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- From: Jakub Sitnicki <[email protected]>
- Date: Wed, 26 Jun 2024 19:51:26 +0200
- Subject: [PATCH] udp: Allow GSO transmit from devices with no checksum offload
- 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 <[email protected]>
- Reviewed-by: Willem de Bruijn <[email protected]>
- Link: https://patch.msgid.link/[email protected]
- Signed-off-by: Jakub Kicinski <[email protected]>
- ---
- --- a/net/ipv4/udp.c
- +++ b/net/ipv4/udp.c
- @@ -942,8 +942,7 @@ static int udp_send_skb(struct sk_buff *
- 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;
- }
- --- a/net/ipv4/udp_offload.c
- +++ b/net/ipv4/udp_offload.c
- @@ -361,6 +361,14 @@ struct sk_buff *__udp_gso_segment(struct
- 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;
- --- a/net/ipv6/udp.c
- +++ b/net/ipv6/udp.c
- @@ -1261,8 +1261,7 @@ static int udp_v6_send_skb(struct sk_buf
- 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;
- }
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