| Message ID | 20240626-linux-udpgso-v2-1-422dfcbd6b48@cloudflare.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 10154dbded6d6a2fecaebdfda206609de0f121a9 |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | Lift UDP_SEGMENT restriction for egress via device w/o csum offload | expand |
Jakub Sitnicki wrote: > Today sending a UDP GSO packet from a TUN device results in an EIO error: > > import fcntl, os, struct > from socket import * > > TUNSETIFF = 0x400454CA > IFF_TUN = 0x0001 > IFF_NO_PI = 0x1000 > UDP_SEGMENT = 103 > > tun_fd = os.open("/dev/net/tun", os.O_RDWR) > ifr = struct.pack("16sH", b"tun0", IFF_TUN | IFF_NO_PI) > fcntl.ioctl(tun_fd, TUNSETIFF, ifr) > > os.system("ip addr add 192.0.2.1/24 dev tun0") > os.system("ip link set dev tun0 up") > > s = socket(AF_INET, SOCK_DGRAM) > s.setsockopt(SOL_UDP, UDP_SEGMENT, 1200) > s.sendto(b"x" * 3000, ("192.0.2.2", 9)) # EIO > > This is due to a check in the udp stack if the egress device offers > checksum offload. While TUN/TAP devices, by default, don't advertise this > capability because it requires support from the TUN/TAP reader. > > However, the GSO stack has a software fallback for checksum calculation, > which we can use. This way we don't force UDP_SEGMENT users to handle the > EIO error and implement a segmentation fallback. > > Lift the restriction so that UDP_SEGMENT can be used with any egress > device. We also need to adjust the UDP GSO code to match the GSO stack > expectation about ip_summed field, as set in commit 8d63bee643f1 ("net: > avoid skb_warn_bad_offload false positives on UFO"). Otherwise we will hit > the bad offload check. > > Users should, however, expect a potential performance impact when > batch-sending packets with UDP_SEGMENT without checksum offload on the > egress device. In such case the packet payload is read twice: first during > the sendmsg syscall when copying data from user memory, and then in the GSO > stack for checksum computation. This double memory read can be less > efficient than a regular sendmsg where the checksum is calculated during > the initial data copy from user memory. > > Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> Reviewed-by: Willem de Bruijn <willemb@google.com>
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index d08bf16d476d..ed97df6af14d 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -938,8 +938,7 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4, kfree_skb(skb); return -EINVAL; } - if (skb->ip_summed != CHECKSUM_PARTIAL || is_udplite || - dst_xfrm(skb_dst(skb))) { + if (is_udplite || dst_xfrm(skb_dst(skb))) { kfree_skb(skb); return -EIO; } diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index 59448a2dbf2c..aa2e0a28ca61 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c @@ -357,6 +357,14 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, else uh->check = gso_make_checksum(seg, ~check) ? : CSUM_MANGLED_0; + /* On the TX path, CHECKSUM_NONE and CHECKSUM_UNNECESSARY have the same + * meaning. However, check for bad offloads in the GSO stack expects the + * latter, if the checksum was calculated in software. To vouch for the + * segment skbs we actually need to set it on the gso_skb. + */ + if (gso_skb->ip_summed == CHECKSUM_NONE) + gso_skb->ip_summed = CHECKSUM_UNNECESSARY; + /* update refcount for the packet */ if (copy_dtor) { int delta = sum_truesize - gso_skb->truesize; diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c index b56f0b9f4307..b5456394cc67 100644 --- a/net/ipv6/udp.c +++ b/net/ipv6/udp.c @@ -1257,8 +1257,7 @@ static int udp_v6_send_skb(struct sk_buff *skb, struct flowi6 *fl6, kfree_skb(skb); return -EINVAL; } - if (skb->ip_summed != CHECKSUM_PARTIAL || is_udplite || - dst_xfrm(skb_dst(skb))) { + if (is_udplite || dst_xfrm(skb_dst(skb))) { kfree_skb(skb); return -EIO; }
Today sending a UDP GSO packet from a TUN device results in an EIO error: import fcntl, os, struct from socket import * TUNSETIFF = 0x400454CA IFF_TUN = 0x0001 IFF_NO_PI = 0x1000 UDP_SEGMENT = 103 tun_fd = os.open("/dev/net/tun", os.O_RDWR) ifr = struct.pack("16sH", b"tun0", IFF_TUN | IFF_NO_PI) fcntl.ioctl(tun_fd, TUNSETIFF, ifr) os.system("ip addr add 192.0.2.1/24 dev tun0") os.system("ip link set dev tun0 up") s = socket(AF_INET, SOCK_DGRAM) s.setsockopt(SOL_UDP, UDP_SEGMENT, 1200) s.sendto(b"x" * 3000, ("192.0.2.2", 9)) # EIO This is due to a check in the udp stack if the egress device offers checksum offload. While TUN/TAP devices, by default, don't advertise this capability because it requires support from the TUN/TAP reader. However, the GSO stack has a software fallback for checksum calculation, which we can use. This way we don't force UDP_SEGMENT users to handle the EIO error and implement a segmentation fallback. Lift the restriction so that UDP_SEGMENT can be used with any egress device. We also need to adjust the UDP GSO code to match the GSO stack expectation about ip_summed field, as set in commit 8d63bee643f1 ("net: avoid skb_warn_bad_offload false positives on UFO"). Otherwise we will hit the bad offload check. Users should, however, expect a potential performance impact when batch-sending packets with UDP_SEGMENT without checksum offload on the egress device. In such case the packet payload is read twice: first during the sendmsg syscall when copying data from user memory, and then in the GSO stack for checksum computation. This double memory read can be less efficient than a regular sendmsg where the checksum is calculated during the initial data copy from user memory. Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> --- net/ipv4/udp.c | 3 +-- net/ipv4/udp_offload.c | 8 ++++++++ net/ipv6/udp.c | 3 +-- 3 files changed, 10 insertions(+), 4 deletions(-)