linux-kernel - Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module removal

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Date:   Thu, 5 Sep 2019 08:08:32 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     jikos@...nel.org, Joe Lawrence <joe.lawrence@...hat.com>,
        Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org,
        live-patching@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] livepatch: Clear relocation targets on a module
 removal

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.

> > >       + 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