Subject: Re: [RFC bpf-next] bpf: Use prog->active instead of bpf_prog_active for kprobe_multi

From: Jiri Olsa <[email-masked]>
Date: Mon, 13 Jun 2022 14:36:38 +0200

On Sat, Jun 11, 2022 at 01:53:26PM -0700, Alexei Starovoitov wrote:
> On Fri, Jun 10, 2022 at 10:58:50AM -0700, Andrii Nakryiko wrote:
> > On Thu, Jun 9, 2022 at 3:03 PM Alexei Starovoitov
> > <[email-masked]> wrote:
> > >
> > > On Thu, Jun 9, 2022 at 11:27 AM Andrii Nakryiko
> > > <[email-masked]> wrote:
> > > >
> > > > On Tue, Jun 7, 2022 at 9:29 PM Alexei Starovoitov
> > > > <[email-masked]> wrote:
> > > > >
> > > > > On Tue, May 31, 2022 at 4:24 PM Andrii Nakryiko
> > > > > <[email-masked]> wrote:
> > > > > >
> > > > > > On Wed, May 25, 2022 at 4:40 AM Jiri Olsa <[email-masked]> wrote:
> > > > > > >
> > > > > > > hi,
> > > > > > > Alexei suggested to use prog->active instead global bpf_prog_active
> > > > > > > for programs attached with kprobe multi [1].
> > > > > > >
> > > > > > > AFAICS this will bypass bpf_disable_instrumentation, which seems to be
> > > > > > > ok for some places like hash map update, but I'm not sure about other
> > > > > > > places, hence this is RFC post.
> > > > > > >
> > > > > > > I'm not sure how are kprobes different to trampolines in this regard,
> > > > > > > because trampolines use prog->active and it's not a problem.
> > > > > > >
> > > > > > > thoughts?
> > > > > > >
> > > > > >
> > > > > > Let's say we have two kernel functions A and B? B can be called from
> > > > > > BPF program though some BPF helper, ok? Now let's say I have two BPF
> > > > > > programs kprobeX and kretprobeX, both are attached to A and B. With
> > > > > > using prog->active instead of per-cpu bpf_prog_active, what would be
> > > > > > the behavior when A is called somewhere in the kernel.
> > > > > >
> > > > > > 1. A is called
> > > > > > 2. kprobeX is activated for A, calls some helper which eventually calls B
> > > > > >   3. kprobeX is attempted to be called for B, but is skipped due to prog->active
> > > > > >   4. B runs
> > > > > >   5. kretprobeX is activated for B, calls some helper which eventually calls B
> > > > > >     6. kprobeX is ignored (prog->active > 0)
> > > > > >     7. B runs
> > > > > >     8. kretprobeX is ignored (prog->active > 0)
> > > > > > 9. kretprobeX is activated for A, calls helper which calls B
> > > > > >   10. kprobeX is activated for B
> > > > > >     11. kprobeX is ignored (prog->active > 0)
> > > > >
> > > > > not correct. kprobeX actually runs.
> > > > > but the end result is correct.
> > > > >
> > > >
> > > > Right, it was a long sequence, but you got the idea :)
> 
> The above analysis was actually incorrect.
> There are three kprobe flavors: int3, opt, ftrace.
> while multi-kprobe is based on fprobe.
> kretprobe can be traditional and rethook based.
> In all of these mechanisms there is at least ftrace_test_recursion_trylock()
> and for kprobes there is kprobe_running (per-cpu current_kprobe) filter
> that acts as bpf_prog_active.
> 
> So this:
> 1. kprobeX for A
> 2. kretprobeX for B
> 3. kretprobeX for A
> 4. kprobeX for B
> doesn't seem possible.
> Unless there is reproducer of above behavior there is no point using above
> as a design argument.

yes, I just experimentally verified ;-) I have a selftest with new test
helper doing Andrii's scenario (with kprobes on ftrace) and kprobe_running
check will take care of the entry side:

        if (kprobe_running()) {
                kprobes_inc_nmissed_count(p);

and as a results kretprobe won't be installed as well

> 
> > > > > It's awful. We have to fix it.
> > > >
> > > > You can call it "a fix" if you'd like, but it's changing a very
> > > > user-visible behavior and guarantees on which users relied for a
> > > > while. So even if we switch to per-prog protection it will have to be
> > > > some sort of opt-in (flag, new program type, whatever).
> > >
> > > No opt-in allowed for fixes and it's a bug fix.
> > > No one should rely on broken kernel behavior.
> > > If retsnoop rely on that it's really retsnoop's fault.
> > 
> > No point in arguing if we can't even agree on whether this is a bug or
> > not, sorry.
> > 
> > Getting kretprobe invocation out of the blue without getting
> > corresponding kprobe invocation first (both of which were successfully
> > attached) seems like more of a bug to me. But perhaps that's a matter
> > of subjective opinion.
> 
> The issue of kprobe/kretprobe mismatch was known for long time.
> First maxactive was an issue. It should be solved by rethook now.
> Then kprobe/kretprobe attach is not atomic.
> bpf prog attaching kprobe and kretprobe to the same func cannot assume
> that they will always pair. bcc scripts had to deal with this.
> 
> Say, kprobe/kretprobe will become fentry/fexit like with prog->active only.
> If retsnoop wants to do its own per-cpu prog_active counter it will
> prevent out-of-order fentry/fexit for the case when the same prog
> is attached to before-bpf-func and during-bpf-func. Only retsnoop's progs
> will miss during-bpf-func events. Such policy decisions is localized to one tool.
> All other users will see the events they care about.
> kprobe/kretprobe/fprobe run handlers with preemption disabled which makes
> these mechanisms unfriendly to RT. Their design shows that they're not suitable
> for always-on running. When bpf+kprobe was introduced 7 years ago it wasn't
> meant to be 24-7 either. bpf_prog_active is modeled like current_kprobe.
> It was addressing the deadlock issue with spinlocks in maps.
> Recursion was not an issue.
> Sadly kprobe/kretprobe/fprobe look unfixable in this form. Too much work
> needs to be done to enable something like:
> user A attaches prog A to func X. X runs, prog A runs with migration disabled.
> Preemption. Something else starts on this cpu. Another user B attaching prog B
> to func Y should see its prog being executed.
> With kprobes it looks impossible. While fentry was designed with this use case
> in mind. Note it's not about sleepable progs. Normal bpf progs can be preempted.
> 
> Back to Jiri's question whether we can remove bpf_prog_active from
> trace_call_bpf.  Yes. We can and we should. It will allow bperf to collect
> stack traces that include bpf progs. It's an important fix. Incorrect retsnoop
> assumptions about kprobes will not be affected.

which bperf tool are you talking about (I found 2)?

and given that the kprobe layer is effectively doing the bpf_prog_active check,
what's the benefit of the change then?

thanks,
jirka