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

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Date:	Thu, 11 Oct 2012 14:34:00 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	James Hogan <james.hogan@...tec.com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH] scsi: make struct scsi_varlen_cdb_hdr packed

> > 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).

It's not a bug for the alignment rules of the processor as far as I can
see. The architectural definition is perfectly entitled to have tail
padding in this case.

The x86 equivalent would be

struct foo {
	double x;
	int a;
};


which is *NOT* 12 bytes long.

It is indeed a portability bug in the scsi layer.

Alan
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