linux-kernel - [PATCH] [v3] x86, pkeys: fix siginfo ABI breakage from new field

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Message-Id: <20160229221733.DC2C56B7@viggo.jf.intel.com>
Date:	Mon, 29 Feb 2016 14:17:33 -0800
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org
Cc:	Dave Hansen <dave@...1.net>, dave.hansen@...ux.intel.com,
	sfr@...b.auug.org.au, akpm@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu, hpa@...or.com,
	peterz@...radead.org, linux-next@...r.kernel.org, deller@....de
Subject: [PATCH] [v3] x86, pkeys: fix siginfo ABI breakage from new field


This responds to the feedback from Ingo that we should be using
explicitly-sized types and fixes a typo in the patch description.

--

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 '__u32'.  The __u32 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.

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>
Acked-by: Stehen 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	2016-02-29 09:22:45.327228965 -0800
+++ b/include/uapi/asm-generic/siginfo.h	2016-02-29 09:22:45.333229241 -0800
@@ -98,7 +98,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _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	2016-02-29 09:22:45.330229103 -0800
+++ b/arch/mips/include/uapi/asm/siginfo.h	2016-02-29 09:22:45.333229241 -0800
@@ -93,7 +93,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _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	2016-02-29 09:22:45.331229149 -0800
+++ b/arch/ia64/include/uapi/asm/siginfo.h	2016-02-29 09:22:45.333229241 -0800
@@ -70,7 +70,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _pkey;
 			};
 		} _sigfault;
 
_