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

From: Kees Cook <[email-masked]>
Date: Thu, 10 Aug 2023 11:46:11 -0700

On Thu, Aug 10, 2023 at 06:35:03AM -0400, Przemek Kitszel wrote:
> 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 <[email-masked]>
> ---
> 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.)

> +
> +/**
> + * 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.

-- 
Kees Cook