From: Kees Cook <kees@xxxxxxxxxx>
Date: Thu, 31 Jul 2025 10:05:47 -0700
> On Thu, Jul 31, 2025 at 01:37:34PM +0100, Simon Horman wrote:
>> While I appreciate the desire for improved performance and nicer code
>> generation. I think the idea of writing 64 bits of data to the
>> address of a 32 bit member of a structure goes against the direction
>> of hardening work by Kees and others.
>
> Agreed: it's better to avoid obscuring these details from the compiler
> so it can have an "actual" view of the object sizes involved.
>
>> Indeed, it seems to me this is the kind of thing that struct_group()
>> aims to avoid.
>>
>> In this case struct group() doesn't seem like the best option,
>> because it would provide a 64-bit buffer that we can memcpy into.
>> But it seems altogether better to simply assign u64 value to a u64 member.
>
> Agreed: with struct_group you get a sized pointer, and while you can
> provide a struct tag to make it an assignable object, it doesn't make
> too much sense here.
>
>> So I'm wondering if an approach along the following lines is appropriate
>> (Very lightly compile tested only!).
>>
>> And yes, there is room for improvement of the wording of the comment
>> I included below.
>>
>> diff --git a/include/net/libeth/xdp.h b/include/net/libeth/xdp.h
>> index f4880b50e804..a7d3d8e44aa6 100644
>> --- a/include/net/libeth/xdp.h
>> +++ b/include/net/libeth/xdp.h
>> @@ -1283,11 +1283,7 @@ static inline void libeth_xdp_prepare_buff(struct libeth_xdp_buff *xdp,
>> const struct page *page = __netmem_to_page(fqe->netmem);
>>
>> #ifdef __LIBETH_WORD_ACCESS
>> - static_assert(offsetofend(typeof(xdp->base), flags) -
>> - offsetof(typeof(xdp->base), frame_sz) ==
>> - sizeof(u64));
>> -
>> - *(u64 *)&xdp->base.frame_sz = fqe->truesize;
>> + xdp->base.frame_sz_le_qword = fqe->truesize;
>> #else
>> xdp_init_buff(&xdp->base, fqe->truesize, xdp->base.rxq);
>> #endif
>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>> index b40f1f96cb11..b5eedeb82c9b 100644
>> --- a/include/net/xdp.h
>> +++ b/include/net/xdp.h
>> @@ -85,8 +85,19 @@ struct xdp_buff {
>> void *data_hard_start;
>> struct xdp_rxq_info *rxq;
>> struct xdp_txq_info *txq;
>> - u32 frame_sz; /* frame size to deduce data_hard_end/reserved tailroom*/
>> - u32 flags; /* supported values defined in xdp_buff_flags */
>> + union {
>> + /* Allow setting frame_sz and flags as a single u64 on
>> + * little endian systems. This may may give optimal
>> + * performance. */
>> + u64 frame_sz_le_qword;
>> + struct {
>> + /* Frame size to deduce data_hard_end/reserved
>> + * tailroom. */
>> + u32 frame_sz;
>> + /* Supported values defined in xdp_buff_flags. */
>> + u32 flags;
>> + };
>> + };
>> };
>
> Yeah, this looks like a nice way to express this, and is way more
> descriptive than "(u64 *)&xdp->base.frame_sz" :)
Sounds good to me!
Let me send v4 where I'll fix this.
Thanks,
Olek
