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Subject[PATCH 4.15 105/122] nospec: Allow index argument to have const-qualified type
4.15-stable review patch.  If anyone has any objections, please let me know.


From: Rasmus Villemoes <>

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).

Signed-off-by: Rasmus Villemoes <>
Signed-off-by: Dan Williams <>
Acked-by: Linus Torvalds <>
Cc: Andy Lutomirski <>
Cc: Arjan van de Ven <>
Cc: Borislav Petkov <>
Cc: Dave Hansen <>
Cc: David Woodhouse <>
Cc: Greg Kroah-Hartman <>
Cc: Josh Poimboeuf <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Cc: Will Deacon <>
Signed-off-by: Ingo Molnar <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -72,7 +72,6 @@ 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 */

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