Re: [RFC PATCH bpf-next v2 1/4] tracing/kprobe: bpf: Check error injectable event is on function entry

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 28 Dec 2017 17:03:24 -0800
Alexei Starovoitov <ast@fb.com> wrote:

> On 12/28/17 12:20 AM, Masami Hiramatsu wrote:
> > On Wed, 27 Dec 2017 20:32:07 -0800
> > Alexei Starovoitov <ast@fb.com> wrote:
> >
> >> On 12/27/17 8:16 PM, Steven Rostedt wrote:
> >>> On Wed, 27 Dec 2017 19:45:42 -0800
> >>> Alexei Starovoitov <ast@fb.com> wrote:
> >>>
> >>>> I don't think that's the case. My reading of current
> >>>> trace_kprobe_ftrace() -> arch_check_ftrace_location()
> >>>> is that it will not be true for old mcount case.
> >>>
> >>> In the old mcount case, you can't use ftrace to return without calling
> >>> the function. That is, no modification of the return ip, unless you
> >>> created a trampoline that could handle arbitrary stack frames, and
> >>> remove them from the stack before returning back to the function.
> >>
> >> correct. I was saying that trace_kprobe_ftrace() won't let us do
> >> bpf_override_return with old mcount.
> >
> > No, trace_kprobe_ftrace() just checks the given address will be
> > managed by ftrace. you can see arch_check_ftrace_location() in kernel/kprobes.c.
> >
> > FYI, CONFIG_KPROBES_ON_FTRACE depends on DYNAMIC_FTRACE_WITH_REGS, and
> > DYNAMIC_FTRACE_WITH_REGS doesn't depend on CC_USING_FENTRY.
> > This means if you compile kernel with old gcc and enable DYNAMIC_FTRACE,
> > kprobes uses ftrace on mcount address which is NOT the entry point
> > of target function.
> 
> ok. fair enough. I think we can gate the feature to !mcount only.
> 
> > On the other hand, changing IP feature has been implemented originaly
> > by kprobes with int3 (sw breakpoint). This means you can use kprobes
> > at correct address (the entry address of the function) you can hijack
> > the function, as jprobe did.
> >
> >>>> As far as the rest of your arguments it very much puzzles me that
> >>>> you claim that this patch suppose to work based on historical
> >>>> reasoning whereas you did NOT test it.
> >>>
> >>> I believe that Masami is saying that the modification of the IP from
> >>> kprobes has been very well tested. But I'm guessing that you still want
> >>> a test case for using kprobes in this particular instance. It's not the
> >>> implementation of modifying the IP that you are worried about, but the
> >>> implementation of BPF using it in this case. Right?
> >>
> >> exactly. No doubt that old code works.
> >> But it doesn't mean that bpf_override_return() will continue to
> >> work in kprobes that are not ftrace based.
> >> I suspect Josef's existing test case will cover this situation.
> >> Probably only special .config is needed to disable ftrace, so
> >> "kprobe on entry but not ftrace" check will kick in.
> >
> > Right. If you need to test it, you can run Josef's test case without
> > CONFIG_DYNAMIC_FTRACE.
> 
> It should be obvious that the person who submits the patch
> must run the tests.
> 
> >> But I didn't get an impression that this situation was tested.
> >> Instead I see only logical reasoning that it's _supposed_ to work.
> >> That's not enough.
> >
> > OK, so would you just ask me to run samples/bpf ?
> 
> Please run Josef's test in the !ftrace setup.

Yes, I'll add the result of the test case.

Thank you,


-- 
Masami Hiramatsu <mhiramat@kernel.org>