Messages in this thread | | | Date | Fri, 28 Jul 2023 15:43:26 -0700 | From | Kees Cook <> | Subject | Re: [PATCH net-next 1/3] virtchnl: fix fake 1-elem arrays in structs allocated as `nents + 1` - 1 |
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On Fri, Jul 28, 2023 at 05:52:05PM +0200, Alexander Lobakin wrote: > The two most problematic virtchnl structures are virtchnl_rss_key and > virtchnl_rss_lut. Their "flex" arrays have the type of u8, thus, when > allocating / checking, the actual size is calculated as `sizeof + > nents - 1 byte`. But their sizeof() is not 1 byte larger than the size > of such structure with proper flex array, it's two bytes larger due to > the padding. That said, their size is always 1 byte larger unless > there are no tail elements -- then it's +2 bytes. > Add virtchnl_struct_size() macro which will handle this case (and later > other cases as well). Make its calling conv the same as we call > struct_size() to allow it to be drop-in, even though it's unlikely to > become possible to switch to generic API. The macro will calculate a > proper size of a structure with a flex array at the end, so that it > becomes transparent for the compilers, but add the difference from the > old values, so that the real size of sorta-ABI-messages doesn't change. > Use it on the allocation side in IAVF and the receiving side (defined > as static inline in virtchnl.h) for the mentioned two structures.
This all looks workable, but it's a unique solution in the kernel. That is fine, of course, but would it be easier to maintain/test if it went with the union style solutions?
struct foo { ... union { type legacy_padding; DECLARE_FLEX_ARRAY(type, member); }; };
Then the size doesn't change and "member" can still be used. (i.e. no collateral changes needed.)
-Kees
-- Kees Cook
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