Subject: Re: [PATCH v17 bpf-next 20/23] net: xdp: introduce bpf_xdp_pointer utility routine

From: Lorenzo Bianconi <[email-masked]>
Date: Mon, 8 Nov 2021 17:48:42 +0100

> On Thu,  4 Nov 2021 18:35:40 +0100 Lorenzo Bianconi wrote:
> > Similar to skb_header_pointer, introduce bpf_xdp_pointer utility routine
> > to return a pointer to a given position in the xdp_buff if the requested
> > area (offset + len) is contained in a contiguous memory area otherwise it
> > will be copied in a bounce buffer provided by the caller.
> > Similar to the tc counterpart, introduce the two following xdp helpers:
> > - bpf_xdp_load_bytes
> > - bpf_xdp_store_bytes
> > 
> > Signed-off-by: Lorenzo Bianconi <[email-masked]>
> 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 386dd2fffded..534305037ad7 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -3840,6 +3840,135 @@ static const struct bpf_func_proto bpf_xdp_adjust_head_proto = {
> >  	.arg2_type	= ARG_ANYTHING,
> >  };
> >  
> > +static void bpf_xdp_copy_buf(struct xdp_buff *xdp, u32 offset,
> > +			     u32 len, void *buf, bool flush)
> > +{
> > +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> > +	u32 headsize = xdp->data_end - xdp->data;
> > +	u32 count = 0, frame_offset = headsize;
> > +	int i = 0;
> > +
> > +	if (offset < headsize) {
> > +		int size = min_t(int, headsize - offset, len);
> > +		void *src = flush ? buf : xdp->data + offset;
> > +		void *dst = flush ? xdp->data + offset : buf;
> > +
> > +		memcpy(dst, src, size);
> > +		count = size;
> > +		offset = 0;
> > +	}
> > +
> > +	while (count < len && i < sinfo->nr_frags) {
> 
> nit: for (i = 0; ...; i++) ?

ack, I will fix it in v18

> 
> > +		skb_frag_t *frag = &sinfo->frags[i++];
> > +		u32 frag_size = skb_frag_size(frag);
> > +
> > +		if  (offset < frame_offset + frag_size) {
> 
> nit: double space after if

ack, I will fix it in v18
> 
> > +			int size = min_t(int, frag_size - offset, len - count);
> > +			void *addr = skb_frag_address(frag);
> > +			void *src = flush ? buf + count : addr + offset;
> > +			void *dst = flush ? addr + offset : buf + count;
> > +
> > +			memcpy(dst, src, size);
> > +			count += size;
> > +			offset = 0;
> > +		}
> > +		frame_offset += frag_size;
> > +	}
> > +}
> > +
> > +static void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset,
> > +			     u32 len, void *buf)
> > +{
> > +	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> > +	u32 size = xdp->data_end - xdp->data;
> > +	void *addr = xdp->data;
> > +	int i;
> > +
> > +	if (unlikely(offset > 0xffff))
> > +		return ERR_PTR(-EFAULT);
> > +
> > +	if (offset + len > xdp_get_buff_len(xdp))
> > +		return ERR_PTR(-EINVAL);
> 
> I don't think it breaks anything but should we sanity check len?
> Maybe make the test above (offset | len) > 0xffff -> EFAULT?

ack, I will add it in v18

> 
> > +	if (offset < size) /* linear area */
> > +		goto out;
> > +
> > +	offset -= size;
> > +	for (i = 0; i < sinfo->nr_frags; i++) { /* paged area */
> > +		u32 frag_size = skb_frag_size(&sinfo->frags[i]);
> > +
> > +		if  (offset < frag_size) {
> > +			addr = skb_frag_address(&sinfo->frags[i]);
> > +			size = frag_size;
> > +			break;
> > +		}
> > +		offset -= frag_size;
> > +	}
> > +
> > +out:
> > +	if (offset + len < size)
> > +		return addr + offset; /* fast path - no need to copy */
> > +
> > +	if (!buf) /* no copy to the bounce buffer */
> > +		return NULL;
> > +
> > +	/* slow path - we need to copy data into the bounce buffer */
> > +	bpf_xdp_copy_buf(xdp, offset, len, buf, false);
> > +	return buf;
> > +}
> > +
> > +BPF_CALL_4(bpf_xdp_load_bytes, struct xdp_buff *, xdp, u32, offset,
> > +	   void *, buf, u32, len)
> > +{
> > +	void *ptr;
> > +
> > +	ptr = bpf_xdp_pointer(xdp, offset, len, buf);
> > +	if (IS_ERR(ptr))
> > +		return PTR_ERR(ptr);
> > +
> > +	if (ptr != buf)
> > +		memcpy(buf, ptr, len);
> 
> Maybe we should just call out to bpf_xdp_copy_buf() like store does
> instead of putting one but not the other inside bpf_xdp_pointer().
> 
> We'll have to refactor this later for the real bpf_xdp_pointer,
> I'd lean on the side of keeping things symmetric for now.

ack, I agree. I will move bpf_xdp_copy_buf out of bpf_xdp_pointer so
bpf_xdp_load_bytes and bpf_xdp_store_bytes are symmetric

Regards,
Lorenzo

> 
> > +	return 0;
> > +}
> 
> > +BPF_CALL_4(bpf_xdp_store_bytes, struct xdp_buff *, xdp, u32, offset,
> > +	   void *, buf, u32, len)
> > +{
> > +	void *ptr;
> > +
> > +	ptr = bpf_xdp_pointer(xdp, offset, len, NULL);
> > +	if (IS_ERR(ptr))
> > +		return PTR_ERR(ptr);
> > +
> > +	if (!ptr)
> > +		bpf_xdp_copy_buf(xdp, offset, len, buf, true);
> > +	else
> > +		memcpy(ptr, buf, len);
> > +
> > +	return 0;
> > +}