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>
Link: https://lore.kernel.org/lkml/202008151007.EF679DF@keescook/
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 93fcef105061bf4ec007e3a7ffa397f3f3bbbf53..f1c4e7b56bd93a949a2346e01243d2dd8339ae7c 100644 (file)
--- 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 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
@@ -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_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
 
@@ -190,21 +200,20 @@
 })
 
 
-#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 */
 
@@ -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_overflow(({           \
        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.