Re: [PATCH RESEND bpf-next 14/15] net, xdp: allow metadata > 32

Jesper Dangaard Brouer
2023-05-15T16:17:10Z

On 12/05/2023 17.26, Larysa Zaremba wrote:
> From: Aleksander Lobakin <aleksander.lobakin@intel.com>
>
> When using XDP hints, metadata sometimes has to be much bigger
> than 32 bytes. Relax the restriction, allow metadata larger than 32 bytes
> and make __skb_metadata_differs() work with bigger lengths.
>
> Now size of metadata is only limited by the fact it is stored as u8
> in skb_shared_info, so maximum possible value is 255.

I'm confused, IIRC the metadata area isn't stored "in skb_shared_info".
The maximum possible size is limited by the XDP headroom, which is also
shared/limited with/by xdp_frame. I must be reading the sentence wrong,
somehow.

> Other important
> conditions, such as having enough space for xdp_frame building, are already
> checked in bpf_xdp_adjust_meta().
>
> The requirement of having its length aligned to 4 bytes is still
> valid.
>
> Signed-off-by: Aleksander Lobakin <aleksander.lobakin@intel.com>
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> ---
> include/linux/skbuff.h | 13 ++++++++-----
> include/net/xdp.h | 7 ++++++-
> 2 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 8ddb4af1a501..afcd372aecdf 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -4219,10 +4219,13 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
> {
> const void *a = skb_metadata_end(skb_a);
> const void *b = skb_metadata_end(skb_b);
> - /* Using more efficient varaiant than plain call to memcmp(). */
> -#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) && BITS_PER_LONG == 64
> u64 diffs = 0;
>
> + if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
> + BITS_PER_LONG != 64)
> + goto slow;
> +
> + /* Using more efficient variant than plain call to memcmp(). */
> switch (meta_len) {
> #define __it(x, op) (x -= sizeof(u##op))
> #define __it_diff(a, b, op) (*(u##op *)__it(a, op)) ^ (*(u##op *)__it(b, op))
> @@ -4242,11 +4245,11 @@ static inline bool __skb_metadata_differs(const struct sk_buff *skb_a,
> fallthrough;
> case 4: diffs |= __it_diff(a, b, 32);
> break;
> + default:
> +slow:
> + return memcmp(a - meta_len, b - meta_len, meta_len);
> }
> return diffs;
> -#else
> - return memcmp(a - meta_len, b - meta_len, meta_len);
> -#endif
> }
>
> static inline bool skb_metadata_differs(const struct sk_buff *skb_a,
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 0fbd25616241..f48723250c7c 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -370,7 +370,12 @@ xdp_data_meta_unsupported(const struct xdp_buff *xdp)
>
> static inline bool xdp_metalen_invalid(unsigned long metalen)
> {
> - return (metalen & (sizeof(__u32) - 1)) || (metalen > 32);
> + typeof(metalen) meta_max;
> +
> + meta_max = type_max(typeof_member(struct skb_shared_info, meta_len));
> + BUILD_BUG_ON(!__builtin_constant_p(meta_max));
> +
> + return !IS_ALIGNED(metalen, sizeof(u32)) || metalen > meta_max;
> }
>
> struct xdp_attachment_info {