netdev - Re: [RFC net-next 1/2] overflow: add DECLARE

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Message-ID: <572fb95a-7806-0ed1-00e3-6a7796273946@intel.com>
Date: Fri, 4 Aug 2023 17:44:05 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Przemek Kitszel <przemyslaw.kitszel@...el.com>
CC: Kees Cook <keescook@...omium.org>, Jacob Keller
	<jacob.e.keller@...el.com>, <intel-wired-lan@...ts.osuosl.org>,
	<netdev@...r.kernel.org>
Subject: Re: [RFC net-next 1/2] overflow: add DECLARE_FLEX() for on-stack
 allocs

From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Date: Fri, 4 Aug 2023 15:47:48 +0200

> 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 :/

You only need to care about -O2 and -Os, since only those 2 are
officially supported by Kbuild. Did you mean it doesn't work on -Os as well?

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

Thanks,
Olek