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Subject[PATCH] overflow: Add __must_check attribute to check_*() helpers
Since the destination variable of the check_*_overflow() helpers will
contain a wrapped value on failure, it would be best to make sure callers
really did check the return result of the helper. Adjust the macros to use
a bool-wrapping static inline that is marked with __must_check. This means
the macros can continue to have their type-agnostic behavior while gaining
the function attribute (that cannot be applied directly to macros).

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/overflow.h | 51 +++++++++++++++++++++++-----------------
1 file changed, 30 insertions(+), 21 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 93fcef105061..ef7d538c2d08 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -43,6 +43,16 @@
#define is_non_negative(a) ((a) > 0 || (a) == 0)
#define is_negative(a) (!(is_non_negative(a)))

+/*
+ * Allows to effectively us apply __must_check to a macro so we can have
+ * both the type-agnostic benefits of the macros while also being able to
+ * enforce that the return value is, in fact, checked.
+ */
+static inline bool __must_check __must_check_bool(bool condition)
+{
+ return unlikely(condition);
+}
+
#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
/*
* For simplicity and code hygiene, the fallback code below insists on
@@ -52,32 +62,32 @@
* alias for __builtin_add_overflow, but add type checks similar to
* below.
*/
-#define check_add_overflow(a, b, d) ({ \
+#define check_add_overflow(a, b, d) __must_check_bool(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_add_overflow(__a, __b, __d); \
-})
+}))

-#define check_sub_overflow(a, b, d) ({ \
+#define check_sub_overflow(a, b, d) __must_check_bool(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_sub_overflow(__a, __b, __d); \
-})
+}))

-#define check_mul_overflow(a, b, d) ({ \
+#define check_mul_overflow(a, b, d) __must_check_bool(({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
typeof(d) __d = (d); \
(void) (&__a == &__b); \
(void) (&__a == __d); \
__builtin_mul_overflow(__a, __b, __d); \
-})
+}))

#else

@@ -190,21 +200,20 @@
})


-#define check_add_overflow(a, b, d) \
- __builtin_choose_expr(is_signed_type(typeof(a)), \
- __signed_add_overflow(a, b, d), \
- __unsigned_add_overflow(a, b, d))
+#define check_add_overflow(a, b, d) \
+ __must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+ __signed_add_overflow(a, b, d), \
+ __unsigned_add_overflow(a, b, d)))

-#define check_sub_overflow(a, b, d) \
- __builtin_choose_expr(is_signed_type(typeof(a)), \
- __signed_sub_overflow(a, b, d), \
- __unsigned_sub_overflow(a, b, d))
-
-#define check_mul_overflow(a, b, d) \
- __builtin_choose_expr(is_signed_type(typeof(a)), \
- __signed_mul_overflow(a, b, d), \
- __unsigned_mul_overflow(a, b, d))
+#define check_sub_overflow(a, b, d) \
+ __must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+ __signed_sub_overflow(a, b, d), \
+ __unsigned_sub_overflow(a, b, d)))

+#define check_mul_overflow(a, b, d) \
+ __must_check_bool(__builtin_choose_expr(is_signed_type(typeof(a)), \
+ __signed_mul_overflow(a, b, d), \
+ __unsigned_mul_overflow(a, b, d)))

#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */

@@ -227,7 +236,7 @@
* '*d' will hold the results of the attempted shift, but is not
* considered "safe for use" if false is returned.
*/
-#define check_shl_overflow(a, s, d) ({ \
+#define check_shl_overflow(a, s, d) __must_check_bool(({ \
typeof(a) _a = a; \
typeof(s) _s = s; \
typeof(d) _d = d; \
@@ -237,7 +246,7 @@
*_d = (_a_full << _to_shift); \
(_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
(*_d >> _to_shift) != _a); \
-})
+}))

/**
* array_size() - Calculate size of 2-dimensional array.
--
2.25.1

--
Kees Cook

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