From mboxrd@z Thu Jan  1 00:00:00 1970
From: Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
Date: Fri, 9 Nov 2018 16:26:32 +0900
Message-ID: <20181109162632.9a3694eea8ac8056302a8262@kernel.org>
References: <20181101083551.3805-2-cyphar@cyphar.com>
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Cc: Aleksa Sarai <cyphar@cyphar.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Brendan Gregg <bgregg@netflix.com>,
        Christian Brauner <christian@brauner.io>,
        Aleksa Sarai <asarai@suse.de>, netd
To: Josh Poimboeuf <jpoimboe@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
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On Thu, 8 Nov 2018 08:44:37 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote:
> > On 2018-11-08, Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > I will attach what I have at the moment to hopefully explain what the
> > > issue I've found is (re-using the kretprobe architecture but with the
> > > shadow-stack idea).
> > 
> > Here is the patch I have at the moment (it works, except for the
> > question I have about how to handle the top-level pt_regs -- I've marked
> > that code with XXX).
> > 
> > -- 
> > Aleksa Sarai
> > Senior Software Engineer (Containers)
> > SUSE Linux GmbH
> > <https://www.cyphar.com/>
> > 
> > --8<---------------------------------------------------------------------
> > 
> > Since the return address is modified by kretprobe, the various unwinders
> > can produce invalid and confusing stack traces. ftrace mostly solved
> > this problem by teaching each unwinder how to find the original return
> > address for stack trace purposes. This same technique can be applied to
> > kretprobes by simply adding a pointer to where the return address was
> > replaced in the stack, and then looking up the relevant
> > kretprobe_instance when a stack trace is requested.
> > 
> > [WIP: This is currently broken because the *first entry* will not be
> >       overwritten since it looks like the stack pointer is different
> >       when we are provided pt_regs. All other addresses are correctly
> >       handled.]
> 
> When you see this problem, what does regs->ip point to?  If it's
> pointing to generated code, then we don't _currently_ have a way of
> dealing with that.  If it's pointing to a real function, we can fix that
> with unwind hints.

As I replied, If the stackdump is called from kretprobe event, regs->ip
always points trampoline function. Otherwise (maybe from kprobe event,
or panic, BUG etc.) it always be the address which the event occurs.

So fixing regs->ip is correct.

Thank you,


-- 
Masami Hiramatsu <mhiramat@kernel.org>