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

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Message-ID: <5118EA3B.9010904@imgtec.com>
Date:	Mon, 11 Feb 2013 12:55:23 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	<linux-scsi@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed

Hi James,

On 11/10/12 15:10, James Hogan wrote:
> On 11/10/12 13:58, James Bottomley wrote:
>> 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).
> 
> This was my initial thought, and I share your feeling that this isn't
> ideal compiler behaviour, however it's pretty much set in stone as part
> of our ABI now. Having talked to one of our compiler folk about it
> here's what he had to say:
>> In GCC 4.2 the following backends have STRUCTURE_SIZE_BOUNDARY set to >=32 which will trigger this behaviour:
>>
>> Alpha+unicos
>> Arm prior to AAPCS
>> Mt
>>
>> This is within standards to my knowledge.

What are the chances of getting this patch accepted? I believe it is
correct and the compiler behaviour is within standards, and hopefully
arch/metag will be merged for v3.9 so without this patch the build of
osd_initiator.c will be broken on an in-tree architecture.

Thanks
James

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