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

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

Signed-off-by: Laurent Vivier --- 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