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.

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

---

Thanks for the reviews Willem and Jakub.

On 2025-09-01 09:45, Willem de Bruijn wrote:

...

...

+int main(void) +{

• struct in6_addr addr = {

•       .s6_addr[15] = 0x01, /* ::1 */
  

• };
• struct sockaddr_in6 sa = {

•       .sin6_family = AF_INET6,
  

•       .sin6_addr = addr,
  

•       .sin6_port = 9      /* port 9/udp (DISCARD) */
  

htons

addr is already initialized in network byte order (BE) here.

My point was about port

Verified with:

    char ip6[INET6_ADDRSTRLEN];
    inet_ntop(AF_INET6, &(sa.sin6_addr), ip6, INET6_ADDRSTRLEN);
    printf("The address is %s\n", ip6);

which prints "The address is ::1"

All other suggestions adopted in v5.

v5 changes:

• disable_dad: delete - not needed for lo
• main: simplify failure paths
• main: char -> static char buf
• setup: remove pointless return value
• setup: remove unused variable fd
• setup: merge with interface_up() to simplify
• setup: check all system call return values
• remove no longer used headers

v4 changes:

• fix "else should follow close brace" (checkpatch ERROR)

v3 changes:

• add usleep instead of busy polling on sendmsg
• simplify error handling by using error() and leaving cleanup to O/S
• use loopback interface - don't bother creating TAP
• send to localhost (::1)

v2 changes:

• remove superfluous namespace calls - unshare(2) suffices
• remove usleep(). Don't wait for the interface to be ready, just send, and handle the (less likely) error case by retrying.
• set destination address only once
• document our use of the IPv6 link-local source address
• send to port 9 (DISCARD) instead of 4242 (DONT PANIC)
• ensure sockets are closed on failure paths
• use KSFT exit codes for clarity

v4: https://lore.kernel.org/netdev/20250901123757.13112-1-bacs@librecast.net v3: https://lore.kernel.org/netdev/20250901112248.5218-1-bacs@librecast.net v2: https://lore.kernel.org/netdev/20250831102908.14655-1-bacs@librecast.net v1: https://lore.kernel.org/netdev/20250825092548.4436-3-bacs@librecast.net

tools/testing/selftests/net/.gitignore | 1 + tools/testing/selftests/net/Makefile | 1 + .../selftests/net/ipv6_fragmentation.c | 115 ++++++++++++++++++ 3 files changed, 117 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 eef0b8f8a7b0..276e0481d996 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 TEST_PROGS += route_hint.sh # YNL files, must be before "include ..lib.mk" diff --git a/tools/testing/selftests/net/ipv6_fragmentation.c b/tools/testing/selftests/net/ipv6_fragmentation.c new file mode 100644 index 000000000000..b76ce7b713fc --- /dev/null +++ b/tools/testing/selftests/net/ipv6_fragmentation.c @@ -0,0 +1,115 @@ +// 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 by sending an IPv6 UDP packet to
• • localhost (::1) on the loopback interface from the autoconfigured link-local
• • address.
• • sendmsg(2) returns bytes sent correctly 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 <error.h> +#include <net/if.h> +#include <netinet/in.h> +#include <sched.h> +#include <stdio.h> +#include <sys/ioctl.h> +#include <sys/socket.h> +#include <unistd.h> +#include "../kselftest.h"

+#define MTU 1500 +#define LARGER_THAN_MTU 8192

+static void setup(void) +{

• struct ifreq ifr = {

• .ifr_name = "lo"
  

• };
• int ctl;
• /* we need to set MTU, so do this in a namespace to play nicely */
• if (unshare(CLONE_NEWNET) == -1)

• error(KSFT_FAIL, errno, "unshare");
  

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

• error(KSFT_FAIL, errno, "socket");
  

• /* ensure MTU is smaller than what we plan to send */
• ifr.ifr_mtu = MTU;
• if (ioctl(ctl, SIOCSIFMTU, &ifr) == -1)

• error(KSFT_FAIL, errno, "ioctl: set MTU");
  

• /* bring up interface */
• if (ioctl(ctl, SIOCGIFFLAGS, &ifr) == -1)

• error(KSFT_FAIL, errno, "ioctl SIOCGIFFLAGS");
  

• ifr.ifr_flags = ifr.ifr_flags | IFF_UP;
• if (ioctl(ctl, SIOCSIFFLAGS, &ifr) == -1)

• error(KSFT_FAIL, errno, "ioctl: bring interface up");
  

• if (close(ctl) == -1)

• error(KSFT_FAIL, errno, "close");
  

+}

+int main(void) +{

• struct in6_addr addr = {

• .s6_addr[15] = 0x01, /* ::1 */
  

• };
• struct sockaddr_in6 sa = {

• .sin6_family = AF_INET6,
  

• .sin6_addr = addr,
  

• .sin6_port = 9      /* port 9/udp (DISCARD) */
  

• };
• static 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 err = KSFT_FAIL;
• int s;
• printf("Testing IPv6 fragmentation\n");
• setup();
• s = socket(AF_INET6, SOCK_DGRAM, 0);

+send_again:

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

• /* if interface wasn't ready, try again */
  

• if (errno == EADDRNOTAVAIL) {
  

• 	usleep(1000);
  

• 	goto send_again;
  

• }
  

• error(KSFT_FAIL, errno, "sendmsg");
  

• } else if (rc != LARGER_THAN_MTU) {

• error(KSFT_FAIL, errno, "sendmsg returned %zi, expected %i",
  

• 		rc, LARGER_THAN_MTU);
  

• }
• printf("[PASS] sendmsg() returned %zi\n", rc);
• err = KSFT_PASS;

err is no longer needed, just return KSFT_PASS below

• close(s);

reminder to check return value of all library and system calls

• return err;

+}

base-commit: cd8a4cfa6bb43a441901e82f5c222dddc75a18a3

2.49.1