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: <20171227231644.168abc0f@vmware.local.home>
Date:   Wed, 27 Dec 2017 23:16:44 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <ast@...com>
Cc:     Masami Hiramatsu <mhiramat@...nel.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 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.

> 
> 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?

-- Steve