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

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Message-ID: <CAD-N9QV5_91A6n5QcrafmQRbqH_qzFRatno-6z0i7q-V9VnLzg@mail.gmail.com>
Date:   Mon, 14 Jun 2021 22:19:10 +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 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.

BTW, I only found another possible memory leak after some manual code
review [1]. However, it is not the root cause for this crash.

[1] https://lkml.org/lkml/2021/6/10/1297

>
> would work.
>
>
> With regards,
> Pavel Skripkin