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

[Laurent Vivier <lvivier@redhat.com>]

On 3/24/26 04:53, David Gibson wrote:
> 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.

The problem appears in the subsequent patches of the series.
So at this point we prevent the problem, don't fix it.

> 
>> 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

I can extract the patch from this series to make this improvement on the current code.

Thanks,
Laurent

> 
> 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
>>
>