linux-kernel - [PATCH] overflow: Allow non-type arg to type_max() and type

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Message-Id: <20240301062221.work.840-kees@kernel.org>
Date: Thu, 29 Feb 2024 22:22:26 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Kees Cook <keescook@...omium.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] overflow: Allow non-type arg to type_max() and type_min()

A common use of type_max() is to find the max for the type of a
variable. Using the pattern type_max(typeof(var)) is needlessly
verbose. Instead, since typeof(type) == type we can just explicitly
call typeof() on the argument to type_max() and type_min(). Add
wrappers for readability.

We can do some replacements right away:

$ git grep '\btype_\(min\|max\)(typeof' | wc -l
11

Signed-off-by: Kees Cook <keescook@...omium.org>
---
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.kernel.org
---
 include/linux/overflow.h | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index dede374832c9..e1f4814a2401 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -31,8 +31,10 @@
  * credit to Christian Biere.
  */
 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
-#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
-#define type_min(T) ((T)((T)-type_max(T)-(T)1))
+#define __type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
+#define type_max(t)	__type_max(typeof(t))
+#define __type_min(T) ((T)((T)-type_max(T)-(T)1))
+#define type_min(t)	__type_min(typeof(t))
 
 /*
  * Avoids triggering -Wtype-limits compilation warning,
@@ -207,10 +209,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 
 #define __overflows_type_constexpr(x, T) (			\
 	is_unsigned_type(typeof(x)) ?				\
-		(x) > type_max(typeof(T)) :			\
+		(x) > type_max(T) :				\
 	is_unsigned_type(typeof(T)) ?				\
-		(x) < 0 || (x) > type_max(typeof(T)) :		\
-	(x) < type_min(typeof(T)) || (x) > type_max(typeof(T)))
+		(x) < 0 || (x) > type_max(T) :			\
+	(x) < type_min(T) || (x) > type_max(T))
 
 #define __overflows_type(x, T)		({	\
 	typeof(T) v = 0;			\
-- 
2.34.1