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selftests: net: add test for ipv6 fragmentation

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

Commit a18dfa9925 ("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>
This commit is contained in:
Brett A C Sheffield
2025-09-03 15:46:01 +00:00
committed by Paolo Abeni
parent d67ca09ca3
commit aeb8d48ea9
3 changed files with 116 additions and 0 deletions
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1
tools/testing/selftests/net/.gitignore vendored
View File

@@ -16,6 +16,7 @@ ip_local_port_range
ipsec
ipv6_flowlabel
ipv6_flowlabel_mgr
ipv6_fragmentation
log.txt
msg_oob
msg_zerocopy

1
tools/testing/selftests/net/Makefile
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@@ -118,6 +118,7 @@ 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"

114
tools/testing/selftests/net/ipv6_fragmentation.c Normal file
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@@ -0,0 +1,114 @@
// 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 = htons(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 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);
if (close(s) == -1)
error(KSFT_FAIL, errno, "close");
return KSFT_PASS;
}