Re: [PATCH v5 11/29] tcp: Convert tcp_tap_handler() to use iov_tail

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 17 Apr 2025 18:51:18 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> Use packet_data() and extract headers using IOV_REMOVE_HEADER()
> and iov_peek_header_() rather than packet_get().
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  tcp.c | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 9c6bc529205e..67d656a1cf97 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -310,6 +310,16 @@
>  #include "tcp_buf.h"
>  #include "tcp_vu.h"
>  
> +/*
> + * The size of TCP header (including options) is given by doff (Data Offset)
> + * that is a 4-bit value specifying the number of 32-bit words in the header.
> + * The maximum value of doff is 15 [(1 << 4) - 1].
> + * The maximum length in bytes of options is 15 minus the number of 32-bit
> + * words in the minimal TCP header (5) multiplied by the length of a 32-bit
> + * word (4).

Now that you explain why, I guess it would be nicer to use the same
calculation for OPTLEN_MAX (pre-existing in some sense, but doing
16 - 6 becomes more confusing now).

> + */
> +#define OPTLEN_MAX (((1UL << 4) - 6) * 4UL)
> +
>  #ifndef __USE_MISC
>  /* From Linux UAPI, missing in netinet/tcp.h provided by musl */
>  struct tcp_repair_opt {
> @@ -1957,7 +1967,10 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  {
>  	struct tcp_tap_conn *conn;
>  	const struct tcphdr *th;
> -	size_t optlen, len;
> +	char optsc[OPTLEN_MAX];
> +	struct iov_tail data;
> +	size_t optlen, l4len;
> +	struct tcphdr thc;
>  	const char *opts;
>  	union flow *flow;
>  	flow_sidx_t sidx;
> @@ -1966,15 +1979,19 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  
>  	(void)pif;
>  
> -	th = packet_get(p, idx, 0, sizeof(*th), &len);
> +	if (!packet_data(p, idx, &data))
> +		return 1;
> +
> +	l4len = iov_tail_size(&data);
> +
> +	th = IOV_REMOVE_HEADER(&data, thc);
>  	if (!th)
>  		return 1;
> -	len += sizeof(*th);
>  
>  	optlen = th->doff * 4UL - sizeof(*th);
>  	/* Static checkers might fail to see this: */
> -	optlen = MIN(optlen, ((1UL << 4) /* from doff width */ - 6) * 4UL);
> -	opts = packet_get(p, idx, sizeof(*th), optlen, NULL);
> +	optlen = MIN(optlen, OPTLEN_MAX);
> +	opts = (char *)iov_remove_header_(&data, &optsc[0], optlen, 1);
>  
>  	sidx = flow_lookup_af(c, IPPROTO_TCP, PIF_TAP, af, saddr, daddr,
>  			      ntohs(th->source), ntohs(th->dest));
> @@ -1986,7 +2003,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  			tcp_conn_from_tap(c, af, saddr, daddr, th,
>  					  opts, optlen, now);
>  		else
> -			tcp_rst_no_conn(c, af, saddr, daddr, flow_lbl, th, len);
> +			tcp_rst_no_conn(c, af, saddr, daddr, flow_lbl, th, l4len);
>  		return 1;
>  	}
>  
> @@ -1994,7 +2011,7 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
>  	ASSERT(pif_at_sidx(sidx) == PIF_TAP);
>  	conn = &flow->tcp;
>  
> -	flow_trace(conn, "packet length %zu from tap", len);
> +	flow_trace(conn, "packet length %zu from tap", l4len);
>  
>  	if (th->rst) {
>  		conn_event(c, conn, CLOSED);

-- 
Stefano