linux-kernel - [PATCH stable 5.4 1/3] overflow: Add __must_check attribute to check

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Message-Id: <20250306010756.719024-2-florian.fainelli@broadcom.com>
Date: Wed,  5 Mar 2025 17:07:54 -0800
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Keith Busch <kbusch@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Subject: [PATCH stable 5.4 1/3] overflow: Add __must_check attribute to check_*() helpers

From: Kees Cook <keescook@...omium.org>

commit 9b80e4c4ddaca3501177ed41e49d0928ba2122a8 upstream

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@...musvillemoes.dk>
Link: https://lore.kernel.org/lkml/202008151007.EF679DF@keescook/
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Florian Fainelli <florian.fainelli@...adcom.com>
---
 include/linux/overflow.h | 39 ++++++++++++++++++++++++---------------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 63e7c77ba942..35af574d006f 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -44,6 +44,16 @@
 #define is_non_negative(a) ((a) > 0 || (a) == 0)
 #define is_negative(a) (!(is_non_negative(a)))
 
+/*
+ * Allows for effectively applying __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_overflow(bool overflow)
+{
+	return unlikely(overflow);
+}
+
 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
 /*
  * For simplicity and code hygiene, the fallback code below insists on
@@ -53,32 +63,32 @@
  * 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_overflow(({	\
 	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_overflow(({	\
 	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_overflow(({	\
 	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
 
@@ -191,21 +201,20 @@
 })
 
 
-#define check_add_overflow(a, b, d)					\
+#define check_add_overflow(a, b, d)	__must_check_overflow(		\
 	__builtin_choose_expr(is_signed_type(typeof(a)),		\
 			__signed_add_overflow(a, b, d),			\
-			__unsigned_add_overflow(a, b, d))
+			__unsigned_add_overflow(a, b, d)))
 
-#define check_sub_overflow(a, b, d)					\
+#define check_sub_overflow(a, b, d)	__must_check_overflow(		\
 	__builtin_choose_expr(is_signed_type(typeof(a)),		\
 			__signed_sub_overflow(a, b, d),			\
-			__unsigned_sub_overflow(a, b, d))
+			__unsigned_sub_overflow(a, b, d)))
 
-#define check_mul_overflow(a, b, d)					\
+#define check_mul_overflow(a, b, d)	__must_check_overflow(		\
 	__builtin_choose_expr(is_signed_type(typeof(a)),		\
 			__signed_mul_overflow(a, b, d),			\
-			__unsigned_mul_overflow(a, b, d))
-
+			__unsigned_mul_overflow(a, b, d)))
 
 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 
@@ -228,7 +237,7 @@
  * '*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_overflow(({		\
 	typeof(a) _a = a;						\
 	typeof(s) _s = s;						\
 	typeof(d) _d = d;						\
@@ -238,7 +247,7 @@
 	*_d = (_a_full << _to_shift);					\
 	(_to_shift != _s || is_negative(*_d) || is_negative(_a) ||	\
 	(*_d >> _to_shift) != _a);					\
-})
+}))
 
 /**
  * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
-- 
2.34.1