linux-kernel - Re: scsi: address leak in the error path of discard page allocation

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Message-ID: <20100702105241.GD26318@lst.de>
Date:	Fri, 2 Jul 2010 12:52:41 +0200
From:	Christoph Hellwig <hch@....de>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	James.Bottomley@...e.de, snitzer@...hat.com, axboe@...nel.dk,
	hch@....de, linux-scsi@...r.kernel.org, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: scsi: address leak in the error path of discard page allocation

On Fri, Jul 02, 2010 at 01:53:14PM +0900, FUJITA Tomonori wrote:
> Seems that scsi-ml calls scsi_unprep_request() for not-prepped
> requests in scsi_init_io error path. So we could move that
> scsi_unprep_request() to the error path in scsi_prep_return(). Then we
> can free discard page in the single place.
> 
> Applying the rule strictly is fine by me too; we remove
> scsi_unprep_request() in scsi_init_io error path and clean up things
> in each prep function's error path.

That would be my preference.  Making sure a function cleans up all
allocations / state changes on errors means code is a lot fragile and
easier to understand.

> Btw, blk_clear_request_payload() is necessary?
> 
> Making sure that a request is clean is not a bad idea but if we hit
> BLKPREP_KILL or BLKPREP_DEFER, we call
> blk_end_request(). blk_end_request() can free a request properly even
> if we don't do something like blk_clear_request_payload?

For BLKPREP_KILL we do call __blk_end_request_all, but for
BLKPREP_DEFER we don't.  In that case we just leave it on the queue for
a later retry.  So we either have to clean it up, or leave the detect
the case of a partially constructed command in ->prep_fn.  I think
cleaning up properly and having defined state when entering ->prep_fn is
the better variant.
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