Re: [PATCH v5 3/8] tap: refactor packets handling functions

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  5 Jun 2024 17:21:24 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> Consolidate pool_tap4() and pool_tap6() into pool_flush_all(),
> and tap4_handler() and tap6_handler() into tap_handler_all().
> Create a generic packet_add_all() to consolidate packet
> addition logic and reduce code duplication.
> 
> The purpose is to ease the export of these functions to use
> them with the vhost-user backend.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  tap.c | 113 +++++++++++++++++++++++++++++++++-------------------------
>  tap.h |   7 ++++
>  2 files changed, 71 insertions(+), 49 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 2ea08491a51f..5fb3cb83f3d2 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -920,6 +920,61 @@ append:
>  	return in->count;
>  }
>  
> +/**
> + * pool_flush() - Flush both IPv4 and IPv6 packet pools
> + */
> +void pool_flush_all(void)
> +{
> +	pool_flush(pool_tap4);
> +	pool_flush(pool_tap6);
> +}
> +
> +/**
> + * tap_handler_all() - IPv4/IPv4 and ARP packet handler for tap file descriptor

IPv4/IPv6

> + * @c:		Execution context
> + * @now:	Current timestamp
> + */
> +void tap_handler_all(struct ctx *c, const struct timespec *now)

I wonder if this shouldn't be named tap_handler() instead. As we
already have tap_handler_passt() and tap_handler_pasta(), it's not
immediately clear what "all" refers to.

> +{
> +	tap4_handler(c, pool_tap4, now);
> +	tap6_handler(c, pool_tap6, now);
> +}
> +
> +/**
> + * packet_add_all_do() - Add a packet to the appropriate TAP pool

A couple of remarks here:

- it's a bit unexpected that this is still in tap.c (it adds packets to
  a pool, it should be in packet.c judging by this name/description).
  If we call it tap_queue_packet(), then it probably makes more sense?

- this does more than adding a packet to a pool. It's probably useless
  to describe in detail what this does, as the function body is anyway
  rather short and clear, but the current description could be a bit
  misleading. What about "Queue/capture packet, update notion of
  guest MAC address"?

- what happens if you just call packet_add() from here, without dealing
  with 'func' and 'line'? I think it's fine to print in tracing output
  name and lines from this function, instead of the ones from the
  caller. It's obvious who the caller is

