Re: [Intel-wired-lan] [RFC net-next 1/2] overflow: add DECLARE_FLEX() for on-stack allocs

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 8/2/23 00:31, Kees Cook wrote:

[...]

> Initially I was struggling to make __counted_by work, but it seems we can
> use an initializer for that member, as long as we don't touch the flexible
> array member in the initializer. So we just need to add the counted-by
> member to the macro, and use a union to do the initialization. And if
> we take the address of the union (and not the struct within it), the
> compiler will see the correct object size with __builtin_object_size:
> 
> #define DEFINE_FLEX(type, name, flex, counter, count) \
>      union { \
>          u8   bytes[struct_size_t(type, flex, count)]; \
>          type obj; \
>      } name##_u __aligned(_Alignof(type)) = { .obj.counter = count }; \
>      /* take address of whole union to get the correct __builtin_object_size */ \
>      type *name = (type *)&name##_u
> 
> i.e. __builtin_object_size(name, 1) (as used by FORTIFY_SOURCE, etc)
> works correctly here, but breaks (sees a zero-sized flex array member)
> if this macro ends with:
> 
>      type *name = &name##_u.obj

__builtin_object_size(name, 0) works fine for both versions (with and 
without .obj at the end)

however it does not work for builds without -O2 switch, so 
struct_size_t() is rather a way to go :/

> 
> 
> -Kees
> 
> [1] https://git.kernel.org/linus/dd06e72e68bcb4070ef211be100d2896e236c8fb
> 

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