Re: [PATCH 1/7] tcp: pass ipv4h checksum, not a pointer to the checksum

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Mar 23, 2026 at 05:52:53PM +0100, Laurent Vivier wrote:
> tcp_fill_headers() takes a pointer to a previously computed IPv4 header
> checksum to avoid recalculating it when the payload length doesn't
> change. A subsequent patch makes tcp_fill_headers() access ip4h via
> with_header() which scopes it to a temporary variable, so a pointer to
> ip4h->check would become dangling after the with_header() block.

Oof, that kind of indicates the dangers with the with_header()
structure.  Is that change merged already?  If not we should probably
fix it before merge rather than after the fact.

> Pass the checksum by value as an int instead, using -1 as the sentinel
> to indicate that the checksum should be computed from scratch (replacing
> the NULL pointer sentinel). As the checksum is a uint16_t, -1 cannot be
> a valid checksum value in an int.

That said, passing this by value I think is cleaner than the pointer,
regardless of other reasons.  Would it also make sense to flag
no_tcp_csum with an additional special value (a #define to -2, or
whatever) instead of using an extra parameter?  Logically the checksum
parameter would be:
	CALCULATE | UNNEEDED | specific value

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  tcp.c          |  7 +++----
>  tcp_buf.c      | 10 +++++-----
>  tcp_internal.h |  3 +--
>  tcp_vu.c       | 12 ++++++------
>  4 files changed, 15 insertions(+), 17 deletions(-)
> 
> diff --git a/tcp.c b/tcp.c
> index b14586249c4e..158a5be0327e 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -953,8 +953,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
>  			struct ethhdr *eh,
>  			struct iphdr *ip4h, struct ipv6hdr *ip6h,
>  			struct tcphdr *th, struct iov_tail *payload,
> -			const uint16_t *ip4_check, uint32_t seq,
> -			bool no_tcp_csum)
> +			int ip4_check, uint32_t seq, bool no_tcp_csum)
>  {
>  	const struct flowside *tapside = TAPFLOW(conn);
>  	size_t l4len = iov_tail_size(payload) + sizeof(*th);
> @@ -974,8 +973,8 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
>  		ip4h->saddr = src4->s_addr;
>  		ip4h->daddr = dst4->s_addr;
>  
> -		if (ip4_check)
> -			ip4h->check = *ip4_check;
> +		if (ip4_check != -1)
> +			ip4h->check = ip4_check;
>  		else
>  			ip4h->check = csum_ip4_header(l3len, IPPROTO_TCP,
>  						      *src4, *dst4);
> diff --git a/tcp_buf.c b/tcp_buf.c
> index 41965b107567..bc0f58dd7a5e 100644
> --- a/tcp_buf.c
> +++ b/tcp_buf.c
> @@ -172,7 +172,7 @@ static void tcp_l2_buf_pad(struct iovec *iov)
>   */
>  static void tcp_l2_buf_fill_headers(const struct ctx *c,
>  				    struct tcp_tap_conn *conn,
> -				    struct iovec *iov, const uint16_t *check,
> +				    struct iovec *iov, int check,
>  				    uint32_t seq, bool no_tcp_csum)
>  {
>  	struct iov_tail tail = IOV_TAIL(&iov[TCP_IOV_PAYLOAD], 1, 0);
> @@ -233,7 +233,7 @@ int tcp_buf_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  	if (flags & KEEPALIVE)
>  		seq--;
>  
> -	tcp_l2_buf_fill_headers(c, conn, iov, NULL, seq, false);
> +	tcp_l2_buf_fill_headers(c, conn, iov, -1, seq, false);
>  
>  	tcp_l2_buf_pad(iov);
>  
> @@ -270,7 +270,7 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  			    ssize_t dlen, int no_csum, uint32_t seq, bool push)
>  {
>  	struct tcp_payload_t *payload;
> -	const uint16_t *check = NULL;
