linux-kernel - Re: BUG: Null pointer dereference in fs/open.c

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Message-ID: <Pine.LNX.4.64.0704242231270.5493@p4.localdomain>
Date:	Tue, 24 Apr 2007 22:46:09 +0200 (CEST)
From:	Peter Osterlund <petero2@...ia.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	William Heimbigner <icxcnika@....tar.cc>,
	linux-kernel@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>
Subject: Re: BUG: Null pointer dereference in fs/open.c

On Mon, 23 Apr 2007, Andrew Morton wrote:

> Try this:
>
> --- a/drivers/block/pktcdvd.c~packet-fix-error-handling
> +++ a/drivers/block/pktcdvd.c
> @@ -777,7 +777,8 @@ static int pkt_generic_packet(struct pkt
> 		rq->cmd_flags |= REQ_QUIET;
>
> 	blk_execute_rq(rq->q, pd->bdev->bd_disk, rq, 0);
> -	ret = rq->errors;
> +	if (rq->errors)
> +		ret = -EIO;
> out:
> 	blk_put_request(rq);
> 	return ret;
> _
>
>
> The packet driver was assuming that request.errors is an errno, but it
> isn't - it's some sort of diagnostic bitfield thing.  Now why would the
> packet driver have though that?  Let's go read the comments:
...
> Well there's your root cause right there.
>
> I don't know why this wasn't oopsing in eariler kernels.  Perhaps something
> else is broken.  Please test this urgently.

The code used to return -EIO until commit 
406c9b605cbc45151c03ac9a3f95e9acf050808c, which was commited 2007-01-05, 
so that would explain why older kernels didn't crash.

> What the heck _is_ in request.errors?

According to linux/Documentation/block/request.txt, it is an error 
counter. The info in that text file would probably do a lot more good as 
comments in the structure definition though.

> Should the packet driver even be looking at it?

I think so. How else is it supposed to know if the request failed?

-- 
Peter Osterlund - petero2@...ia.com
http://web.telia.com/~u89404340
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