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

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Date:	Fri, 26 Feb 2016 09:44:00 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dave Hansen <dave@...1.net>, linux-kernel@...r.kernel.org
CC:	dave.hansen@...ux.intel.com, sfr@...b.auug.org.au,
	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

On February 26, 2016 9:34:27 AM PST, Dave Hansen <dave@...1.net> wrote:
>
>From: Dave Hansen <dave.hansen@...ux.intel.com>
>
>Stephen Rothwell reported:
>
>	http://lkml.kernel.org/r/20160226164406.065a1ffc@canb.auug.org.au
>
>that the Memory Protection Keys patches from the tip tree broke
>a build-time check on an ARM build because they changed the ABI
>of siginfo.
>
>A u64 was used for the protection key field in siginfo.  When the
>containing union was aligned, this u64 unioned nicely with the
>two 'void *'s in _addr_bnd.  But, on 32-bit, if the union was
>unaligned, the u64 might grow the size of the union, breaking the
>ABI for subsequent fields.
>
>To fix this, we replace the u64 with an 'unsigned long'.  The long
>is guaranteed to union well with the pointers from _addr_bnd.  It
>is also plenty large enough to store the 16-bit pkey we have today
>on x86.  This also has the advantage that it allows existing 64-bit
>userspace to keep working without modification.
>
>I also shouldn't have been using a u64 in a userspace API to begin
>with.
>
>Fixes: cd0ea35ff551 ("signals, pkeys: Notify userspace about protection
>key faults")
>Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
>Cc: Stephen Rothwell <sfr@...b.auug.org.au>
>Cc: Andrew Morton <akpm@...ux-foundation.org>
>Cc: Thomas Gleixner <tglx@...utronix.de>
>Cc: Ingo Molnar <mingo@...e.hu>
>Cc: "H. Peter Anvin" <hpa@...or.com>
>Cc: Peter Zijlstra <peterz@...radead.org>
>Cc: linux-next@...r.kernel.org
>Cc: linux-kernel@...r.kernel.org
>Cc: Helge Deller <deller@....de>
>---
>
> b/arch/ia64/include/uapi/asm/siginfo.h |    2 +-
> b/arch/mips/include/uapi/asm/siginfo.h |    2 +-
> b/include/uapi/asm-generic/siginfo.h   |    2 +-
> 3 files changed, 3 insertions(+), 3 deletions(-)
>
>diff -puN include/uapi/asm-generic/siginfo.h~pkeys-101-fix-siginfo
>include/uapi/asm-generic/siginfo.h
>---
>a/include/uapi/asm-generic/siginfo.h~pkeys-101-fix-siginfo	2016-02-26
>08:50:47.760659292 -0800
>+++ b/include/uapi/asm-generic/siginfo.h	2016-02-26 08:52:08.591330838
>-0800
>@@ -98,7 +98,7 @@ typedef struct siginfo {
> 					void __user *_upper;
> 				} _addr_bnd;
> 				/* used when si_code=SEGV_PKUERR */
>-				u64 _pkey;
>+				unsigned long _pkey;
> 			};
> 		} _sigfault;
> 
>diff -puN arch/mips/include/uapi/asm/siginfo.h~pkeys-101-fix-siginfo
>arch/mips/include/uapi/asm/siginfo.h
>---
>a/arch/mips/include/uapi/asm/siginfo.h~pkeys-101-fix-siginfo	2016-02-26
>08:51:50.357502608 -0800
>+++ b/arch/mips/include/uapi/asm/siginfo.h	2016-02-26
>08:51:55.206722873 -0800
>@@ -93,7 +93,7 @@ typedef struct siginfo {
> 					void __user *_upper;
> 				} _addr_bnd;
> 				/* used when si_code=SEGV_PKUERR */
>-				u64 _pkey;
>+				unsigned long _pkey;
> 			};
> 		} _sigfault;
> 
>diff -puN arch/ia64/include/uapi/asm/siginfo.h~pkeys-101-fix-siginfo
>arch/ia64/include/uapi/asm/siginfo.h
>---
>a/arch/ia64/include/uapi/asm/siginfo.h~pkeys-101-fix-siginfo	2016-02-26
>08:51:50.413505152 -0800
>+++ b/arch/ia64/include/uapi/asm/siginfo.h	2016-02-26
>08:52:00.806977252 -0800
>@@ -70,7 +70,7 @@ typedef struct siginfo {
> 					void __user *_upper;
> 				} _addr_bnd;
> 				/* used when si_code=SEGV_PKUERR */
>-				u64 _pkey;
>+				unsigned long _pkey;
> 			};
> 		} _sigfault;
> 
>_

__u64 is okay, "unsigned long" is really messy in the presence of 32-on-64 bit ABIs...
-- 
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