Re: [Qemu-devel] [PATCHv7 4/9] slirp: Factorizing tcpiphdr structure with an union

[Thomas Huth <thuth@redhat.com>]

On 14.02.2016 18:47, Samuel Thibault wrote:
> From: Guillaume Subiron <maethor@subiron.org>
> 
> This patch factorizes the tcpiphdr structure to put the IPv4 fields in
> an union, for addition of version 6 in further patch.
> Using some macros, retrocompatibility of the existing code is assured.
> 
> This patch also fixes the SLIRP_MSIZE and margin computation in various
> functions, and makes them compatible with the new tcpiphdr structure,
> whose size will be bigger than sizeof(struct tcphdr) + sizeof(struct ip)
> 
> Signed-off-by: Guillaume Subiron <maethor@subiron.org>
> Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> ---
>  slirp/if.h         |  4 ++--
>  slirp/mbuf.c       |  3 ++-
>  slirp/slirp.c      | 15 ++++++++-------
>  slirp/socket.c     | 13 ++++++++++++-
>  slirp/tcp_input.c  | 31 ++++++++++++++++++++-----------
>  slirp/tcp_output.c | 18 +++++++++++++-----
>  slirp/tcp_subr.c   | 31 ++++++++++++++++++++++---------
>  slirp/tcpip.h      | 31 +++++++++++++++++++++++--------
>  8 files changed, 102 insertions(+), 44 deletions(-)
> 
> diff --git a/slirp/if.h b/slirp/if.h
> index 3327023..c7a5c57 100644
> --- a/slirp/if.h
> +++ b/slirp/if.h
> @@ -17,7 +17,7 @@
>  #define IF_MRU 1500
>  #define	IF_COMP IF_AUTOCOMP	/* Flags for compression */
>  
> -/* 2 for alignment, 14 for ethernet, 40 for TCP/IP */
> -#define IF_MAXLINKHDR (2 + 14 + 40)
> +/* 2 for alignment, 14 for ethernet */
> +#define IF_MAXLINKHDR (2 + ETH_HLEN)
>  
>  #endif
> diff --git a/slirp/mbuf.c b/slirp/mbuf.c
> index c959758..f081c69 100644
> --- a/slirp/mbuf.c
> +++ b/slirp/mbuf.c
> @@ -24,7 +24,8 @@
>   * Find a nice value for msize
>   * XXX if_maxlinkhdr already in mtu

Maybe you should now remove the XXX line, now that the size of the
TCP/IP headers is counted via IF_MTU instead?

>   */
> -#define SLIRP_MSIZE (IF_MTU + IF_MAXLINKHDR + offsetof(struct mbuf, m_dat) + 6)
> +#define SLIRP_MSIZE\
> +    (offsetof(struct mbuf, m_dat) + IF_MAXLINKHDR + TCPIPHDR_DELTA + IF_MTU)
>  
>  void
>  m_init(Slirp *slirp)
...
> diff --git a/slirp/tcp_input.c b/slirp/tcp_input.c
> index 5f845da..26b0c8b 100644
> --- a/slirp/tcp_input.c
> +++ b/slirp/tcp_input.c
> @@ -256,11 +256,6 @@ tcp_input(struct mbuf *m, int iphlen, struct socket *inso)
>  	}
>  	slirp = m->slirp;
>  
> -	/*
> -	 * Get IP and TCP header together in first mbuf.
> -	 * Note: IP leaves IP header in first mbuf.
> -	 */
> -	ti = mtod(m, struct tcpiphdr *);
>  	if (iphlen > sizeof(struct ip )) {
>  	  ip_stripoptions(m, (struct mbuf *)0);
>  	  iphlen=sizeof(struct ip );
> @@ -277,14 +272,28 @@ tcp_input(struct mbuf *m, int iphlen, struct socket *inso)
>  	save_ip.ip_len+= iphlen;
>  
>  	/*
> +	 * Get IP and TCP header together in first mbuf.
> +	 * Note: IP leaves IP header in first mbuf.
> +	 */
> +	m->m_data -= sizeof(struct tcpiphdr) - (sizeof(struct ip)
> +	                                     + sizeof(struct tcphdr));
> +	m->m_len += sizeof(struct tcpiphdr) - (sizeof(struct ip)
> +	                                    + sizeof(struct tcphdr));

I'm somewhat having a hard time to understand the  "+ sizeof(struct
tcphdr))" here.

