On Fri, Aug 01, 2025 at 03:17:42PM +0200, Alexander Lobakin wrote:
> From: Alexander Lobakin <aleksander.loba...@intel.com>
> Date: Fri, 1 Aug 2025 15:12:43 +0200
>
> > From: Kees Cook <k...@kernel.org>
> > 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.
>
> Note: would it be okay if I send v4 with this fix when the window opens,
> while our validation will retest v3 from Tony's tree in meantine? It's a
> cosmetic change anyway and does not involve any functional changes.
If this is directed at me, yeah, I don't see any high urgency here.
--
Kees Cook
