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

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Date:   Mon, 15 May 2023 18:17:02 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Larysa Zaremba <larysa.zaremba@...el.com>, bpf@...r.kernel.org
Cc:     brouer@...hat.com, Stanislav Fomichev <sdf@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Tony Nguyen <anthony.l.nguyen@...el.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        linux-kernel@...r.kernel.org,
        Aleksander Lobakin <aleksander.lobakin@...el.com>
Subject: Re: [PATCH RESEND bpf-next 14/15] net, xdp: allow metadata > 32



On 12/05/2023 17.26, Larysa Zaremba wrote:
> From: Aleksander Lobakin <aleksander.lobakin@...el.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@...el.com>
> Signed-off-by: Larysa Zaremba <larysa.zaremba@...el.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 {