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

[Alexei Starovoitov]

On 12/28/17 12:20 AM, Masami Hiramatsu wrote:
> On Wed, 27 Dec 2017 20:32:07 -0800
> Alexei Starovoitov <ast@xxxxxx> wrote:
>
> > On 12/27/17 8:16 PM, Steven Rostedt wrote:
> > > On Wed, 27 Dec 2017 19:45:42 -0800
> > > Alexei Starovoitov <ast@xxxxxx> 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.