nospec: Allow index argument to have const-qualified type

The last expression in a statement expression need not be a bare
variable, quoting gcc docs

  The last thing in the compound statement should be an expression
  followed by a semicolon; the value of this subexpression serves as the
  value of the entire construct.

and we already use that in e.g. the min/max macros which end with a
ternary expression.

This way, we can allow index to have const-qualified type, which will in
some cases avoid the need for introducing a local copy of index of
non-const qualified type. That, in turn, can prevent readers not
familiar with the internals of array_index_nospec from wondering about
the seemingly redundant extra variable, and I think that's worthwhile
considering how confusing the whole _nospec business is.

The expression _i&_mask has type unsigned long (since that is the type
of _mask, and the BUILD_BUG_ONs guarantee that _i will get promoted to
that), so in order not to change the type of the whole expression, add
a cast back to typeof(_i).

Signed-off-by: Rasmus Villemoes <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/151881604837.17395.10812767547837568328.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <[email protected]>

include/linux/nospec.h

@@ -52,7 +52,6 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 	BUILD_BUG_ON(sizeof(_i) > sizeof(long));			\
 	BUILD_BUG_ON(sizeof(_s) > sizeof(long));			\
 									\
-	_i &= _mask;							\
-	_i;								\
+	(typeof(_i)) (_i & _mask);					\
 })
 #endif /* _LINUX_NOSPEC_H */