In the tcp_output.c code, there is this:

	m->m_data += sizeof(struct tcpiphdr) - sizeof(struct tcphdr)
	                                     - sizeof(struct ip);

So with my limited point of view, I'd rather expect this here in
tcp_input.c:

	m->m_data -= sizeof(struct tcpiphdr) - (sizeof(struct ip)
	                                     - sizeof(struct tcphdr));
i.e. "-" instead of "+" here ----------------^

Could you maybe elaborate a little bit on the above calculation? Or is
it just a bug?

> +	ti = mtod(m, struct tcpiphdr *);
> +
> +	/*
>  	 * Checksum extended TCP header and data.
>  	 */
> -	tlen = ((struct ip *)ti)->ip_len;
> -        tcpiphdr2qlink(ti)->next = tcpiphdr2qlink(ti)->prev = NULL;
> -        memset(&ti->ti_i.ih_mbuf, 0 , sizeof(struct mbuf_ptr));
> -	ti->ti_x1 = 0;
> +	tlen = ip->ip_len;
> +	tcpiphdr2qlink(ti)->next = tcpiphdr2qlink(ti)->prev = NULL;
> +	memset(&ti->ih_mbuf, 0 , sizeof(struct mbuf_ptr));
> +	memset(&ti->ti, 0, sizeof(ti->ti));
> +	ti->ti_x0 = 0;
> +	ti->ti_src = save_ip.ip_src;
> +	ti->ti_dst = save_ip.ip_dst;
> +	ti->ti_pr = save_ip.ip_p;
>  	ti->ti_len = htons((uint16_t)tlen);
> -	len = sizeof(struct ip ) + tlen;
> +	len = ((sizeof(struct tcpiphdr) - sizeof(struct tcphdr)) + tlen);
>  	if(cksum(m, len)) {
>  	  goto drop;
>  	}
> @@ -1475,7 +1484,7 @@ tcp_mss(struct tcpcb *tp, u_int offer)
>  	DEBUG_ARG("tp = %p", tp);
>  	DEBUG_ARG("offer = %d", offer);
>  
> -	mss = min(IF_MTU, IF_MRU) - sizeof(struct tcpiphdr);
> +	mss = min(IF_MTU, IF_MRU) - sizeof(struct tcphdr) + sizeof(struct ip);
>  	if (offer)
>  		mss = min(mss, offer);
>  	mss = max(mss, 32);
> diff --git a/slirp/tcp_output.c b/slirp/tcp_output.c
> index 34e4d2e..7fc6a87 100644
> --- a/slirp/tcp_output.c
> +++ b/slirp/tcp_output.c
> @@ -448,15 +448,23 @@ send:
>  	 */
>  	m->m_len = hdrlen + len; /* XXX Needed? m_len should be correct */
>  
> -    {
> +	struct tcpiphdr tcpiph_save = *(mtod(m, struct tcpiphdr *));
> +	m->m_data += sizeof(struct tcpiphdr) - sizeof(struct tcphdr)
> +	                                     - sizeof(struct ip);
> +	m->m_len  -= sizeof(struct tcpiphdr) - sizeof(struct tcphdr)
> +	                                     - sizeof(struct ip);
> +	struct ip *ip = mtod(m, struct ip *);
>  
> -	((struct ip *)ti)->ip_len = m->m_len;
> +	ip->ip_len = m->m_len;
> +	ip->ip_dst = tcpiph_save.ti_dst;
> +	ip->ip_src = tcpiph_save.ti_src;
> +	ip->ip_p = tcpiph_save.ti_pr;
>  
> -	((struct ip *)ti)->ip_ttl = IPDEFTTL;
> -	((struct ip *)ti)->ip_tos = so->so_iptos;
> +	ip->ip_ttl = IPDEFTTL;
> +	ip->ip_tos = so->so_iptos;
>  
>  	error = ip_output(so, m);
> -    }
> +
>  	if (error) {
>  out:
>  		return (error);

 Thomas