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Date:   Sun, 11 Nov 2018 20:53:51 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Alexander Popov <alex.popov@linux.com>
Cc:     kernel-hardening@lists.openwall.com,
        Kees Cook <keescook@chromium.org>,
        Jann Horn <jannh@google.com>, Ingo Molnar <mingo@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, Jan Kara <jack@suse.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Masami Hiramatsu <mhiramat@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] stackleak: Disable ftrace for stackleak.c
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On Sun, 11 Nov 2018 13:19:45 +0300
Alexander Popov <alex.popov@linux.com> wrote:

> On 11.11.2018 2:30, Steven Rostedt wrote:
> > On Sun, 11 Nov 2018 01:05:30 +0300
> > Alexander Popov <alex.popov@linux.com> wrote:
> >   
> >> The stackleak_erase() function is called on the trampoline stack at the
> >> end of syscall. This stack is not big enough for ftrace operations,
> >> e.g. it can be overflowed if we enable kprobe_events for stackleak_erase().  
> > 
> > Is the issue with kprobes or with function tracing? Because this stops
> > function tracing which I only want disabled if function tracing itself
> > is an issue, not for other things that may use the function tracing
> > infrastructure.  
> 
> Hello Steven,
> 
> I believe that stackleak erasing is not compatible with function tracing itself.
> That's what the kernel testing robot has hit:
> https://www.openwall.com/lists/kernel-hardening/2018/11/09/1
> 
> I used kprobe_events just to reproduce the problem:
> https://www.openwall.com/lists/kernel-hardening/2018/11/09/4

Have you tried adding a "notrace" to stackleak_erase()?

Not tracing the entire file is a bit of overkill. There's no reason
ftrace can't trace stack_erasing_sysctl() or perhaps even
stackleak_track_stack() as that may be very interesting to trace.

-- Steve