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 Wed, 27 Dec 2017 19:45:42 -0800
Alexei Starovoitov <ast@fb.com> wrote:

> On 12/27/17 6:34 PM, Masami Hiramatsu wrote:
> > On Wed, 27 Dec 2017 14:46:24 -0800
> > Alexei Starovoitov <ast@fb.com> wrote:
> >
> >> On 12/26/17 9:56 PM, Masami Hiramatsu wrote:
> >>> On Tue, 26 Dec 2017 17:57:32 -0800
> >>> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> >>>
> >>>> On Tue, Dec 26, 2017 at 04:46:59PM +0900, Masami Hiramatsu wrote:
> >>>>> Check whether error injectable event is on function entry or not.
> >>>>> Currently it checks the event is ftrace-based kprobes or not,
> >>>>> but that is wrong. It should check if the event is on the entry
> >>>>> of target function. Since error injection will override a function
> >>>>> to just return with modified return value, that operation must
> >>>>> be done before the target function starts making stackframe.
> >>>>>
> >>>>> As a side effect, bpf error injection is no need to depend on
> >>>>> function-tracer. It can work with sw-breakpoint based kprobe
> >>>>> events too.
> >>>>>
> >>>>> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> >>>>> ---
> >>>>>  kernel/trace/Kconfig        |    2 --
> >>>>>  kernel/trace/bpf_trace.c    |    6 +++---
> >>>>>  kernel/trace/trace_kprobe.c |    8 +++++---
> >>>>>  kernel/trace/trace_probe.h  |   12 ++++++------
> >>>>>  4 files changed, 14 insertions(+), 14 deletions(-)
> >>>>>
> >>>>> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> >>>>> index ae3a2d519e50..6400e1bf97c5 100644
> >>>>> --- a/kernel/trace/Kconfig
> >>>>> +++ b/kernel/trace/Kconfig
> >>>>> @@ -533,9 +533,7 @@ config FUNCTION_PROFILER
> >>>>>  config BPF_KPROBE_OVERRIDE
> >>>>>  	bool "Enable BPF programs to override a kprobed function"
> >>>>>  	depends on BPF_EVENTS
> >>>>> -	depends on KPROBES_ON_FTRACE
> >>>>>  	depends on HAVE_KPROBE_OVERRIDE
> >>>>> -	depends on DYNAMIC_FTRACE_WITH_REGS
> >>>>>  	default n
> >>>>>  	help
> >>>>>  	 Allows BPF to override the execution of a probed function and
> >>>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> >>>>> index f6d2327ecb59..d663660f8392 100644
> >>>>> --- a/kernel/trace/bpf_trace.c
> >>>>> +++ b/kernel/trace/bpf_trace.c
> >>>>> @@ -800,11 +800,11 @@ int perf_event_attach_bpf_prog(struct perf_event *event,
> >>>>>  	int ret = -EEXIST;
> >>>>>
> >>>>>  	/*
> >>>>> -	 * Kprobe override only works for ftrace based kprobes, and only if they
> >>>>> -	 * are on the opt-in list.
> >>>>> +	 * Kprobe override only works if they are on the function entry,
> >>>>> +	 * and only if they are on the opt-in list.
> >>>>>  	 */
> >>>>>  	if (prog->kprobe_override &&
> >>>>> -	    (!trace_kprobe_ftrace(event->tp_event) ||
> >>>>> +	    (!trace_kprobe_on_func_entry(event->tp_event) ||
> >>>>>  	     !trace_kprobe_error_injectable(event->tp_event)))
> >>>>>  		return -EINVAL;
> >>>>>
> >>>>> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> >>>>> index 91f4b57dab82..265e3e27e8dc 100644
> >>>>> --- a/kernel/trace/trace_kprobe.c
> >>>>> +++ b/kernel/trace/trace_kprobe.c
> >>>>> @@ -88,13 +88,15 @@ static nokprobe_inline unsigned long trace_kprobe_nhit(struct trace_kprobe *tk)
> >>>>>  	return nhit;
> >>>>>  }
> >>>>>
> >>>>> -int trace_kprobe_ftrace(struct trace_event_call *call)
> >>>>> +bool trace_kprobe_on_func_entry(struct trace_event_call *call)
> >>>>>  {
> >>>>>  	struct trace_kprobe *tk = (struct trace_kprobe *)call->data;
> >>>>> -	return kprobe_ftrace(&tk->rp.kp);
> >>>>> +
> >>>>> +	return kprobe_on_func_entry(tk->rp.kp.addr, tk->rp.kp.symbol_name,
> >>>>> +				    tk->rp.kp.offset);
> >>>>
> >>>> That would be nice, but did you test this?
> >>>
> >>> Yes, because the jprobe, which was only official user of modifying execution
> >>> path using kprobe, did same way to check. (and kretprobe also does it)
> >>>
> >>>> My understanding that kprobe will restore all regs and
> >>>> here we need to override return ip _and_ value.
> >>>
> >>> yes, no problem. kprobe restore all regs from pt_regs, including regs->ip.
> >>>
> >>>> Could you add a patch with the test the way Josef did
> >>>> or describe the steps to test this new mode?
> >>>
> >>> Would you mean below patch? If so, it should work without any change.
> >>>
> >>>  [PATCH v10 4/5] samples/bpf: add a test for bpf_override_return
> >>
> >> yeah. I expect bpf_override_return test to work as-is.
> >> I'm asking for the test for new functionality added by this patch.
> >> In particular kprobe on func entry without ftrace.
> >> How did you test it?
> >
> > This function is used in kretprobe and jprobe. Jprobe was the user of
> > "modifying instruction pointer to another function" in kprobes.
> > If it doesn't work, jprobe also doesn't work, this means you can not
> > modify IP by kprobes anymore.
> > Anyway, until linux-4.13, that was well tested by kprobe smoke test.
> >
> >> and how I can repeat the test?
> >> I'm still not sure that it works correctly.
> >
> > That works correctly because it checks given address is on the entry
> > point (the 1st instruction) of a function, using kallsyms.
> >
> > The reason why I made another flag for ftrace was, there are 2 modes
> > for ftrace dynamic instrumentation, fentry and mcount.
> > With new fentry mode, ftrace will be put on the first instruction
> > of the function, so it will work as you expected.
> > With traditional gcc mcount, ftrace will be called after making call
> > frame for _mcount(). This means if you modify ip, it will not work
> > or cause a trouble because _mcount call frame is still on the stack.
> >
> > So, current ftrace-based checker doesn't work, it depends on the case.
> > Of course, in most case, kernel will be build in new gcc which
> > supports fentry, but there is no guarantee.
> 
> 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.
> 
> 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.

No, even with this patch, I meant, you can still test it with Josef's
testcase, since this is just backend change. No frontend changes.

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>