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

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Date:	Thu, 28 Apr 2016 16:18:41 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Miroslav Benes <mbenes@...e.cz>, 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, 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. 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.

-- 
Josh