linux-kernel - [PATCH 3.2 098/153] nospec: Allow index argument to have const-qualified type

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Date:   Wed, 30 May 2018 11:52:41 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Borislav Petkov" <bp@...en8.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Ingo Molnar" <mingo@...nel.org>,
        "Dan Williams" <dan.j.williams@...el.com>,
        linux-arch@...r.kernel.org, "Will Deacon" <will.deacon@....com>,
        "David Woodhouse" <dwmw2@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Arjan van de Ven" <arjan@...ux.intel.com>,
        "Rasmus Villemoes" <linux@...musvillemoes.dk>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>
Subject: [PATCH 3.2 098/153] nospec: Allow index argument to have
 const-qualified type

3.2.102-rc1 review patch.  If anyone has any objections, please let me know.

------------------

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

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 <linux@...musvillemoes.dk>
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: David Woodhouse <dwmw2@...radead.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Deacon <will.deacon@....com>
Cc: linux-arch@...r.kernel.org
Link: http://lkml.kernel.org/r/151881604837.17395.10812767547837568328.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/nospec.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/include/linux/nospec.h
+++ b/include/linux/nospec.h
@@ -53,7 +53,6 @@ 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 */