On 9/26/22 3:37 AM, Kees Cook wrote:
Add overflows_type() to test if a variable or constant value would
overflow another variable or type. This can be used as a constant
expression for static_assert() (which requires a constant
expression[1][2]) when used on constant values. This must be constructed
manually, since __builtin_add_overflow() does not produce a constant
expression[3].
Additionally adds __castable_to_type(), similar to __same_type(), for
checking if a constant value will fit in a given type (i.e. it could
be cast to the type without overflow).
Add unit tests for overflows_type(), __same_type(), and
__castable_to_type() to the existing KUnit "overflow" test.
[1] https://en.cppreference.com/w/c/language/_Static_assert
[2] C11 standard (ISO/IEC 9899:2011): 6.7.10 Static assertions
[3] https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html
6.56 Built-in Functions to Perform Arithmetic with Overflow Checking
Built-in Function: bool __builtin_add_overflow (type1 a, type2 b,
type3 *res)
Cc: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Tom Rix <trix@xxxxxxxxxx>
Cc: Daniel Latypov <dlatypov@xxxxxxxxxx>
Cc: Vitor Massaru Iha <vitor@xxxxxxxxxxx>
Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
Cc: llvm@xxxxxxxxxxxxxxx
Co-developed-by: Gwan-gyeong Mun <gwan-gyeong.mun@xxxxxxxxx>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
include/linux/compiler.h | 1 +
include/linux/overflow.h | 48 +++++
lib/overflow_kunit.c | 393 ++++++++++++++++++++++++++++++++++++++-
3 files changed, 441 insertions(+), 1 deletion(-)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 7713d7bcdaea..c631107e93b1 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -244,6 +244,7 @@ static inline void *offset_to_ptr(const int *off)
* bool and also pointer types.
*/
#define is_signed_type(type) (((type)(-1)) < (__force type)1)
+#define is_unsigned_type(type) (!is_signed_type(type))
/*
* This is needed in functions which generate the stack canary, see
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 19dfdd74835e..c8cbeae5f4f8 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -127,6 +127,54 @@ static inline bool __must_check __must_check_overflow(bool overflow)
(*_d >> _to_shift) != _a); \
}))
+#define __overflows_type_constexpr(x, T) ( \
+ is_unsigned_type(typeof(x)) ? \
+ (x) > type_max(typeof(T)) ? 1 : 0 \
+ : is_unsigned_type(typeof(T)) ? \
+ (x) < 0 || (x) > type_max(typeof(T)) ? 1 : 0 \
+ : (x) < type_min(typeof(T)) || \
+ (x) > type_max(typeof(T)) ? 1 : 0 )
+
+#define __overflows_type(x, T) ({ \
+ typeof(T) v = 0; \
+ check_add_overflow((x), v, &v); \
+})
+
+/**
+ * overflows_type - helper for checking the overflows between value, variables,
+ * or data type
+ *
+ * @n: source constant value or variable to be checked
+ * @T: destination variable or data type proposed to store @x
+ *
+ * Compares the @x expression for whether or not it can safely fit in
+ * the storage of the type in @T. @x and @T can have different types.
+ * If @x is a conxtant expression, this will also resolve to a constant
+ * expression.
+ *
+ * Returns: true if overflow can occur, false otherwise.
+ */
+#define overflows_type(n, T) \
+ __builtin_choose_expr(__is_constexpr(n), \
+ __overflows_type_constexpr(n, T), \
+ __overflows_type(n, T))
+
+/**
+ * __castable_to_type - like __same_type(), but also allows for casted literals
+ *
+ * @n: variable or constant value
+ * @T: data type or variable
+ *
+ * Unlike the __same_type() macro, this allows a constant value as the
+ * first argument. If this value would not overflow into an assignment
+ * of the second argument's type, it returns true. Otherwise, this falls
+ * back to __same_type().
+ */
+#define __castable_to_type(n, T) \
+ __builtin_choose_expr(__is_constexpr(n), \
+ !__overflows_type_constexpr(n, T), \
+ __same_type(n, T))
+
This name is fine, but I prefer the __same_typable you suggested as a
comment in the previous patch better, what do you think?
( __castable_to_type(n, T); The macro name seems to handle if type
casting is possible to the second argument type from the first argument
variable. )
G.G.
