linux-kernel - Re: [BUG REPORT] NULL pointer dereference in jdb2_journal_grab_journal

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Date:	Mon, 25 Jan 2016 08:41:36 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>, Jan Kara <jack@...e.com>,
	linux-ext4@...r.kernel.org
Subject: Re: [BUG REPORT] NULL pointer dereference in jdb2_journal_grab_journal_head
 (RDI)

On 1/25/16, Jan Kara <jack@...e.cz> wrote:
> On Sat 23-01-16 09:42:52, Jeff Merkey wrote:
>> If I leave the system in the debugger console overnight with all the
>> processors suspended for about 8 hours, then type go, the following
>> bug shows up during file I/O.  This particular bug showed up while
>> using git to update some branches.
>>
>> I have only seen this bug once and I attempted to reproduce it to get
>> a trace dump but have not been able to trigger it again.  The NULL
>> pointer is RDI set to NULL while trying to obtain a lock.
>>
>> (2)> .z grab_journal
>> ffffffffa00bb740 t jbd2_journal_grab_journal_head [jbd2]
>> (2)> u ffffffffa00bb740
>> jbd2|jbd2_journal_grab_journal_head:
>> 0xffffffffa00bb740 0F1F440000      nop    DWORD PTR [rax+rax]=0x0
>> 0xffffffffa00bb745 55              push   rbp
>> 0xffffffffa00bb746 4889E5          mov    rbp,rsp
>> <<<<<<<<<<<<   Crashes here with RDI set to NULL
>> 0xffffffffa00bb749 F00FBA2F18      lock bts DWORD PTR [rdi]=0x0,0x18
>> <<<<<<<<<<<<
>
> Thanks for report. Ok, this means jbd2_journal_grab_journal_head() got
> called with 'bh == NULL'. That is certainly wrong but unless we know a full
> stack trace, it's hard to guess what went wrong.
>
> 								Honza
>
> --
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
>


I have a system setup to get a better trace if it triggers again.
When it happened the first time I was not able to get a good dump.  If
it triggers again, I'll send you the trace.   So far it has not
triggered.

:-)

Jeff