linux-kernel - Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb

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Date:	Thu, 11 Oct 2012 12:01:48 +0200
From:	Bart Van Assche <bvanassche@....org>
To:	James Hogan <james.hogan@...tec.com>
CC:	"James E.J. Bottomley" <JBottomley@...allels.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed

On 10/11/12 11:15, James Hogan wrote:
> The struct scsi_varlen_cdb_hdr is expected to be exactly 10 bytes when
> used in struct osd_cdb_head, but it isn't marked as packed. Some
> architectures will round the struct size up which triggers BUILD_BUG_ON
> compile errors in osd_initiator.c when the outer structs are unexpected
> sizes. This is fixed by marking struct scsi_varlen_cdb_hdr as __packed.
>
> Signed-off-by: James Hogan <james.hogan@...tec.com>
> ---
>   include/scsi/scsi.h |    2 +-
>   1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/include/scsi/scsi.h b/include/scsi/scsi.h
> index 66216c1..3beaef3 100644
> --- a/include/scsi/scsi.h
> +++ b/include/scsi/scsi.h
> @@ -198,7 +198,7 @@ struct scsi_varlen_cdb_hdr {
>   	__u8 additional_cdb_length;         /* total cdb length - 8 */
>   	__be16 service_action;
>   	/* service specific data follows */
> -};
> +} __packed;
>
>   static inline unsigned
>   scsi_varlen_cdb_length(const void *hdr)

Hello James,

Are you aware that __packed can also be used on individual struct 
members and that doing so has a lower performance penalty than using the 
__packed attribute on an entire struct ? See e.g. <linux/sysv_fs.h> for 
an example.

Bart.
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