linux-kernel - Re: [PATCH] x86, pkeys: fix siginfo ABI breakage from new field

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Message-ID: <20160228102629.0ef4ef4f@canb.auug.org.au>
Date:	Sun, 28 Feb 2016 10:26:29 +1100
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Dave Hansen <dave@...1.net>, Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
	akpm@...ux-foundation.org, tglx@...utronix.de, mingo@...e.hu,
	peterz@...radead.org, linux-next@...r.kernel.org, deller@....de
Subject: Re: [PATCH] x86, pkeys: fix siginfo ABI breakage from new field

Hi H.,

On Sat, 27 Feb 2016 11:35:08 -0800 "H. Peter Anvin" <hpa@...or.com> wrote:
>
> On February 27, 2016 11:16:44 AM PST, Dave Hansen <dave@...1.net> wrote:
> >On 02/27/2016 03:41 AM, Ingo Molnar wrote:  
> >> * Stephen Rothwell <sfr@...b.auug.org.au> wrote:  
> >>> > On Fri, 26 Feb 2016 09:44:00 -0800 "H. Peter Anvin"  
> ><hpa@...or.com> wrote:  
> >>>> > > __u64 is okay, "unsigned long" is really messy in the presence  
> >of 32-on-64 bit ABIs...  
> >>> > 
> >>> > Yeah, but unfortunately, any 64 bit scalar type here will change  
> >the  
> >>> > alignment of the enclosing unions on (some) 32 bit platforms and  
> >thus  
> >>> > break the ABI.  
> >> Then a different solution has to be found.  
> >
> >I've acked Stephen's initial patch changing the 'u64' to an 'int'.  x86
> >only needs 4 bits, and in the remote chance that a future
> >implementation
> >needed more space, we could easily add a second 32-bit field "_pkey_hi"
> >or something that wouldn't have the alignment issues of a true 64-bit
> >type.
> >
> >How should we get Stephen's patch in to the tip tree?  
> 
> u32?

It would have to be __u32, but we already use int and unsigned int
extensively in the siginfo structure (which are both always assumed to
be 32 bits).  So "unsigned int" probably makes most sense.

I will submit that patch - with Dave's Ack.
-- 
Cheers,
Stephen Rothwell