Patch "nospec: Allow index argument to have const-qualified type" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

nospec: Allow index argument to have const-qualified type

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
nospec-allow-index-argument-to-have-const-qualified-type.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Rasmus Villemoes <[email protected]>
Date: Fri, 16 Feb 2018 13:20:48 -0800
Subject: nospec: Allow index argument to have const-qualified type

From: Rasmus Villemoes <[email protected]>

commit b98c6a160a057d5686a8c54c79cc6c8c94a7d0c8 upstream.

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).

---
 include/linux/nospec.h |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

@@ -66,7 +66,6 @@ static inline unsigned long array_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 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/nospec-allow-index-argument-to-have-const-qualified-type.patch