From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jens Axboe <axboe@suse.de>
Subject: Re: [PATCH]: Fix erroneous rq->buffer = NULL in ide-io.c:ide_dma_timeout_retry
Date: Tue, 4 Jan 2005 20:04:03 +0100
Message-ID: <20050104190403.GW2825@suse.de>
References: <41DAAC7D.7060002@sgi.com>
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To: Prarit Bhargava <prarit@sgi.com>
Cc: linux-ide@vger.kernel.org

On Tue, Jan 04 2005, Prarit Bhargava wrote:
> Hello,
> 
> I have found an IDE bug in the IDE DMA timeout function,
> ide-io.c: ide_dma_timeout_retry erroneously sets the first_rq->buffer = 
> NULL.
> 
> ide_dma_timeout_retry will be called whenever a command is issued, times 
> out,
> and the drive is waiting for DMA.  The function, ide_dma_timeout_retry,
> un-busies the hardware group and attempts to clean up the current request.
> As part of this cleanup the current failed first_rq->buffer is set to NULL.
> 
> However, as part of this retry process first_rq is retried up to 3 times in
> PIO mode (with DMA off).
> 
> During the retry, ide-cd.c: cdrom_start_read is called, which in turn
> calls, restore_request.  restore_request references first_rq->buffer 
> (which is
> NULL) in order to calculate hard_cur_sectors, hard_nr_sectors, etc.
> 
> ie) All of these values will be bogus because of the first_rq->buffer = 
> NULL.
> 
> This request will fail and the IDE core will enter error handling.  IDE
> core generates a new request, sense_rq, in order to request sense.  Attached
> this request is a back pointer to the original first_rq request.
> 
>        ie) sense_rq->buffer = first_rq
> 
> Eventually ide-cd.c:cdrom_end_request is called on sense_rq, and
> then ide-io.c:ide_end_dequeued_request is called on first_rq.  Note
> that ide_end_dequeued_request is called with the bogus values from first_rq.
> 
> The return value essentially depends on the return value of
> ll_rw_blk.c:__end_that_request_first.  The arguements to this function 
> include
> nr_sectors, which as noted above, is bogus.
> 
> This leads to a return of 1 from ll_rw_blk.c:__end_that_request_first
> which eventually leads to an erroneous call to BUG() in
> ide-cd.c:cdrom_end_request.
> 
> I have forced this issue to occur by modifying code to effectively DMA
> timeout on CDROM accesses on i686 and ia64 platforms.  I hit the bug
> 100% of the time.
> 
> It appears that the modification should be to rid the ide-io.c code of
> the rq->buffer = NULL call.
> 
> Patch is based off of latest BK linux-2.5 as of 2005-01-04 09:00.
> 
> --- linux-2.5.orig/drivers/ide/ide-io.c 2005-01-04 09:31:45.000000000 -0500
> +++ linux-2.5/drivers/ide/ide-io.c      2005-01-04 09:32:23.000000000 -0500
> @@ -1205,21 +1205,20 @@
>        HWGROUP(drive)->rq = NULL;
> 
>        rq->errors = 0;
> 
>        if (!rq->bio)
>                goto out;
> 
>        rq->sector = rq->bio->bi_sector;
>        rq->current_nr_sectors = bio_iovec(rq->bio)->bv_len >> 9;
>        rq->hard_cur_sectors = rq->current_nr_sectors;
> -       rq->buffer = NULL;

Probably safer to do

	rq->buffer = bio_data(rq->bio);

-- 
Jens Axboe