linux-kernel - Re: Suggestions on how to debug kernel crashes where printk and gdb both does not work

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Date:   Mon, 14 Jun 2021 23:04:03 +0800
From:   Dongliang Mu <mudongliangabcd@...il.com>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     alex.aring@...il.com, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-wpan@...r.kernel.org, netdev@...r.kernel.org,
        stefan@...enfreihafen.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        syzbot+b80c9959009a9325cdff@...kaller.appspotmail.com,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: Suggestions on how to debug kernel crashes where printk and gdb
 both does not work

On Mon, Jun 14, 2021 at 10:47 PM Pavel Skripkin <paskripkin@...il.com> wrote:
>
> On Mon, 14 Jun 2021 22:40:55 +0800
> Dongliang Mu <mudongliangabcd@...il.com> wrote:
>
> > On Mon, Jun 14, 2021 at 10:25 PM Pavel Skripkin
> > <paskripkin@...il.com> wrote:
> > >
> > > On Mon, 14 Jun 2021 22:19:10 +0800
> > > Dongliang Mu <mudongliangabcd@...il.com> wrote:
> > >
> > > > On Mon, Jun 14, 2021 at 9:34 PM Pavel Skripkin
> > > > <paskripkin@...il.com> wrote:
> > > > >
> > > > > On Mon, 14 Jun 2021 21:22:43 +0800
> > > > > Dongliang Mu <mudongliangabcd@...il.com> wrote:
> > > > >
> > > > > > Dear kernel developers,
> > > > > >
> > > > > > I was trying to debug the crash - memory leak in
> > > > > > hwsim_add_one [1] recently. However, I encountered a
> > > > > > disgusting issue: my breakpoint and printk/pr_alert in the
> > > > > > functions that will be surely executed do not work. The stack
> > > > > > trace is in the following. I wrote this email to ask for some
> > > > > > suggestions on how to debug such cases?
> > > > > >
> > > > > > Thanks very much. Looking forward to your reply.
> > > > > >
> > > > >
> > > > > Hi, Dongliang!
> > > > >
> > > > > This bug is not similar to others on the dashboard. I spent some
> > > > > time debugging it a week ago. The main problem here, that memory
> > > > > allocation happens in the boot time:
> > > > >
> > > > > > [<ffffffff84359255>] kernel_init+0xc/0x1a7 init/main.c:1447
> > > > >
> > > >
> > > > Oh, nice catch. No wonder why my debugging does not work. :(
> > > >
> > > > > and reproducer simply tries to
> > > > > free this data. You can use ftrace to look at it. Smth like
> > > > > this:
> > > > >
> > > > > $ echo 'hwsim_*' > $TRACE_DIR/set_ftrace_filter
> > > >
> > > > Thanks for your suggestion.
> > > >
> > > > Do you have any conclusions about this case? If you have found
> > > > out the root cause and start writing patches, I will turn my
> > > > focus to other cases.
> > >
> > > No, I had some busy days and I have nothing about this bug for now.
> > > I've just traced the reproducer execution and that's all :)
> > >
> > > I guess, some error handling paths are broken, but Im not sure
> >
> > In the beginning, I agreed with you. However, after I manually checked
> > functions: hwsim_probe (initialization) and  hwsim_remove (cleanup),
> > then things may be different. The cleanup looks correct to me. I would
> > like to debug but stuck with the debugging process.
> >
> > And there is another issue: the cleanup function also does not output
> > anything or hit the breakpoint. I don't quite understand it since the
> > cleanup is not at the boot time.
> >
> > Any idea?
> >
>
> Output from ftrace (syzkaller repro):
>
> root@...kaller:~# cat /sys/kernel/tracing/trace
> # tracer: function_graph
> #
> # CPU  DURATION                  FUNCTION CALLS
> # |     |   |                     |   |   |   |
>  1)               |  hwsim_del_radio_nl() {
>  1)               |    hwsim_del() {
>  1)               |      hwsim_edge_unsubscribe_me() {
>  1) ! 310.041 us  |        hwsim_free_edge();
>  1) ! 665.221 us  |      }
>  1) * 52999.05 us |    }
>  1) * 53035.38 us |  }
>
> Cleanup function is not the case, I think :)

It seems like I spot the incorrect cleanup function (hwsim_remove is
the right one is in my mind). Let me learn how to use ftrace to log
the executed functions and then discuss this case with you guys.

>
>
>
> With regards,
> Pavel Skripkin