Re: [PATCH net-next] selftests: net: add test for ipv6 fragmentation

Thanks for the review, Willem.

On 2025-08-25 12:16, Willem de Bruijn wrote:

Brett A C Sheffield wrote:

...

Add selftest for the IPv6 fragmentation regression which affected several stable kernels.

Commit a18dfa9925b9 ("ipv6: save dontfrag in cork") was backported to stable without some prerequisite commits. This caused a regression when sending IPv6 UDP packets by preventing fragmentation and instead returning -1 (EMSGSIZE).

Add selftest to check for this issue by attempting to send a packet larger than the interface MTU. The packet will be fragmented on a working kernel, with sendmsg(2) correctly returning the expected number of bytes sent. When the regression is present, sendmsg returns -1 and sets errno to EMSGSIZE.

Signed-off-by: Brett A C Sheffield bacs@librecast.net Link: https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com

Thanks for adding a regression test for this.

No problem. I wrote a test for myself when bisecting the problem back in June - makes sense to convert it to a selftest.

...

+/* we need to set MTU, so do this in a namespace to play nicely */ +static int create_namespace(void) +{

• const char *netns_path = "/proc/self/ns/net";
• int fd;
• if (unshare(CLONE_NEWNET) != 0) {

• perror("unshare");
  

• return -1;
  

• }

Is this not sufficient to move the current process in its own netns?

Yes. Yes it is. Apparently I did not read the man page properly.

...

• fd = open(netns_path, O_RDONLY);
• if (fd == -1) {

• perror("open");
  

• return -1;
  

• }
• if (setns(fd, CLONE_NEWNET)) {

• perror("setns");
  

• return -1;
  

• }
• return 0;

+}

+static int setup(void) +{

• struct ifreq ifr = {0};
• char ifname[IFNAMSIZ];
• int fd = -1;
• int ctl;
• if (create_namespace() == -1)

• return -1;
  

• ctl = socket(AF_LOCAL, SOCK_STREAM, 0);
• if (ctl == -1)

• return -1;
  

• memset(ifname, 0, sizeof(ifname));
• fd = create_interface(ctl, ifname, &ifr);
• if (fd == -1)

• goto err_close_ctl;
  

• if (disable_dad(ifname) == -1)

• goto err_close_fd;
  

• if (interface_up(ctl, ifname, &ifr) == -1)

• goto err_close_fd;
  

• if (set_mtu(ctl, ifname, &ifr) == -1)

• goto err_close_fd;
  

• usleep(10000); /* give interface a moment to wake up */

This may be racy. Wait on a more explicit signal? E.g., /sys/class/net/$DEV/operstate.

Good thinking. I'll try that.

...

• struct msghdr msg = {

• .msg_iov = &iov,
  

• .msg_iovlen = 1,
  

• .msg_name = (struct sockaddr *)&sa,
  

• .msg_namelen = sizeof(sa),
  

• };
• ssize_t rc;
• int ns_fd;
• int s;
• printf("Testing IPv6 fragmentation\n");
• ns_fd = setup();
• if (ns_fd == -1)

• return 1;
  

• s = socket(AF_INET6, SOCK_DGRAM, 0);
• msg.msg_name = (struct sockaddr *)&sa;
• msg.msg_namelen = sizeof(sa);

nit: duplicate?

Well spotted. Will fix.

Also, no local address is set. This uses the IPv6 auto assigned address?

Correct. The test sends to a link-local scope multicast group from the autoconf link-local address. I'll clarify that in the comments at the top of the test.

...

• rc = sendmsg(s, &msg, 0);
• if (rc == -1) {

• perror("send");
  

• return 1;
  

Probably want to cleanup state both on success and failure.

Ack.

Could use KSFT_.. exit codes, though 0/1 works just as well for kselftests in practice.

Ok.

...

• } else if (rc != LARGER_THAN_MTU) {

• fprintf(stderr, "send() returned %zi\n", rc);
  

• return 1;
  

• }
• close(s);
• close(ns_fd);
• return 0;

+}

2.49.1

Thanks again - expect a v2 when I have that cleaned up and re-tested.

Cheers,

Brett --