[PATCH 3/5] overflow: Introduce add_would_overflow()

Kees Cook
2024-01-29T18:34:12Z

For instances where only the overflow needs to be checked (and the sum
isn't used), provide the new helper add_would_overflow(), which is
a wrapper for check_add_overflow().

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/overflow.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 210e5602e89b..3c46c648d2e8 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -104,6 +104,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
__builtin_add_overflow(__filter_integral(a), b, \
__filter_ptrint(d))))

+/**
+ * add_would_overflow() - Check if an addition would overflow
+ * @var: variable to add to that is checked for overflow
+ * @offset: value to add
+ *
+ * Returns true if the sum would overflow.
+ *
+ * To keep a copy of the sum when the addition doesn't overflow, use
+ * check_add_overflow() instead.
+ */
+#define add_would_overflow(var, offset) \
+ __must_check_overflow(({ \
+ typeof(var) __result; \
+ check_add_overflow(var, offset, &__result); \
+ }))
+
/**
* check_sub_overflow() - Calculate subtraction with overflow checking
* @a: minuend; value to subtract from
--
2.34.1