Introduce __flex_counter() which wraps __builtin_counted_by_ref(),
as newly introduced by GCC[1] and Clang[2]. Use of __flex_counter()
allows access to the counter member of a struct's flexible array member
when it has been annotated with __counted_by().
Introduce typeof_flex_counter(), can_set_flex_counter(), and
set_flex_counter() to provide the needed _Generic() wrappers to get sane
results out of __flex_counter().
For example, with:
struct foo {
int counter;
short array[] __counted_by(counter);
} *p;
__flex_counter(p->array) will resolve to: &p->counter
typeof_flex_counter(p->array) will resolve to "int". (If p->array was not
annotated, it would resolve to "size_t".)
can_set_flex_counter(p->array, COUNT) is the same as:
COUNT <= type_max(p->counter) && COUNT >= type_min(p->counter)
(If p->array was not annotated it would return true since everything
fits in size_t.)
set_flex_counter(p->array, COUNT) is the same as:
p->counter = COUNT;
(It is a no-op if p->array is not annotated with __counted_by().)
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
---
Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Nick Desaulniers <nick.desaulniers+lkml@xxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Przemek Kitszel <przemyslaw.kitszel@xxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
---
include/linux/compiler_types.h | 31 +++++++++++++++++++++++++++++
include/linux/overflow.h | 36 ++++++++++++++++++++++++++++++++++
2 files changed, 67 insertions(+)
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 981cc3d7e3aa..8b45ecfad5b1 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -453,6 +453,37 @@ struct ftrace_likely_data {
#define __annotated(var, attr) (false)
#endif
+/*
+ * Optional: only supported since gcc >= 15, clang >= 19
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fcounted_005fby_005fref
+ * clang: https://github.com/llvm/llvm-project/pull/102549
+ */
+#if __has_builtin(__builtin_counted_by_ref)
+/**
+ * __flex_counter() - Get pointer to counter member for the given
+ * flexible array, if it was annotated with __counted_by()
+ * @FAM: Pointer to flexible array member of an addressable struct instance
+ *
+ * For example, with:
+ *
+ * struct foo {
+ * int counter;
+ * short array[] __counted_by(counter);
+ * } *p;
+ *
+ * __flex_counter(p->array) will resolve to &p->counter.
+ *
+ * Note that Clang may not allow this to be assigned to a separate
+ * variable; it must be used directly.
+ *
+ * If p->array is unannotated, this returns (void *)NULL.
+ */
+#define __flex_counter(FAM) __builtin_counted_by_ref(FAM)
+#else
+#define __flex_counter(FAM) ((void *)NULL)
+#endif
+
/*
* Some versions of gcc do not mark 'asm goto' volatile:
*
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 0c7e3dcfe867..e2b81cb5576e 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -440,4 +440,40 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
#define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
_DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
+/**
+ * typeof_flex_counter() - Return the type of the counter variable of a given
+ * flexible array member annotated by __counted_by().
+ * @FAM: Pointer to the flexible array member within a given struct.
+ *
+ * Returns "size_t" if no annotation exists.
+ */
+#define typeof_flex_counter(FAM) \
+ typeof(_Generic(__flex_counter(FAM), \
+ void *: (size_t)0, \
+ default: *__flex_counter(FAM)))
+
+/** can_set_flex_counter() - Check if the counter associated with the given
+ * flexible array member can represent a value.
+ * @FAM: Pointer to the flexible array member within a given struct.
+ * @COUNT: Value to check against the __counted_by annotated @FAM's counter.
+ */
+#define can_set_flex_counter(FAM, COUNT) \
+ (!overflows_type(COUNT, typeof_flex_counter(FAM)))
+
+/**
+ * set_flex_counter() - Set the counter associated with the given flexible
+ * array member that has been annoated by __counted_by().
+ * @FAM: Pointer to the flexible array member within a given struct.
+ * @COUNT: Value to store to the __counted_by annotated @FAM's counter.
+ *
+ * This is a no-op if no annotation exists. Count needs to be checked with
+ * can_set_flex_counter(FAM, COUNT) before using this function.
+ */
+#define set_flex_counter(FAM, COUNT) \
+({ \
+ *_Generic(__flex_counter(FAM), \
+ void *: &(size_t){ 0 }, \
+ default: __flex_counter(FAM)) = (COUNT); \
+})
+
#endif /* __LINUX_OVERFLOW_H */
--
2.34.1
