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

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Message-ID: <b6057235-a1f4-a461-a2d5-295e964249ea@fb.com>
Date:   Thu, 28 Dec 2017 17:03:24 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Josef Bacik <jbacik@...com>, <mingo@...hat.com>,
        <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <ast@...nel.org>,
        <kernel-team@...com>, <daniel@...earbox.net>,
        <linux-btrfs@...r.kernel.org>, <darrick.wong@...cle.com>,
        Josef Bacik <josef@...icpanda.com>,
        Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [RFC PATCH bpf-next v2 1/4] tracing/kprobe: bpf: Check error
 injectable event is on function entry

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