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

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Message-ID: <20160125123059.GB24938@quack.suse.cz>
Date:	Mon, 25 Jan 2016 13:30:59 +0100
From:	Jan Kara <jack@...e.cz>
To:	Jeff Merkey <linux.mdb@...il.com>
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 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