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Date:   Fri, 4 Jan 2019 19:05:44 +0100
From:   Stefano Brivio <sbrivio@redhat.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        syzbot <syzbot+4ad25edc7a33e4ab91e0@syzkaller.appspotmail.com>,
        David Miller <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Subject: Re: kernel panic: stack is corrupted in udp4_lib_lookup2
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On Fri, 4 Jan 2019 18:26:16 +0100
Dmitry Vyukov <dvyukov@google.com> wrote:

> On Fri, Jan 4, 2019 at 6:14 PM Stefano Brivio <sbrivio@redhat.com> wrote:
> >
> > On Fri, 4 Jan 2019 12:05:04 +0100
> > Dmitry Vyukov <dvyukov@google.com> wrote:
> >
> > > I've added these as tests:
> > >
> > > https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/341
> > > https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/342
> > > https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/343
> > > https://github.com/google/syzkaller/blob/master/pkg/report/testdata/linux/report/344
> > >
> > > Will try to figure out how to distinguish them from true corrupted
> > > reports. Usually when Call Trace does not have any frames, it's a sign
> > > of a corrupted report, and in other crashes we see the same report but
> > > with a stack trace. But some stack-corruption-related reliably don't
> > > have stack traces (not corrupted). But then some other
> > > stack-corruption-related crashes do have stack traces, and for these
> > > no stack trace again means a corrupted kernel output. Amusingly this
> > > is one of the most complex parts of syzkaller.  
> >
> > I'm not sure how complicated that would be, but what about some metric
> > based on valid symbol names being reported?  
> 
> Please elaborate. What do you mean by "valid symbol names"?

I mean a symbol name listed in /proc/kallsyms on the running system.

This is usually my minimum threshold for "I can do something with this
report" -- which doesn't mean it's necessarily valid, but well, if you
have that, it means that at least something worked in the reporting,
and you can at least start having a look at a specific function.

> Note that corrupted output detection solves 2 problems:
> 1. Do we think the output is truncated to the point of being not useful?
> E.g. sometimes kernel produces just 1 line:
> 
> general protection fault: 0000 [#1] PREEMPT SMP KASAN
> 
> This is sure a crash, but it's not too useful to report.

Sure. In those tests above you have:
- 341: udp6_lib_lookup2+0x622, handle_irq+0x2cb
- 342: __sanitizer_cov_trace_pc+0x8, handle_irq+0x2cb
- 343: __udp6_lib_err, etc.
- 344: __udp6_lib_lookup+0x1d, etc.

and this makes all those reports at least minimally useful.

> 2. Do we have any reasons to think we extracted bogus crash identity?
> E.g. crash intermixed with output from another thread so that we say
> "something-bad in function foo", when in fact function foo come from
> output of the second non-crashing thread.

Okay, this looks way more complicated :)

-- 
Stefano