~8.5% of functions use jump labels.

Obviously some functions may use more than one jump label so this isn't
exactly bulletproof math.  But it gives a rough idea of how widespread
they are.

> 
> > > >       + How often new problematic features appear?
> > > 
> > > I'm not exactly sure what you mean, but it seems that anytime we add a
> > > new feature, we have to try to wrap our heads around how it interacts
> > > with the weirdness of late module patching.
> > 
> > I agree that we need to think about it and it makes complications.
> > Anyway, I think that these are never the biggest problems.
> > 
> > I would be more concerned about arch-specific features that might need
> > special handling in the livepatch code. Everyone talks only about
> > alternatives and jump_labels that were added long time ago.
> 
> Jump labels have been around for many years, but we somehow missed
> implementing klp.arch for them.  As I said this resulted in panics.
> 
> There may be other similar cases lurking, both in x86 and other arches.
> It's not a comforting thought!
> 
> And each case requires special klp code in addition to the real code.
> 
> -- 
> Josh

-- 
Josh

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-19 12:28 [RFC PATCH 0/2] " Miroslav Benes
2019-07-19 12:28 ` [PATCH 1/2] livepatch: Nullify obj->mod in klp_module_coming()'s error path Miroslav Benes
2019-07-28 19:45   ` Josh Poimboeuf
2019-08-19 11:26     ` Petr Mladek
2019-07-19 12:28 ` [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal Miroslav Benes
2019-07-22  9:33   ` Petr Mladek
2019-08-14 12:33     ` Miroslav Benes
2019-07-28 20:04   ` Josh Poimboeuf
2019-08-14 11:06     ` Miroslav Benes
2019-08-14 15:12       ` Josh Poimboeuf
2019-08-16  9:46         ` Petr Mladek
2019-08-22 22:36           ` Josh Poimboeuf
2019-08-23  8:13             ` Petr Mladek
2019-08-26 14:54               ` Josh Poimboeuf
2019-08-27 15:05                 ` Joe Lawrence
2019-08-27 15:37                   ` Josh Poimboeuf
2019-09-02 16:13                 ` Miroslav Benes
2019-09-02 17:05                   ` Joe Lawrence
2019-09-03 13:02                     ` Miroslav Benes
2019-09-04  8:49                       ` Petr Mladek
2019-09-04 16:26                         ` Joe Lawrence
2019-09-05  2:50                         ` Josh Poimboeuf
2019-09-05 11:09                           ` Petr Mladek
2019-09-05 11:19                             ` Jiri Kosina
2019-09-05 13:23                               ` Josh Poimboeuf
2019-09-05 13:31                                 ` Jiri Kosina
2019-09-05 13:42                                   ` Josh Poimboeuf
2019-09-05 11:39                             ` Joe Lawrence
2019-09-05 13:08                             ` Josh Poimboeuf
2019-09-05 13:15                               ` Josh Poimboeuf [this message]
2019-09-05 13:52                                 ` Petr Mladek
2019-09-05 14:28                                   ` Josh Poimboeuf
2019-09-05 12:03                           ` Miroslav Benes
2019-09-05 12:35                             ` Josh Poimboeuf
2019-09-05 12:49                               ` Miroslav Benes
2019-09-05 11:52                         ` Miroslav Benes
2019-09-05  2:32                       ` Josh Poimboeuf
2019-09-05 12:16                         ` Miroslav Benes
2019-09-05 12:54                           ` Josh Poimboeuf
2019-09-06 12:51                             ` Miroslav Benes
2019-09-06 15:38                               ` Joe Lawrence
2019-09-06 16:45                               ` Josh Poimboeuf
2019-08-26 13:44         ` Nicolai Stange
2019-08-26 15:02           ` Josh Poimboeuf

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