overflow: add DEFINE_FLEX() for on-stack allocs

Add DEFINE_FLEX() macro for on-stack allocations of structs with
flexible array member.

Expose __struct_size() macro outside of fortify-string.h, as it could be
used to read size of structs allocated by DEFINE_FLEX().
Move __member_size() alongside it.
-Kees

Using underlying array for on-stack storage lets us to declare
known-at-compile-time structures without kzalloc().

Actual usage for ice driver is in following patches of the series.

Missing __has_builtin() workaround is moved up to serve also assembly
compilation with m68k-linux-gcc, see [1].
Error was (note the .S file extension):
In file included from ../include/linux/linkage.h:5,
                 from ../arch/m68k/fpsp040/skeleton.S:40:
../include/linux/compiler_types.h:331:5: warning: "__has_builtin" is not defined, evaluates to 0 [-Wundef]
  331 | #if __has_builtin(__builtin_dynamic_object_size)
      |     ^~~~~~~~~~~~~
../include/linux/compiler_types.h:331:18: error: missing binary operator before token "("
  331 | #if __has_builtin(__builtin_dynamic_object_size)
      |                  ^

[1] https://lore.kernel.org/netdev/202308112122.OuF0YZqL-lkp@intel.com/
Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Link: https://lore.kernel.org/r/20230912115937.1645707-2-przemyslaw.kitszel@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/compiler_types.h

@@ -2,6 +2,15 @@
 #ifndef __LINUX_COMPILER_TYPES_H
 #define __LINUX_COMPILER_TYPES_H
 
+/*
+ * __has_builtin is supported on gcc >= 10, clang >= 3 and icc >= 21.
+ * In the meantime, to support gcc < 10, we implement __has_builtin
+ * by hand.
+ */
+#ifndef __has_builtin
+#define __has_builtin(x) (0)
+#endif
+
 #ifndef __ASSEMBLY__
 
 /*
@@ -134,17 +143,6 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 # define __preserve_most
 #endif
 
-/* Builtins */
-
-/*
- * __has_builtin is supported on gcc >= 10, clang >= 3 and icc >= 21.
- * In the meantime, to support gcc < 10, we implement __has_builtin
- * by hand.
- */
-#ifndef __has_builtin
-#define __has_builtin(x) (0)
-#endif
-
 /* Compiler specific macros. */
 #ifdef __clang__
 #include <linux/compiler-clang.h>
@@ -352,6 +350,18 @@ struct ftrace_likely_data {
 # define __realloc_size(x, ...)
 #endif
 
+/*
+ * When the size of an allocated object is needed, use the best available
+ * mechanism to find it. (For cases where sizeof() cannot be used.)
+ */
+#if __has_builtin(__builtin_dynamic_object_size)
+#define __struct_size(p)	__builtin_dynamic_object_size(p, 0)
+#define __member_size(p)	__builtin_dynamic_object_size(p, 1)
+#else
+#define __struct_size(p)	__builtin_object_size(p, 0)
+#define __member_size(p)	__builtin_object_size(p, 1)
+#endif
+
 #ifndef asm_volatile_goto
 #define asm_volatile_goto(x...) asm goto(x)
 #endif

include/linux/fortify-string.h

@@ -93,13 +93,9 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
 #if __has_builtin(__builtin_dynamic_object_size)
 #define POS			__pass_dynamic_object_size(1)
 #define POS0			__pass_dynamic_object_size(0)
-#define __struct_size(p)	__builtin_dynamic_object_size(p, 0)
-#define __member_size(p)	__builtin_dynamic_object_size(p, 1)
 #else
 #define POS			__pass_object_size(1)
 #define POS0			__pass_object_size(0)
-#define __struct_size(p)	__builtin_object_size(p, 0)
-#define __member_size(p)	__builtin_object_size(p, 1)
 #endif
 
 #define __compiletime_lessthan(bounds, length)	(	\

include/linux/overflow.h

@@ -309,4 +309,39 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 #define struct_size_t(type, member, count)					\
 	struct_size((type *)NULL, member, count)
 
+/**
+ * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
+ * Enables caller macro to pass (different) initializer.
+ *
+ * @type: structure type name, including "struct" keyword.
+ * @name: Name for a variable to define.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array; must be compile-time const.
+ * @initializer: initializer expression (could be empty for no init).
+ */
+#define _DEFINE_FLEX(type, name, member, count, initializer)			\
+	_Static_assert(__builtin_constant_p(count),				\
+		       "onstack flex array members require compile-time const count"); \
+	union {									\
+		u8 bytes[struct_size_t(type, member, count)];			\
+		type obj;							\
+	} name##_u initializer;							\
+	type *name = (type *)&name##_u
+
+/**
+ * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
+ * flexible array member.
+ *
+ * @type: structure type name, including "struct" keyword.
+ * @name: Name for a variable to define.
+ * @member: Name of the array member.
+ * @count: Number of elements in the array; must be compile-time const.
+ *
+ * Define a zeroed, on-stack, instance of @type structure with a trailing
+ * flexible array member.
+ * Use __struct_size(@name) to get compile-time size of it afterwards.
+ */
+#define DEFINE_FLEX(type, name, member, count)			\
+	_DEFINE_FLEX(type, name, member, count, = {})
+
 #endif /* __LINUX_OVERFLOW_H */