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 8d63bee6

 ("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>
Link: https://patch.msgid.link/20240626-linux-udpgso-v2-1-422dfcbd6b48@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>