Re: [PATCH v8 7/8] vhost-user: add vhost-user

On 17/10/2024 02:10, Stefano Brivio wrote:
...
 
  +/**
 + * tcp_vu_prepare() - Prepare the packet header
 + * @c:		Execution context
 + * @conn:	Connection pointer
 + * @first:	Pointer to the array of IO vectors
 + * @dlen:	Packet data length
 + * @check:	Checksum, if already known
 + */
 +static void tcp_vu_prepare(const struct ctx *c,
 +			   struct tcp_tap_conn *conn, struct iovec *first,
 +			   size_t dlen, const uint16_t **check)
 +{
 +	const struct flowside *toside = TAPFLOW(conn);
 +	char *base = first->iov_base;
 +	struct ethhdr *eh;
 +
 +	/* we guess the first iovec provided by the guest can embed
 +	 * all the headers needed by L2 frame
 +	 */ 
 What happens if it doesn't (buggy guest)? Do we have a way to make
sure
 it's the case? I guess it's more straightforward to do this in
 tcp_vu_data_from_sock() where we check and set iov_len (even though the
 implication of VIRTIO_NET_F_MRG_RXBUF isn't totally clear to me). 
According to spec, minimum size of a buffer is 1526 bytes
(https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html…)

So if the guest is buggy, we will write in the guest memory out of the (buggy) provided 
buffer, and we can crash the guest. But it's what happens to a buggy guest.

We can't fix the guest, IMO passt should crash in this case (add an ASSERT()?).

Thanks,
Laurent