Re: [PATCH v2] livepatch: old_name.number scheme in livepatch sysfs directory

From: Chris J Arges
Date: Thu Nov 05 2015 - 11:07:48 EST


On Thu, Nov 05, 2015 at 09:56:56AM -0600, Josh Poimboeuf wrote:
> On Thu, Nov 05, 2015 at 04:18:12PM +0100, Miroslav Benes wrote:
> > On Wed, 4 Nov 2015, Josh Poimboeuf wrote:
> >
> > > On Wed, Nov 04, 2015 at 10:52:52AM +0100, Miroslav Benes wrote:
> > > > On Tue, 3 Nov 2015, Josh Poimboeuf wrote:
> > > > > > Object entry would be empty for not loaded object. I would not
> > > > > > dare to propose to remove such object entries. It would make things worse.
> > > > >
> > > > > Why would removing an empty object entry make things worse?
> > > >
> > > > I think it all comes down to a question whether the sysfs entries say what
> > > > a patch is capable to patch or what this patch is currently patching in
> > > > the system. I am inclined to the former so the removal would make me
> > > > nervous. But I am not against the second approach. We are still in testing
> > > > mode as far as sysfs is concerned so we can try even harsh changes and see
> > > > how it's gonna go.
> > >
> > > I see your point. This approach only describes what is patched now, but
> > > it doesn't describe what *will* be patched. Ideally we could find a way
> > > to describe both.
> > >
> > > Speaking of harsh changes, here's an idea.
> >
> > Which is the very same you proposed last year when I tried to persuade you
> > to get rid off old_addr and stuff. I called it crazy I remember :D. So
> > here we are again...
>
> Ah, I knew I had entertained the idea before, but I forgot we discussed
> it. It is indeed a little crazy. But this is live patching after all
> ;-)
>
> > > What if we require the patch author to supply the value of 'n' instead
> > > of supplying the symbol address? We could get rid of 'old_addr' as an
> > > input in klp_func and and replace it with 'old_sympos' which has the
> > > value of 'n'. Or alternatively we could require old_name to be of the
> > > format "func,n".
> > >
> > > That would uniquely identify each patched function, even _before_ the
> > > object is loaded.
> >
> > I find it reasonable and we should try it. I think that old_sympos should
> > have this semantics
> >
> > 0 - default, preserve more or less current behaviour. If the symbol is
> > unique there is no problem. If it is not the patching would fail.
> > 1, 2, ... - occurrence of the symbol in kallsyms.
> >
> > The advantage is that if the user does not care and is certain that the
> > symbol is unique he doesn't have to do anything. If the symbol is not
> > unique he still has means how to solve it.
> >
> > Does it make sense?
>
> Sounds good!
>
> > > It would also fix another big problem we have today, where there's no
> > > way to disambiguate duplicate symbols in modules, for both function
> > > addresses and for relocs.
> >
> > True.
> >
> > > It would simplify the code in other places as well: no special handling
> > > for kASLR, no need for klp_verify_vmlinux_symbol() vs
> > > klp_find_object_symbol().
> >
> > Which would be great.
> >
> > > A drawback is that it requires the patch author to do a little more due
> > > diligence when filling out klp_func. But we already require them to be
> > > careful.
> >
> > Yes, I don't think this should be a problem.
> >
> > > Thoughts?
> >
> > Yup, we should try it. I suppose that the order of the symbols in kallsyms
> > table is stable for once-built kernel. It is the order of the symbols in
> > the object files, isn't it? And since each livepatch module is built
> > against the specific kernel there should be no issues with this.
>
> The order of the symbols in an object's symbol table does appear to be
> the same as the order in kallsyms (per-object). So yeah, let's try it.
>
> --
> Josh
>

Great! Using this feedback to create the next patch. I'll post something in the
next few days.

--chris

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