linux-kernel - [PATCH] linux/nospec.h: allow index argument to have const-qualified type

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Date:   Thu, 15 Feb 2018 20:52:07 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] linux/nospec.h: 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).

Cc: stable@...r.kernel.org
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
cc stable because if this is ok, there will probably be future users
relying on this which also get cc'ed to -stable.

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

diff --git a/include/linux/nospec.h b/include/linux/nospec.h
index fbc98e2c8228..132e3f5a2e0d 100644
--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -72,7 +72,6 @@ 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 */
-- 
2.15.1