linux-kernel - Re: linux-next: build failure after merge of the akpm-current tree

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Date:	Fri, 26 Feb 2016 08:14:48 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Helge Deller <deller@....de>
Subject: Re: linux-next: build failure after merge of the akpm-current tree

On 02/25/2016 09:44 PM, Stephen Rothwell wrote:
> The addition of the "u64 _pkey" has presumably changed the alignment of
> the enclosing union on (some) 32 bit platforms and so added padding
> after the si_code field.  This is a user API issue. :-(
> 
> [As an aside, I am pretty sure that we should not be using "u64" in a
> uapi header in any case.]

Ahh, so if _addr_bnd wasn't 64-bit-aligned, the compiler is free to
align it and enlarge the structure.  But, this only applied to
architectures where _addr_bnd wasn't 64-bit-aligned:

Would anybody object to _pkey being an 'unsigned long'?  It would at
least keep existing 64-bit userspace (mostly my tests) from having to
change.

Here's the snippet of the struct in question:

>                         union {
>                                 /* used when si_code=SEGV_BNDERR */
>                                 struct {
>                                         void __user *_lower;
>                                         void __user *_upper;
>                                 } _addr_bnd;
>                                 /* used when si_code=SEGV_PKUERR */
>                                 u64 _pkey;
>                         };