Re: kern/39897: sizeof(struct gpt_hdr);

[Jason Thorpe <thorpej%shagadelic.org@localhost>]

The following reply was made to PR kern/39897; it has been noted by GNATS.

From: Jason Thorpe <thorpej%shagadelic.org@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: kern-bug-people%netbsd.org@localhost,
 gnats-admin%netbsd.org@localhost,
 netbsd-bugs%netbsd.org@localhost,
 jakllsch%kollasch.net@localhost
Subject: Re: kern/39897: sizeof(struct gpt_hdr);
Date: Wed, 12 Nov 2008 14:47:31 -0800

 On Nov 12, 2008, at 2:45 PM, David Laight wrote:
 
 > The following reply was made to PR kern/39897; it has been noted by  
 > GNATS.
 >
 > From: David Laight <david%l8s.co.uk@localhost>
 > To: Jason Thorpe <thorpej%shagadelic.org@localhost>
 > Cc: gnats-bugs%NetBSD.org@localhost
 > Subject: Re: kern/39897: sizeof(struct gpt_hdr);
 > Date: Wed, 12 Nov 2008 22:40:39 +0000
 >
 > On Tue, Nov 11, 2008 at 04:56:14PM -0800, Jason Thorpe wrote:
 >>
 >> On Nov 11, 2008, at 3:55 PM, David Laight wrote:
 >>
 >>> On Tue, Nov 11, 2008 at 07:00:01PM +0000, jakllsch%kollasch.net@localhost  
 >>> wrote:
 >>>>> Synopsis:       sizeof(struct gpt_hdr); is "wrong" on amd64
 >>>
 >>>> Perhaps add a '__packed' attribute to the GPT structures.
 >>>
 >>> There is a typedef for a 64bit int with 32bit alignment somewhere.
 >>> That is better than applying packed to the entire structure.
 >>
 >> No, there's not.
 >>
 >> The problem is likely due to the fact that the struct has 3  
 >> uint32_t's
 >> at the end, rather than 2 or 4.
 >
 > To get the entire structure to have 4-byte alignment, the uint64_t
 > fields need to be marked as having 4-byte alignment.
 > This is done in some of the compat code for i386.
 
 Alignment is not the issue.  Padding at the end is.
 
 The solution to this problem is trivial.  I have a patch to check in.
 
 >
 >
 > However I suspect the gpt_hdr never exists as an array, so the
 > overall size/padding should be handled differently.
 >
 > Otherwise you get rather long code sequences if you have to access
 > the structure members on systems that can't perform misaligned  
 > transfers.
 >
 >      David
 >
 > -- 
 > David Laight: david%l8s.co.uk@localhost
 >
 
 -- thorpej