Re: [PATCH net-next v1 1/7] overflow: add DEFINE_FLEX() for on-stack allocs

On Thu, Aug 10, 2023 at 06:35:03AM -0400, Przemek Kitszel wrote:

quoted

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

Add also const_flex_size() macro, that reads size of structs
allocated by DEFINE_FLEX().

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.

Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
---
v1: change macro name; add macro for size read;
     accept struct type instead of ptr to it; change alignment;
---
  include/linux/overflow.h | 27 +++++++++++++++++++++++++++
  1 file changed, 27 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index f9b60313eaea..21a4410799eb 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h

@@ -309,4 +309,31 @@ 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() - Define a zeroed, on-stack, instance of @type 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 DEFINE_FLEX(type, name, member, count)					\
+	union {									\
+		u8 bytes[struct_size_t(type, member, count)];			\
+		type obj;							\
+	} name##_u __aligned(_Alignof(type)) = {};				\
+	type *name = (type *)&name##_u

We'll need another macro when __counted_by is needed, but yes, if all of
these structs use non-native endian counters, we can't require it in the
base macro. (i.e. not now -- this is fine as-is.)

quoted

+
+/**
+ * const_flex_size() - Get size of on-stack instance of structure with
+ * a trailing flexible array member.
+ *
+ * @name: Name of the variable, the one defined by DEFINE_FLEX() macro above.
+ *
+ * Get size of @name, which is equivalent to struct_size(name, array, count),
+ * but does not require (repeating) last two arguments.
+ */
+#define const_flex_size(name)	__builtin_object_size(name, 1)

Naming is hard. ;) I don't like "const" here (it's not a storage
class). But more importantly, this calculation ("how big is this thing
actually?") gets used a lot in the fortify routines, so I'd prefer
exposing those macros (from fortify-string.h):

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index c88488715a39..4b788fa0c576 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h

@@ -352,6 +352,18 @@ 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

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index da51a83b2829..1e7711185ec6 100644
--- a/include/linux/fortify-string.h
+++ b/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)	(	\

And the way DEFINE_FLEX is built, __struct_size() and __member_size()
will give the same result (which is what I was concerned about for
FORTIFY_SOURCE's use of __member_size not "seeing" the flexible array
members).

In this case, I think using __struct_size() in place of const_flex_size()
in the patch series is the way to go.