Re: kprobes: propagate error from arm_kprobe_ftrace()

[Jessica Yu]

+++ Steven Rostedt [03/11/17 10:03 -0400]:
> On Thu,  2 Nov 2017 17:33:33 +0100
> Jessica Yu <jeyu@xxxxxxxxxx> wrote:
>
> > Improve error handling when arming ftrace-based kprobes. Specifically, if
> > we fail to arm a ftrace-based kprobe, register_kprobe()/enable_kprobe()
> > should report an error instead of success. Previously, this has lead to
> > confusing situations where register_kprobe() would return 0 indicating
> > success, but the kprobe would not be functional if ftrace registration
> > during the kprobe arming process had failed. We should therefore take any
> > errors returned by ftrace into account and propagate this error so that we
> > do not register/enable kprobes that cannot be armed. This can happen if,
> > for example, register_ftrace_function() finds an IPMODIFY conflict (since
> > kprobe_ftrace_ops has this flag set) and returns an error. Such a conflict
> > is possible since livepatches also set the IPMODIFY flag for their ftrace_ops.
> >
> > arm_all_kprobes() keeps its current behavior and attempts to arm all
> > kprobes. It returns the last encountered error and gives a warning if
> > not all kprobes could be armed.
> >
> > This patch is based on Petr Mladek's original patchset (patches 2 and 3)
> > back in 2015, which improved kprobes error handling, found here:
> >
> >    https://lkml.org/lkml/2015/2/26/452
> >
> > However, further work on this had been paused since then and the patches
> > were not upstreamed.
> >
> > Based-on-patches-by: Petr Mladek <pmladek@xxxxxxxx>
> > Signed-off-by: Jessica Yu <jeyu@xxxxxxxxxx>
> > ---
> >  kernel/kprobes.c | 88 ++++++++++++++++++++++++++++++++++++++++----------------
> >  1 file changed, 63 insertions(+), 25 deletions(-)
> >
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index da2ccf142358..f4a094007cb5 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -978,18 +978,27 @@ static int prepare_kprobe(struct kprobe *p)
> >  }
> >
> >  /* Caller must lock kprobe_mutex */
> > -static void arm_kprobe_ftrace(struct kprobe *p)
> > +static int arm_kprobe_ftrace(struct kprobe *p)
> >  {
> > -	int ret;
> > +	int ret = 0;
> >
> >  	ret = ftrace_set_filter_ip(&kprobe_ftrace_ops,
> >  				   (unsigned long)p->addr, 0, 0);
> > -	WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret);
> > -	kprobe_ftrace_enabled++;
> > -	if (kprobe_ftrace_enabled == 1) {
> > +	if (WARN(ret < 0, "Failed to arm kprobe-ftrace at %p (%d)\n", p->addr, ret))
> > +		return ret;
> > +
> > +	if (kprobe_ftrace_enabled == 0) {
> >  		ret = register_ftrace_function(&kprobe_ftrace_ops);
> > -		WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret);
> > +		if (WARN(ret < 0, "Failed to init kprobe-ftrace (%d)\n", ret))
> > +			goto err_ftrace;
> >  	}
> > +
> > +	kprobe_ftrace_enabled++;
> > +	return ret;
> > +
> > +err_ftrace:
> > +	ftrace_set_filter_ip(&kprobe_ftrace_ops, (unsigned long)p->addr, 1, 0);
>
> Hmm, this could have a very nasty side effect. If you remove a function
> from the ops, and it was the last function, an empty ops means to trace
> *all* functions.

Good point, and yes, normally this would be the (undesirable) outcome.

However in this case, kprobes_ftrace_ops has the IPMODIFY flag set, so
ftrace_set_filter_ip() will not allow the removal of a function if it
is the very last function, and it will return an error in
__ftrace_hash_update_ipmodify(). The comment there explains, if
IPMODIFY is set, "return -EINVAL if the new_hash tries to trace all
recs". So I think we are safe here...

> Perhaps you want to add it to the "notrace" list. Which would require
> implementing a ftrace_set_notrace_ip() function. Which I believe is
> what you want. Any function in the notrace hash will have the same
> functions in the filter hash be ignored.

I think this would've been a good alternative if we wanted to protect
against the empty ops case, but IPMODIFY is also incompatible with notrace..
(See: commit f8b8be8a310 and this comment here:
https://www.mail-archive.com/linux-kernel@xxxxxxxxxxxxxxx/msg688632.html)

Speaking of IPMODIFY, a question for Masami - is this flag still
relevant/needed for kprobes, since jprobes has been deprecated
recently? IIRC, IPMODIFY was needed in the first place because jprobes
and livepatch were in direct conflict, but I recall some work being
done in the past to remove the IPMODIFY flag from kprobes, but I don't
think this was ever upstreamed. (See: https://patchwork.kernel.org/patch/5352481/)

Thanks!

Jessica