Re: memory allocation in sg_io()

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Jan 10, 2008 at 02:19:08PM +0100, Oliver Neukum wrote:
> Am Donnerstag, 10. Januar 2008 14:05:25 schrieb Boaz Harrosh:
> > On Thu, Jan 10 2008 at 14:33 +0200, Oliver Neukum <oliver@neukum.org> wrote:
> > > Hi,
> > > 
> > > could you explain to me why this code can get away with allocating the
> > > sense buffer on the stack?
> > > 
> > > static int sg_io(struct file *file, struct request_queue *q,
> > > 		struct gendisk *bd_disk, struct sg_io_hdr *hdr)
> > > {
> > > 	unsigned long start_time;
> > > 	int writing = 0, ret = 0, has_write_perm = 0;
> > > 	struct request *rq;
> > > 	char sense[SCSI_SENSE_BUFFERSIZE];
> 
> Yes, you are doing DMA on the stack.

Nobody does DMA to rq->sense.  ub does a memcpy to it.  cciss does a
memcpy to it.  scsi_lib assigns its own buffer to req->sense, ignoring
the one passed down.

That's how this code gets away with it.

-- 
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