[PATCH] [v4] x86, pkeys: fix siginfo ABI breakage from new field

[PATCH] [v4] x86, pkeys: fix siginfo ABI breakage from new field

[Dave Hansen]

Update changelog with better description of the issue from Ingo.

--

From: Dave Hansen <dave.hansen@linux.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.

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 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@linux.intel.com>
Acked-by: Stehen Rothwell <sfr@canb.auug.org.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-next@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Helge Deller <deller@gmx.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;
 
_

Re: [PATCH] [v4] x86, pkeys: fix siginfo ABI breakage from new field

[Ingo Molnar]

* Dave Hansen <dave@sr71.net> wrote:

> 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.

The 'union well' sentence is really a leftover from the earlier changelog (the 
problem was never about interaction between union members) - a better explantion 
is:

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

I fixed this up locally, no need to resend.

Thanks,

	Ingo