linux-kernel - [PATCH 2/5] compiler.h: Add read_word_at_a

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Message-Id: <20180201180052.32643-2-aryabinin@virtuozzo.com>
Date:   Thu,  1 Feb 2018 21:00:49 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eryu Guan <eguan@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Chris Metcalf <metcalf@...m.mit.edu>,
        David Laight <David.Laight@...lab.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH 2/5] compiler.h: Add read_word_at_a_time() function.

Sometimes we know that it's safe to do potentially out-of-bounds access
because we know it won't cross a page boundary. Still, KASAN will
report this as a bug.

Add read_word_at_a_time() function which is supposed to be used in such
cases. In read_word_at_a_time() KASAN performs relaxed check - only the
first byte of access is validated.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 include/linux/compiler.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index b8414ecf9ba1..0eb5bb0902e9 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -243,6 +243,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  * required ordering.
  */
 #include <asm/barrier.h>
+#include <linux/kasan-checks.h>
 
 #define __READ_ONCE(x, check)						\
 ({									\
@@ -262,6 +263,13 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
  */
 #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
 
+static __no_kasan_or_inline
+unsigned long read_word_at_a_time(const void *addr)
+{
+	kasan_check_read(addr, 1);
+	return *(unsigned long *)addr;
+}
+
 #define WRITE_ONCE(x, val) \
 ({							\
 	union { typeof(x) __val; char __c[1]; } __u =	\
-- 
2.13.6