> +	int check = -1;
>  	struct iovec *iov;
>  
>  	conn->seq_to_tap = seq + dlen;
> @@ -279,9 +279,9 @@ static void tcp_data_to_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  	if (CONN_V4(conn)) {
>  		if (no_csum) {
>  			struct iovec *iov_prev = tcp_l2_iov[tcp_payload_used - 1];
> -			struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;
> +			const struct iphdr *iph = iov_prev[TCP_IOV_IP].iov_base;
>  
> -			check = &iph->check;
> +			check = iph->check;
>  		}
>  		iov[TCP_IOV_IP] = IOV_OF_LVALUE(tcp4_payload_ip[tcp_payload_used]);
>  	} else if (CONN_V6(conn)) {
> diff --git a/tcp_internal.h b/tcp_internal.h
> index d9408852571f..bb7a6629839c 100644
> --- a/tcp_internal.h
> +++ b/tcp_internal.h
> @@ -187,8 +187,7 @@ size_t tcp_fill_headers(const struct ctx *c, struct tcp_tap_conn *conn,
>  			struct ethhdr *eh,
>  			struct iphdr *ip4h, struct ipv6hdr *ip6h,
>  			struct tcphdr *th, struct iov_tail *payload,
> -			const uint16_t *ip4_check, uint32_t seq,
> -			bool no_tcp_csum);
> +			int ip4_check, uint32_t seq, bool no_tcp_csum);
>  
>  int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
>  			  bool force_seq, struct tcp_info_linux *tinfo);
> diff --git a/tcp_vu.c b/tcp_vu.c
> index 3001defb5467..a21ee3499aed 100644
> --- a/tcp_vu.c
> +++ b/tcp_vu.c
> @@ -138,7 +138,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
>  		seq--;
>  
>  	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
> -			 NULL, seq, !*c->pcap);
> +			 -1, seq, !*c->pcap);
>  
>  	if (*c->pcap)
>  		pcap_iov(&flags_elem[0].in_sg[0], 1, VNET_HLEN);
> @@ -283,7 +283,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
>   */
>  static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
>  			   struct iovec *iov, size_t iov_cnt,
> -			   const uint16_t **check, bool no_tcp_csum, bool push)
> +			   int *check, bool no_tcp_csum, bool push)
>  {
>  	const struct flowside *toside = TAPFLOW(conn);
>  	bool v6 = !(inany_v4(&toside->eaddr) && inany_v4(&toside->oaddr));
> @@ -329,7 +329,7 @@ static void tcp_vu_prepare(const struct ctx *c, struct tcp_tap_conn *conn,
>  	tcp_fill_headers(c, conn, eh, ip4h, ip6h, th, &payload,
>  			 *check, conn->seq_to_tap, no_tcp_csum);
>  	if (ip4h)
> -		*check = &ip4h->check;
> +		*check = ip4h->check;
>  }
>  
>  /**
> @@ -350,7 +350,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	size_t hdrlen, fillsize;
>  	int v6 = CONN_V6(conn);
>  	uint32_t already_sent;
> -	const uint16_t *check;
> +	int check;
>  
>  	if (!vu_queue_enabled(vq) || !vu_queue_started(vq)) {
>  		debug("Got packet, but RX virtqueue not usable yet");
> @@ -437,7 +437,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  	 */
>  
>  	hdrlen = tcp_vu_hdrlen(v6);
> -	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
> +	for (i = 0, previous_dlen = -1, check = -1; i < head_cnt; i++) {
>  		struct iovec *iov = &elem[head[i]].in_sg[0];
>  		int buf_cnt = head[i + 1] - head[i];
>  		size_t frame_size = iov_size(iov, buf_cnt);
> @@ -451,7 +451,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
>  
>  		/* The IPv4 header checksum varies only with dlen */
>  		if (previous_dlen != dlen)
> -			check = NULL;
> +			check = -1;
>  		previous_dlen = dlen;
>  
>  		tcp_vu_prepare(c, conn, iov, buf_cnt, &check, !*c->pcap, push);
> -- 
> 2.53.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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