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

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Date:	Thu, 11 Oct 2012 13:58:16 +0100
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	James Hogan <james.hogan@...tec.com>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed

On Thu, 2012-10-11 at 12:32 +0100, James Hogan wrote:
> On 11/10/12 11:24, James Bottomley wrote:
> > On Thu, 2012-10-11 at 10:15 +0100, 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.
> > 
> > What actual problem have you encountered? The structure is {u8[8], u16}
> > which is naturally packed on every architecture I know about.  I've even
> > built osd_initiator without problem on parisc, which has some of the
> > most rigid alignment rules I've seen.
> 
> Hi James,
> 
> The alignment is fine (the offset of the u16 is 8 bytes), but
> unfortunately with the metag port of gcc, sizeof(struct
> scsi_varlen_cdb_hdr) is rounded up to a 4 byte boundary (even though the
> largest data member alignment is only 2 bytes), which is 12 bytes
> instead of 10.

That sounds to be a bug in your compiler ... it shouldn't be rounding up
structure sizes if the structure can fit in 10 bytes.  This isn't
happening in any other architecture that I know of (otherwise we'd have
had a reported build break).

James


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