linux-kernel - [tip:mm/pkeys] mm/pkeys: Fix siginfo ABI breakage caused by new u64 field

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Date:	Thu, 3 Mar 2016 08:53:08 -0800
From:	tip-bot for Dave Hansen <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	dave@...1.net, peterz@...radead.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, deller@....de, sfr@...b.auug.org.au,
	linux-kernel@...r.kernel.org, mingo@...nel.org, hpa@...or.com,
	dave.hansen@...ux.intel.com, tglx@...utronix.de
Subject: [tip:mm/pkeys] mm/pkeys: Fix siginfo ABI breakage caused by new u64
 field

Commit-ID:  16bc7477807393efb5b81f875888ee9221ead3a1
Gitweb:     http://git.kernel.org/tip/16bc7477807393efb5b81f875888ee9221ead3a1
Author:     Dave Hansen <dave.hansen@...ux.intel.com>
AuthorDate: Tue, 1 Mar 2016 04:54:51 -0800
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 3 Mar 2016 16:44:21 +0100

mm/pkeys: Fix siginfo ABI breakage caused by new u64 field

Stephen Rothwell reported this linux-next build failure:

	http://lkml.kernel.org/r/20160226164406.065a1ffc@canb.auug.org.au

... caused by the Memory Protection Keys patches from the tip tree triggering
a newly introduced build-time sanity check on an ARM build, because they changed
the ABI of siginfo in an unexpected way.

If u64 has a natural alignment of 8 bytes (this is rare, most 32-bit
platforms align it to 4 bytes), then the leadup to the _sifields union
matters:

typedef struct siginfo {
        int si_signo;
        int si_errno;
        int si_code;

        union {
	...
        } _sifields;
} __ARCH_SI_ATTRIBUTES siginfo_t;

Note how the first 3 fields give us 12 bytes, so _sifields is not 8
naturally bytes aligned.

Before the _pkey field addition the largest element of _sifields (on
32-bit platforms) was 32 bits. With the u64 added, the minimum alignment
requirement increased to 8 bytes on those (rare) 32-bit platforms. Thus
GCC padded the space after si_code with 4 extra bytes, and shifted all
_sifields offsets by 4 bytes - breaking the ABI of all of those
remaining fields.

On 64-bit platforms this problem was hidden due to _sifields already
having numerous fields with natural 8 bytes alignment (pointers).

To fix this, we replace the u64 with an '__u32'.  The __u32 does not
increase the minimum alignment requirement of the union, and it is
also large enough to store the 16-bit pkey we have today on x86.

Reported-by: Stehen Rothwell <sfr@...b.auug.org.au>
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: Dave Hansen <dave@...1.net>
Cc: Helge Deller <deller@....de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-next@...r.kernel.org
Fixes: cd0ea35ff551 ("signals, pkeys: Notify userspace about protection key faults")
Link: http://lkml.kernel.org/r/20160301125451.02C7426D@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/ia64/include/uapi/asm/siginfo.h | 2 +-
 arch/mips/include/uapi/asm/siginfo.h | 2 +-
 include/uapi/asm-generic/siginfo.h   | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/ia64/include/uapi/asm/siginfo.h b/arch/ia64/include/uapi/asm/siginfo.h
index 0151cfa..f72bf01 100644
--- a/arch/ia64/include/uapi/asm/siginfo.h
+++ b/arch/ia64/include/uapi/asm/siginfo.h
@@ -70,7 +70,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _pkey;
 			};
 		} _sigfault;
 
diff --git a/arch/mips/include/uapi/asm/siginfo.h b/arch/mips/include/uapi/asm/siginfo.h
index 6f4edf0..cc49dc2 100644
--- a/arch/mips/include/uapi/asm/siginfo.h
+++ b/arch/mips/include/uapi/asm/siginfo.h
@@ -93,7 +93,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _pkey;
 			};
 		} _sigfault;
 
diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
index 90384d5..1abaf62 100644
--- a/include/uapi/asm-generic/siginfo.h
+++ b/include/uapi/asm-generic/siginfo.h
@@ -98,7 +98,7 @@ typedef struct siginfo {
 					void __user *_upper;
 				} _addr_bnd;
 				/* used when si_code=SEGV_PKUERR */
-				u64 _pkey;
+				__u32 _pkey;
 			};
 		} _sigfault;