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/aelivduxqd1oq...@karahi.gladserv.com
Signed-off-by: Brett A C Sheffield <b...@librecast.net>
---
Thanks again Willem for the prompt review. I've incorporated your suggested
changes into v3.
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
v2: https://lore.kernel.org/netdev/20250831102908.14655-1-b...@librecast.net
v1: https://lore.kernel.org/netdev/20250825092548.4436-3-b...@librecast.net
tools/testing/selftests/net/.gitignore | 1 +
tools/testing/selftests/net/Makefile | 1 +
.../selftests/net/ipv6_fragmentation.c | 145 ++++++++++++++++++
3 files changed, 147 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..be79a0340826
--- /dev/null
+++ b/tools/testing/selftests/net/ipv6_fragmentation.c
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Author: Brett A C Sheffield <b...@librecast.net>
+ *
+ * Kernel selftest for the IPv6 fragmentation regression which affected stable
+ * kernels:
+ *
+ * https://lore.kernel.org/stable/aelivduxqd1oq...@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 <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>
+#include "../kselftest.h"
+
+#define MTU 1500
+#define LARGER_THAN_MTU 8192
+
+/* bring up interface */
+static int interface_up(int ctl, struct ifreq *ifr)
+{
+ if (ioctl(ctl, SIOCGIFFLAGS, ifr) == -1)
+ error(KSFT_FAIL, errno, "ioctl SIOCGIFFLAGS");
+ 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)
+ error(KSFT_FAIL, errno, "open accept_dad");
+ if (write(fd, "0", 1) != 1)
+ error(KSFT_FAIL, errno, "write accept_dad");
+
+ return close(fd);
+}
+
+static int setup(void)
+{
+ struct ifreq ifr = {
+ .ifr_name = "lo"
+ };
+ int fd = -1;
+ 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");
+
+ disable_dad("lo");
+ interface_up(ctl, &ifr);
+
+ close(ctl);
+ return fd;
+}
+
+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) */
+ };
+ 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 err = KSFT_FAIL;
+ int s;
+
+ printf("Testing IPv6 fragmentation\n");
+ ns_fd = 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;
+ }
+ printf("[FAIL] sendmsg: %s\n", strerror(errno));
+ } else if (rc != LARGER_THAN_MTU) {
+ printf("[FAIL] sendmsg() returned %zi, expected %i\n", rc,
LARGER_THAN_MTU);
+ }
+ else {
+ printf("[PASS] sendmsg() returned %zi\n", rc);
+ err = KSFT_PASS;
+ }
+ close(s);
+ close(ns_fd);
+ return err;
+}
base-commit: 864ecc4a6dade82d3f70eab43dad0e277aa6fc78
--
2.49.1
