linux-kernel - Re: NULL pointer dereference in print_daily_error

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Date:	Tue, 14 Sep 2010 15:46:41 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>, Jan Kara <jack@...e.cz>,
	Eric Sandeen <sandeen@...hat.com>,
	Christoph Hellwig <hch@....de>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: NULL pointer dereference in print_daily_error_info

On Tue, Sep 14, 2010 at 03:51:02PM +0300, Sergey Senozhatsky wrote:
> Hello,
>       
> This patch fixes NULL pointer dereference in print_daily_error_info, when   
> called
> on unmounted fs (EXT4_SB(sb) returns NULL). Deleting error reporting timer  
> in
> ext4_put_super fixes oops.

Good catch!  Thanks for the patch.  I will include this into ext4
tree, and I will probably push it separately to Linus so that it gets
into 2.6.36, since this is a regresssion.

You didn't add a Signed-off-by: line, which is needed for Developer's
Certification of Origin (see section 1, subsection 16 of
Documentation/SubmittingPatches in the Linux source tree).  Can you
send confirmation that it's OK for me to add a Signed-off-by line for
you?  Thanks!!

> By the way, isn't print_daily_error_info racy? Is it safe to call           
> print_daily_error_info
> (by timer event (softirq)) when we'are remounting fs, etc.?

It should be fine.  Remounting doesn't actually change out the struct
superblock.  There is a chance that the information might not be fully
complete if an error is printed exactly as the same time as
print_daily_error_info() is run, but I'm not sure it's worth trying to
protect against that race, since the worst that this will mean is a
confusing report in the /var/log/messages file, and the ext4 error
message will be printed right next to it, which will have all of the
information the system administrator will need.

						- Ted

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