On Thu,  4 Nov 2021 18:35:40 +0100 Lorenzo Bianconi wrote:

quoted

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 <lorenzo@kernel.org>

quoted

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

quoted

+		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

quoted

+			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

quoted

+	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

quoted

+	return 0;
+}

quoted

+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;
+}