On Tue, Mar 24, 2026 at 08:56:48AM +0100, Laurent Vivier wrote: > 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. Oh, sorry, I misread your comment which already said that. > > > 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 > > > > > 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