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

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Message-ID: <20210614163401.52807197@gmail.com>
Date:   Mon, 14 Jun 2021 16:34:01 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Dongliang Mu <mudongliangabcd@...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, 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

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

would work.


With regards,
Pavel Skripkin