Re: [PATCH net-next v5 4/4] vsock/virtio: MSG_ZEROCOPY flag support

On 01.08.2023 16:34, Paolo Abeni wrote:

On Sun, 2023-07-30 at 11:59 +0300, Arseniy Krasnov wrote:

quoted

+static int virtio_transport_fill_skb(struct sk_buff *skb,
+				     struct virtio_vsock_pkt_info *info,
+				     size_t len,
+				     bool zcopy)
+{
+	if (zcopy) {
+		return __zerocopy_sg_from_iter(info->msg, NULL, skb,
+					      &info->msg->msg_iter,
+					      len);
+	} else {

No need for an else statement after 'return'

quoted

+		void *payload;
+		int err;
+
+		payload = skb_put(skb, len);
+		err = memcpy_from_msg(payload, info->msg, len);
+		if (err)
+			return -1;
+
+		if (msg_data_left(info->msg))
+			return 0;
+

This path does not update truesize, evem if it increases the skb len...

Sorry, but what is potential problem here ? In this path I copy data from the user's
buffer to the linear skb (there is no fragged part in this case). I think 'truesize'
is constant in this case - it is SKB_TRUESIZE(length of skb buffer) - there is no need
to update it as 'truesize' does not show amount of data in skb, only real size of
skb's buffer.

For non-linear case, __zerocopy_sg_from_iter() always updates 'sk_wmem_alloc' of the
socket during iterating over frags array.

Also 'skb_set_owner_w()' is called before this code, thus setting 'sk_wmem_alloc' to the
'truesize' value of the skb.

Thanks, Arseniy

quoted

+		return 0;
+	}
+}

[...]

quoted

@@ -214,6 +251,70 @@ static u16 virtio_transport_get_type(struct sock *sk)
 		return VIRTIO_VSOCK_TYPE_SEQPACKET;
 }
 
+static struct sk_buff *virtio_transport_alloc_skb(struct vsock_sock *vsk,
+						  struct virtio_vsock_pkt_info *info,
+						  size_t payload_len,
+						  bool zcopy,
+						  u32 src_cid,
+						  u32 src_port,
+						  u32 dst_cid,
+						  u32 dst_port)
+{
+	struct sk_buff *skb;
+	size_t skb_len;
+
+	skb_len = VIRTIO_VSOCK_SKB_HEADROOM;
+
+	if (!zcopy)
+		skb_len += payload_len;
+
+	skb = virtio_vsock_alloc_skb(skb_len, GFP_KERNEL);
+	if (!skb)
+		return NULL;
+
+	virtio_transport_init_hdr(skb, info, src_cid, src_port,
+				  dst_cid, dst_port,
+				  payload_len);
+
+	/* Set owner here, because '__zerocopy_sg_from_iter()' uses
+	 * owner of skb without check to update 'sk_wmem_alloc'.
+	 */
+	if (vsk)
+		skb_set_owner_w(skb, sk_vsock(vsk));

... which can lead to bad things(TM) if the skb goes trough some later
non trivial processing, due to the above skb_set_owner_w().

Additionally can be the following condition be true:

	vsk == NULL && (info->msg && payload_len > 0) && zcopy

???

If so it looks like skb can go through __zerocopy_sg_from_iter() even
without a prior skb_set_owner_w()...


Cheers,

Paolo