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

[Larysa Zaremba]

On Mon, May 15, 2023 at 06:17:02PM +0200, Jesper Dangaard Brouer wrote:
> 
> 
> On 12/05/2023 17.26, Larysa Zaremba wrote:
> > From: Aleksander Lobakin <aleksander.lobakin@xxxxxxxxx>
> > 
> > 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.

It's not 'metadata is stored as u8', it's 'metadata size is stored as u8' :)
Maybe I should rephrase it better in v2.

> 
> > 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@xxxxxxxxx>
> > Signed-off-by: Larysa Zaremba <larysa.zaremba@xxxxxxxxx>
> > ---
> >   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 {
> 
>