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

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.

---

tools/testing/selftests/net/.gitignore | 1 + tools/testing/selftests/net/Makefile | 1 + .../selftests/net/ipv6_fragmentation.c | 204 ++++++++++++++++++ 3 files changed, 206 insertions(+) create mode 100644 tools/testing/selftests/net/ipv6_fragmentation.c

diff --git a/tools/testing/selftests/net/.gitignore b/tools/testing/selftests/net/.gitignore index 47c293c2962f..3d4b4a53dfda 100644 --- a/tools/testing/selftests/net/.gitignore +++ b/tools/testing/selftests/net/.gitignore @@ -16,6 +16,7 @@ ip_local_port_range ipsec ipv6_flowlabel ipv6_flowlabel_mgr +ipv6_fragmentation log.txt msg_oob msg_zerocopy diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile index b31a71f2b372..f83f91b758ae 100644 --- a/tools/testing/selftests/net/Makefile +++ b/tools/testing/selftests/net/Makefile @@ -117,6 +117,7 @@ TEST_GEN_FILES += tfo TEST_PROGS += tfo_passive.sh TEST_PROGS += broadcast_pmtu.sh TEST_PROGS += ipv6_force_forwarding.sh +TEST_GEN_PROGS += ipv6_fragmentation # YNL files, must be before "include ..lib.mk" YNL_GEN_FILES := busy_poller netlink-dumps diff --git a/tools/testing/selftests/net/ipv6_fragmentation.c b/tools/testing/selftests/net/ipv6_fragmentation.c new file mode 100644 index 000000000000..21e1a3cdc63d --- /dev/null +++ b/tools/testing/selftests/net/ipv6_fragmentation.c @@ -0,0 +1,204 @@ +// SPDX-License-Identifier: GPL-2.0 +/*

• • Author: Brett A C Sheffield bacs@librecast.net
• • Kernel selftest for the IPv6 fragmentation regression which affected
• • stable kernels:
• • https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com
• • 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).
• • This selftest demonstrates the issue. sendmsg returns correctly (8192)
• • on a working kernel, and returns -1 (EMSGSIZE) when the regression is
• • present.
• • The regression was not present in the mainline kernel, but add this test to
• • catch similar breakage in future.
• */

+#define _GNU_SOURCE

+#include <fcntl.h> +#include <linux/if_tun.h> +#include <net/if.h> +#include <netinet/in.h> +#include <sched.h> +#include <stdio.h> +#include <stdlib.h> +#include <string.h> +#include <sys/ioctl.h> +#include <sys/socket.h> +#include <unistd.h>

+#define MTU 1500 +#define LARGER_THAN_MTU 8192

+/* ensure MTU is smaller than what we plan to send */ +static int set_mtu(int ctl, char *ifname, struct ifreq *ifr) +{

• ifr->ifr_mtu = MTU;
• return ioctl(ctl, SIOCSIFMTU, ifr);

+}

+/* bring up interface */ +static int interface_up(int ctl, char *ifname, struct ifreq *ifr) +{

• if (ioctl(ctl, SIOCGIFFLAGS, ifr) == -1) {

• perror("ioctl SIOCGIFFLAGS");
  

• return -1;
  

• }
• ifr->ifr_flags = ifr->ifr_flags | IFF_UP;
• return ioctl(ctl, SIOCSIFFLAGS, ifr);

+}

+/* no need to wait for DAD in our namespace */ +static int disable_dad(char *ifname) +{

• char sysvar[] = "/proc/sys/net/ipv6/conf/%s/accept_dad";
• char fname[IFNAMSIZ + sizeof(sysvar)];
• int fd;
• snprintf(fname, sizeof(fname), sysvar, ifname);
• fd = open(fname, O_WRONLY);
• if (fd == -1) {

• perror("open accept_dad");
  

• return -1;
  

• }
• if (write(fd, "0", 1) != 1) {

• perror("write");
  

• return -1;
  

• }
• return close(fd);

+}

+/* create TAP interface that will be deleted when this process exits */ +static int create_interface(int ctl, char *ifname, struct ifreq *ifr) +{

• int fd;
• fd = open("/dev/net/tun", O_RDWR);
• if (fd == -1) {

• perror("open tun");
  

• return -1;
  

• }
• ifr->ifr_flags = IFF_TAP | IFF_NO_PI;
• if (ioctl(fd, TUNSETIFF, (void *)ifr) == -1) {

• close(fd);
  

• perror("ioctl: TUNSETIFF");
  

• return -1;
  

• }
• strcpy(ifname, ifr->ifr_name);
• return fd;

+}

+/* 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?

• 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.

• goto err_close_ctl;

+err_close_fd:

• close(fd);
• fd = -1;

+err_close_ctl:

• close(ctl);
• return fd;

+}

+int main(void) +{

• /* address doesn't matter, use an IPv6 multicast address for simplicity */
• struct in6_addr addr = {

• .s6_addr[0] = 0xff, /* multicast */
  

• .s6_addr[1] = 0x12, /* set flags (T, link-local) */
  

• };
• struct sockaddr_in6 sa = {

• .sin6_family = AF_INET6,
  

• .sin6_addr = addr,
  

• .sin6_port = 4242
  

• };
• char buf[LARGER_THAN_MTU] = {0};
• struct iovec iov = { .iov_base = buf, .iov_len = sizeof(buf)};
• 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?

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

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

• perror("send");
  

• return 1;
  

Probably want to cleanup state both on success and failure.

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

• } 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