x86/extable: Ensure entries are swapped completely when sorting

The x86 exception table sorting was changed in this recent commit:

  29934b0fb8ff ("x86/extable: use generic search and sort routines")

... to use the arch independent code in lib/extable.c. However, the
patch was mangled somehow on its way into the kernel from the last
version posted at:

  https://lkml.org/lkml/2016/1/27/232

The committed version kind of attempted to incorporate the changes of
contemporary commit done in the x86 tree:

  548acf19234d ("x86/mm: Expand the exception table logic to allow new handling options")

... as in _completely_ _ignoring_ the x86 specific 'handler' member of
struct exception_table_entry. This effectively broke the sorting as
entries will only be partly swapped now.

Fortunately, the x86 Kconfig selects BUILDTIME_EXTABLE_SORT, so the
exception table doesn't need to be sorted at runtime. However, in case
that ever changes, we better not break the exception table sorting just
because of that.

Fix this by providing a swap_ex_entry_fixup() macro that takes care of
the 'handler' member.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1462914422-2911-1-git-send-email-minipli@googlemail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/uaccess.h

@@ -108,6 +108,14 @@ struct exception_table_entry {
 
 #define ARCH_HAS_RELATIVE_EXTABLE
 
+#define swap_ex_entry_fixup(a, b, tmp, delta)			\
+	do {							\
+		(a)->fixup = (b)->fixup + (delta);		\
+		(b)->fixup = (tmp).fixup - (delta);		\
+		(a)->handler = (b)->handler + (delta);		\
+		(b)->handler = (tmp).handler - (delta);		\
+	} while (0)
+
 extern int fixup_exception(struct pt_regs *regs, int trapnr);
 extern bool ex_has_fault_handler(unsigned long ip);
 extern int early_fixup_exception(unsigned long *ip);