linux-kernel - Re: klp: make object/func-walking helpers more robust

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Message-ID: <alpine.LNX.2.00.1604290943001.10299@pobox.suse.cz>
Date:	Fri, 29 Apr 2016 09:48:37 +0200 (CEST)
From:	Miroslav Benes <mbenes@...e.cz>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
cc:	Jessica Yu <jeyu@...hat.com>, jikos@...nel.org, pmladek@...e.com,
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: klp: make object/func-walking helpers more robust

On Thu, 28 Apr 2016, Josh Poimboeuf wrote:

> On Thu, Apr 28, 2016 at 02:21:31PM -0400, Jessica Yu wrote:
> > +++ Miroslav Benes [28/04/16 16:34 +0200]:
> > > Current object-walking helper checks the presence of obj->funcs to
> > > determine the end of objs array in klp_object structure. This is
> > > somewhat fragile because one can easily forget about funcs definition
> > > during livepatch creation. In such a case the livepatch module is
> > > successfully loaded and all objects after the incorrect one are omitted.
> > > This is very confusing. Let's make the helper more robust and check also
> > > for the other external member, name. Thus the helper correctly stops on
> > > an empty item of the array. We need to have a check for obj->funcs in
> > > klp_init_object() to make it work.
> > > 
> > > The same applies to a func-walking helper.
> > > 
> > > As a benefit we'll check for new_func member definition during the
> > > livepatch initialization. There is no such check anywhere in the code
> > > now.
> > > 
> > > Signed-off-by: Miroslav Benes <mbenes@...e.cz>
> > > ---
> > > include/linux/livepatch.h | 6 ++++--
> > > kernel/livepatch/core.c   | 3 +++
> > > 2 files changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
> > > index 0933ca47791c..a93a0b23dc8d 100644
> > > --- a/include/linux/livepatch.h
> > > +++ b/include/linux/livepatch.h
> > > @@ -104,10 +104,12 @@ struct klp_patch {
> > > };
> > > 
> > > #define klp_for_each_object(patch, obj) \
> > > -	for (obj = patch->objs; obj->funcs; obj++)
> > > +	for (obj = patch->objs; obj->funcs || obj->name; obj++)
> > 
> > Remember that for patches to vmlinux, obj->name and obj->mod will also
> > both be NULL. So if someone happens to forget to fill in obj->funcs
> > for a vmlinux patch, we won't catch that case here.

Yes, that is true. My reasoning is that if someone even accidently writes 
{ } somewhere in the middle of the array, there is nothing we can do to 
help :). I consider it improbable whereas an omission of one field is 
possible.

> > Perhaps we need a
> > better way of determining whether we've reached the end of the array,
> > or determining that the struct is truly empty..
> 
> That would be nice, but I'm not sure how we could do that.  I suppose we
> could add a patch->nr_objs field.  But that might arguably be even
> easier for the user to mess up.

Yeah, that is perhaps the only way (ARRAY_SIZE won't work here) besides 
introducing some special mark. I think this is not worth it. I agree it is 
even more error-prone.

The idea behind this patch is that there is at least something we can do 
to help without imposing much on the user.

Miroslav