> + * @c:		Execution context
> + * @l2len:	Total L2 packet length
> + * @p:		Packet buffer
> + * @func:	For tracing: name of calling function, NULL means no trace()
> + * @line:	For tracing: caller line of function call
> + */
> +void packet_add_all_do(struct ctx *c, ssize_t l2len, char *p,
> +		       const char *func, int line)
> +{
> +	const struct ethhdr *eh;
> +
> +	pcap(p, l2len);
> +
> +	eh = (struct ethhdr *)p;
> +
> +	if (memcmp(c->mac_guest, eh->h_source, ETH_ALEN)) {
> +		memcpy(c->mac_guest, eh->h_source, ETH_ALEN);
> +		proto_update_l2_buf(c->mac_guest, NULL);
> +	}
> +
> +	switch (ntohs(eh->h_proto)) {
> +	case ETH_P_ARP:
> +	case ETH_P_IP:
> +		packet_add_do(pool_tap4, l2len, p, func, line);
> +		break;
> +	case ETH_P_IPV6:
> +		packet_add_do(pool_tap6, l2len, p, func, line);
> +		break;
> +	default:
> +		break;
> +	}
> +}
> +
>  /**
>   * tap_sock_reset() - Handle closing or failure of connect AF_UNIX socket
>   * @c:		Execution context
> @@ -946,7 +1001,6 @@ static void tap_sock_reset(struct ctx *c)
>  void tap_handler_passt(struct ctx *c, uint32_t events,
>  		       const struct timespec *now)
>  {
> -	const struct ethhdr *eh;
>  	ssize_t n, rem;
>  	char *p;
>  
> @@ -959,8 +1013,7 @@ redo:
>  	p = pkt_buf;
>  	rem = 0;
>  
> -	pool_flush(pool_tap4);
> -	pool_flush(pool_tap6);
> +	pool_flush_all();
>  
>  	n = recv(c->fd_tap, p, TAP_BUF_FILL, MSG_DONTWAIT);
>  	if (n < 0) {
> @@ -987,38 +1040,18 @@ redo:
>  		/* Complete the partial read above before discarding a malformed
>  		 * frame, otherwise the stream will be inconsistent.
>  		 */
> -		if (l2len < (ssize_t)sizeof(*eh) ||
> +		if (l2len < (ssize_t)sizeof(struct ethhdr) ||
>  		    l2len > (ssize_t)ETH_MAX_MTU)
>  			goto next;
>  
> -		pcap(p, l2len);
> -
> -		eh = (struct ethhdr *)p;
> -
> -		if (memcmp(c->mac_guest, eh->h_source, ETH_ALEN)) {
> -			memcpy(c->mac_guest, eh->h_source, ETH_ALEN);
> -			proto_update_l2_buf(c->mac_guest, NULL);
> -		}
> -
> -		switch (ntohs(eh->h_proto)) {
> -		case ETH_P_ARP:
> -		case ETH_P_IP:
> -			packet_add(pool_tap4, l2len, p);
> -			break;
> -		case ETH_P_IPV6:
> -			packet_add(pool_tap6, l2len, p);
> -			break;
> -		default:
> -			break;
> -		}
> +		packet_add_all(c, l2len, p);
>  
>  next:
>  		p += l2len;
>  		n -= l2len;
>  	}
>  
> -	tap4_handler(c, pool_tap4, now);
> -	tap6_handler(c, pool_tap6, now);
> +	tap_handler_all(c, now);
>  
>  	/* We can't use EPOLLET otherwise. */
>  	if (rem)
> @@ -1043,35 +1076,18 @@ void tap_handler_pasta(struct ctx *c, uint32_t events,
>  redo:
>  	n = 0;
>  
> -	pool_flush(pool_tap4);
> -	pool_flush(pool_tap6);
> +	pool_flush_all();
>  restart:
>  	while ((len = read(c->fd_tap, pkt_buf + n, TAP_BUF_BYTES - n)) > 0) {
> -		const struct ethhdr *eh = (struct ethhdr *)(pkt_buf + n);
>  
> -		if (len < (ssize_t)sizeof(*eh) || len > (ssize_t)ETH_MAX_MTU) {
> +		if (len < (ssize_t)sizeof(struct ethhdr) ||
> +		    len > (ssize_t)ETH_MAX_MTU) {
>  			n += len;
>  			continue;
>  		}
>  
> -		pcap(pkt_buf + n, len);
>  
> -		if (memcmp(c->mac_guest, eh->h_source, ETH_ALEN)) {
> -			memcpy(c->mac_guest, eh->h_source, ETH_ALEN);
> -			proto_update_l2_buf(c->mac_guest, NULL);
> -		}
> -
> -		switch (ntohs(eh->h_proto)) {
> -		case ETH_P_ARP:
> -		case ETH_P_IP:
> -			packet_add(pool_tap4, len, pkt_buf + n);
> -			break;
> -		case ETH_P_IPV6:
> -			packet_add(pool_tap6, len, pkt_buf + n);
> -			break;
> -		default:
> -			break;
> -		}
> +		packet_add_all(c, len, pkt_buf + n);
>  
>  		if ((n += len) == TAP_BUF_BYTES)
>  			break;
> @@ -1082,8 +1098,7 @@ restart:
>  
>  	ret = errno;
>  
> -	tap4_handler(c, pool_tap4, now);
> -	tap6_handler(c, pool_tap6, now);
> +	tap_handler_all(c, now);
>  
>  	if (len > 0 || ret == EAGAIN)
>  		return;
> diff --git a/tap.h b/tap.h
> index 2285a87093f9..3ffb7d6c3a91 100644
> --- a/tap.h
> +++ b/tap.h
> @@ -70,5 +70,12 @@ void tap_handler_passt(struct ctx *c, uint32_t events,
>  		       const struct timespec *now);
>  int tap_sock_unix_open(char *sock_path);
>  void tap_sock_init(struct ctx *c);
> +void pool_flush_all(void);
> +void tap_handler_all(struct ctx *c, const struct timespec *now);
> +
> +void packet_add_all_do(struct ctx *c, ssize_t l2len, char *p,
> +		       const char *func, int line);
> +#define packet_add_all(p, l2len, start)					\
> +	packet_add_all_do(p, l2len, start, __func__, __LINE__)
>  
>  #endif /* TAP_H */

-- 
Stefano