selftests: net: add test for ipv6 fragmentation

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>
Link: https://patch.msgid.link/20250903154925.13481-1-bacs@librecast.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

authored by

Brett A C Sheffield and committed by

Paolo Abeni 9 months ago aeb8d48e d67ca09c

+116

3 changed files

expand all

tools

testing

selftests

net

.gitignore

Makefile

ipv6_fragmentation.c

tools/testing/selftests/net/.gitignore

··· 16 16 ipsec 17 17 ipv6_flowlabel 18 18 ipv6_flowlabel_mgr 19 + ipv6_fragmentation 19 20 log.txt 20 21 msg_oob 21 22 msg_zerocopy

tools/testing/selftests/net/Makefile

··· 118 118 TEST_PROGS += tfo_passive.sh 119 119 TEST_PROGS += broadcast_pmtu.sh 120 120 TEST_PROGS += ipv6_force_forwarding.sh 121 + TEST_GEN_PROGS += ipv6_fragmentation 121 122 TEST_PROGS += route_hint.sh 122 123 123 124 # YNL files, must be before "include ..lib.mk"

+114

tools/testing/selftests/net/ipv6_fragmentation.c

··· 1 + // SPDX-License-Identifier: GPL-2.0 2 + /* 3 + * Author: Brett A C Sheffield <bacs@librecast.net> 4 + * 5 + * Kernel selftest for the IPv6 fragmentation regression which affected stable 6 + * kernels: 7 + * 8 + * https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com 9 + * 10 + * Commit: a18dfa9925b9 ("ipv6: save dontfrag in cork") was backported to stable 11 + * without some prerequisite commits. 12 + * 13 + * This caused a regression when sending IPv6 UDP packets by preventing 14 + * fragmentation and instead returning -1 (EMSGSIZE). 15 + * 16 + * This selftest demonstrates the issue by sending an IPv6 UDP packet to 17 + * localhost (::1) on the loopback interface from the autoconfigured link-local 18 + * address. 19 + * 20 + * sendmsg(2) returns bytes sent correctly on a working kernel, and returns -1 21 + * (EMSGSIZE) when the regression is present. 22 + * 23 + * The regression was not present in the mainline kernel, but add this test to 24 + * catch similar breakage in future. 25 + */ 26 + 27 + #define _GNU_SOURCE 28 + 29 + #include <error.h> 30 + #include <net/if.h> 31 + #include <netinet/in.h> 32 + #include <sched.h> 33 + #include <stdio.h> 34 + #include <sys/ioctl.h> 35 + #include <sys/socket.h> 36 + #include <unistd.h> 37 + #include "../kselftest.h" 38 + 39 + #define MTU 1500 40 + #define LARGER_THAN_MTU 8192 41 + 42 + static void setup(void) 43 + { 44 + struct ifreq ifr = { 45 + .ifr_name = "lo" 46 + }; 47 + int ctl; 48 + 49 + /* we need to set MTU, so do this in a namespace to play nicely */ 50 + if (unshare(CLONE_NEWNET) == -1) 51 + error(KSFT_FAIL, errno, "unshare"); 52 + 53 + ctl = socket(AF_LOCAL, SOCK_STREAM, 0); 54 + if (ctl == -1) 55 + error(KSFT_FAIL, errno, "socket"); 56 + 57 + /* ensure MTU is smaller than what we plan to send */ 58 + ifr.ifr_mtu = MTU; 59 + if (ioctl(ctl, SIOCSIFMTU, &ifr) == -1) 60 + error(KSFT_FAIL, errno, "ioctl: set MTU"); 61 + 62 + /* bring up interface */ 63 + if (ioctl(ctl, SIOCGIFFLAGS, &ifr) == -1) 64 + error(KSFT_FAIL, errno, "ioctl SIOCGIFFLAGS"); 65 + ifr.ifr_flags = ifr.ifr_flags | IFF_UP; 66 + if (ioctl(ctl, SIOCSIFFLAGS, &ifr) == -1) 67 + error(KSFT_FAIL, errno, "ioctl: bring interface up"); 68 + 69 + if (close(ctl) == -1) 70 + error(KSFT_FAIL, errno, "close"); 71 + } 72 + 73 + int main(void) 74 + { 75 + struct in6_addr addr = { 76 + .s6_addr[15] = 0x01, /* ::1 */ 77 + }; 78 + struct sockaddr_in6 sa = { 79 + .sin6_family = AF_INET6, 80 + .sin6_addr = addr, 81 + .sin6_port = htons(9) /* port 9/udp (DISCARD) */ 82 + }; 83 + static char buf[LARGER_THAN_MTU] = {0}; 84 + struct iovec iov = { .iov_base = buf, .iov_len = sizeof(buf) }; 85 + struct msghdr msg = { 86 + .msg_iov = &iov, 87 + .msg_iovlen = 1, 88 + .msg_name = (struct sockaddr *)&sa, 89 + .msg_namelen = sizeof(sa), 90 + }; 91 + ssize_t rc; 92 + int s; 93 + 94 + printf("Testing IPv6 fragmentation\n"); 95 + setup(); 96 + s = socket(AF_INET6, SOCK_DGRAM, 0); 97 + send_again: 98 + rc = sendmsg(s, &msg, 0); 99 + if (rc == -1) { 100 + /* if interface wasn't ready, try again */ 101 + if (errno == EADDRNOTAVAIL) { 102 + usleep(1000); 103 + goto send_again; 104 + } 105 + error(KSFT_FAIL, errno, "sendmsg"); 106 + } else if (rc != LARGER_THAN_MTU) { 107 + error(KSFT_FAIL, errno, "sendmsg returned %zi, expected %i", 108 + rc, LARGER_THAN_MTU); 109 + } 110 + printf("[PASS] sendmsg() returned %zi\n", rc); 111 + if (close(s) == -1) 112 + error(KSFT_FAIL, errno, "close"); 113 + return KSFT_PASS; 114 + }

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