linux-kernel - [PATCH 2/3] nospec: Allow index argument to have const-qualified type

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Date:   Fri, 16 Feb 2018 13:20:48 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     mingo@...nel.org
Cc:     linux-arch@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 2/3] nospec: Allow index argument to have const-qualified
 type

From: Rasmus Villemoes <linux@...musvillemoes.dk>

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
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Will Deacon <will.deacon@....com>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---
 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 d6701e34424f..172a19dc35ab 100644
--- a/include/linux/nospec.h
+++ b/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 */