Re: [PATCH 6/9] livepatch/ftrace: Add recursion protection to the ftrace callback

Miroslav Benes
2020-10-29T15:03:15Z

On Thu, 29 Oct 2020, Petr Mladek wrote:

> On Thu 2020-10-29 14:51:06, Miroslav Benes wrote:
> > On Wed, 28 Oct 2020, Steven Rostedt wrote:
>
> > Hm, I've always thought that we did not need any kind of recursion
> > protection for our callback. It is marked as notrace and it does not call
> > anything traceable. In fact, it does not call anything. I even have a note
> > in my todo list to mark the callback as RECURSION_SAFE :)
>
> Well, it calls WARN_ON_ONCE() ;-)

Oh my, I learned to ignore these. Of course there is printk hidden
everywhere.

> > At the same time, it probably does not hurt and the patch is still better
> > than what we have now without RECURSION_SAFE if I understand the patch set
> > correctly.
>
> And better be on the safe side.

Agreed.

> > > Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> > > Cc: Jiri Kosina <jikos@kernel.org>
> > > Cc: Miroslav Benes <mbenes@suse.cz>
> > > Cc: Petr Mladek <pmladek@suse.com>
> > > Cc: Joe Lawrence <joe.lawrence@redhat.com>
> > > Cc: live-patching@vger.kernel.org
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> > > ---
> > > kernel/livepatch/patch.c | 5 +++++
> > > 1 file changed, 5 insertions(+)
> > >
> > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
> > > index b552cf2d85f8..6c0164d24bbd 100644
> > > --- a/kernel/livepatch/patch.c
> > > +++ b/kernel/livepatch/patch.c
> > > @@ -45,9 +45,13 @@ static void notrace klp_ftrace_handler(unsigned long ip,
> > > struct klp_ops *ops;
> > > struct klp_func *func;
> > > int patch_state;
> > > + int bit;
> > >
> > > ops = container_of(fops, struct klp_ops, fops);
> > >
> > > + bit = ftrace_test_recursion_trylock();
> > > + if (bit < 0)
> > > + return;
> >
> > This means that the original function will be called in case of recursion.
> > That's probably fair, but I'm wondering if we should at least WARN about
> > it.
>
> Yeah, the early return might break the consistency model and
> unexpected things might happen. We should be aware of it.
> Please use:
>
> if (WARN_ON_ONCE(bit < 0))
> return;
>
> WARN_ON_ONCE() might be part of the recursion. But it should happen
> only once. IMHO, it is worth the risk.

Agreed.

Miroslav