Yan Zhai wrote:
> Commit 4094871db1d6 ("udp: only do GSO if # of segs > 1") avoided GSO
> for small packets. But the kernel currently dismisses GSO requests only
> after checking MTU/PMTU on gso_size. This means any packets, regardless
> of their payload sizes, could be dropped when PMTU becomes smaller than
> requested gso_size. We encountered this issue in production and it
> caused a reliability problem that new QUIC connection cannot be
> established before PMTU cache expired, while non GSO sockets still
> worked fine at the same time.
>
> Ideally, do not check any GSO related constraints when payload size is
> smaller than requested gso_size, and return EMSGSIZE instead of EINVAL
> on MTU/PMTU check failure to be more specific on the error cause.
>
> Fixes: 4094871db1d6 ("udp: only do GSO if # of segs > 1")
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> --
> v1->v2: add a missing MTU check when fall back to no GSO mode suggested
> by Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>; Fixed up commit
> message to be more precise.
>
> v1: https://lore.kernel.org/all/Z5cgWh%2F6bRQm9vVU@debian.debian/
> ---
> net/ipv4/udp.c | 28 +++++++++++++++++++---------
> net/ipv6/udp.c | 28 +++++++++++++++++++---------
> tools/testing/selftests/net/udpgso.c | 14 ++++++++++++++
> 3 files changed, 52 insertions(+), 18 deletions(-)
>
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index c472c9a57cf6..0b5010238d05 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1141,9 +1141,20 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
> const int hlen = skb_network_header_len(skb) +
> sizeof(struct udphdr);
>
> + if (datalen <= cork->gso_size) {
> + /*
> + * check MTU again: it's skipped previously when
> + * gso_size != 0
> + */
> + if (hlen + datalen > cork->fragsize) {
> + kfree_skb(skb);
> + return -EMSGSIZE;
> + }
> + goto no_gso;
This is almost the same as the test below.
How about just
if (hlen + min(cork->gso_size, datalen) > cork->fragsize)
And don't bypass the subsequent checks with a goto, or modify the
rest of the code.
> + }
> if (hlen + cork->gso_size > cork->fragsize) {
> kfree_skb(skb);
> - return -EINVAL;
> + return -EMSGSIZE;
> }
> if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
> kfree_skb(skb);
> @@ -1158,17 +1169,16 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4,
> return -EIO;
> }
>
> - if (datalen > cork->gso_size) {
> - skb_shinfo(skb)->gso_size = cork->gso_size;
> - skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> - skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> - cork->gso_size);
> + skb_shinfo(skb)->gso_size = cork->gso_size;
> + skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> + skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> + cork->gso_size);
>
> - /* Don't checksum the payload, skb will get segmented */
> - goto csum_partial;
> - }
> + /* Don't checksum the payload, skb will get segmented */
> + goto csum_partial;
> }
>
> +no_gso:
> if (is_udplite) /* UDP-Lite */
> csum = udplite_csum(skb);
>
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 6671daa67f4f..d97befa7f80d 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1389,9 +1389,20 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
> const int hlen = skb_network_header_len(skb) +
> sizeof(struct udphdr);
>
> + if (datalen <= cork->gso_size) {
> + /*
> + * check MTU again: it's skipped previously when
> + * gso_size != 0
> + */
> + if (hlen + datalen > cork->fragsize) {
> + kfree_skb(skb);
> + return -EMSGSIZE;
> + }
> + goto no_gso;
> + }
> if (hlen + cork->gso_size > cork->fragsize) {
> kfree_skb(skb);
> - return -EINVAL;
> + return -EMSGSIZE;
> }
> if (datalen > cork->gso_size * UDP_MAX_SEGMENTS) {
> kfree_skb(skb);
> @@ -1406,17 +1417,16 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6,
> return -EIO;
> }
>
> - if (datalen > cork->gso_size) {
> - skb_shinfo(skb)->gso_size = cork->gso_size;
> - skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> - skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> - cork->gso_size);
> + skb_shinfo(skb)->gso_size = cork->gso_size;
> + skb_shinfo(skb)->gso_type = SKB_GSO_UDP_L4;
> + skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(datalen,
> + cork->gso_size);
>
> - /* Don't checksum the payload, skb will get segmented */
> - goto csum_partial;
> - }
> + /* Don't checksum the payload, skb will get segmented */
> + goto csum_partial;
> }
>
> +no_gso:
> if (is_udplite)
> csum = udplite_csum(skb);
> else if (udp_get_no_check6_tx(sk)) { /* UDP csum disabled */
> diff --git a/tools/testing/selftests/net/udpgso.c b/tools/testing/selftests/net/udpgso.c
> index 3f2fca02fec5..fb73f1c331fb 100644
> --- a/tools/testing/selftests/net/udpgso.c
> +++ b/tools/testing/selftests/net/udpgso.c
> @@ -102,6 +102,13 @@ struct testcase testcases_v4[] = {
> .gso_len = CONST_MSS_V4,
> .r_num_mss = 1,
> },
> + {
> + /* datalen <= MSS < gso_len: will fall back to no GSO */
> + .tlen = CONST_MSS_V4,
> + .gso_len = CONST_MSS_V4 + 1,
> + .r_num_mss = 0,
> + .r_len_last = CONST_MSS_V4,
> + },
Please also add a test where datalen > MSS < gso_len (with .tfail = true).
> {
> /* send a single MSS + 1B */
> .tlen = CONST_MSS_V4 + 1,
> @@ -205,6 +212,13 @@ struct testcase testcases_v6[] = {
> .gso_len = CONST_MSS_V6,
> .r_num_mss = 1,
> },
> + {
> + /* datalen <= MSS < gso_len: will fall back to no GSO */
> + .tlen = CONST_MSS_V6,
> + .gso_len = CONST_MSS_V6 + 1,
> + .r_num_mss = 0,
> + .r_len_last = CONST_MSS_V6,
> + },
> {
> /* send a single MSS + 1B */
> .tlen = CONST_MSS_V6 + 1,
> --
> 2.30.2
>
>
