Message-ID: <willemdebruijn.kernel.10c80f9e245b5@gmail.com>
Date: Sun, 31 Aug 2025 17:47:46 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Brett A C Sheffield <bacs@...recast.net>, 
 willemdebruijn.kernel@...il.com
Cc: bacs@...recast.net, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 gregkh@...uxfoundation.org, 
 horms@...nel.org, 
 kuba@...nel.org, 
 linux-kernel@...r.kernel.org, 
 linux-kselftest@...r.kernel.org, 
 netdev@...r.kernel.org, 
 pabeni@...hat.com, 
 shuah@...nel.org, 
 willemb@...gle.com
Subject: Re: [PATCH net-next v2] 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.
> 
> Link: https://lore.kernel.org/stable/aElivdUXqd1OqgMY@karahi.gladserv.com
> Signed-off-by: Brett A C Sheffield <bacs@...recast.net>
> ---
> Willem: Thanks for the suggestion to check /sys/class/net/$DEV/operstate
> 
> I did try this but, if I call unshare() and THEN create a TAP interface in the
> new namespace, operstate no longer seems to be visible to the process?
> 
> The process can still read operstate for interfaces in the shared namespace, but
> not ones created in the unshare()d namespace.
> 
> I'm sure I'm doing something wrong there, but after trying a few different
> things including reading operstate from netlink it suddenly occurred to me that
> a simpler and more reliable way to check whether an interface is ready to send
> is to, er, send, and then handle the error for the (unlikely) failure case and
> retry.
> 
> I've incorporated your other review suggestions in this v2. Many thanks.
> 
> 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
> 
> 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        | 189 ++++++++++++++++++
>  3 files changed, 191 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..4ba16bf56a32
> --- /dev/null
> +++ b/tools/testing/selftests/net/ipv6_fragmentation.c
> @@ -0,0 +1,189 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Author: Brett A C Sheffield <bacs@...recast.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 from
> + * the autoconfigured link-local address to an arbritrary multicast group.
> + *
> + * 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 <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
> +
> +/* ensure MTU is smaller than what we plan to send */
> +static int set_mtu(int ctl, struct ifreq *ifr)
> +{
> +	ifr->ifr_mtu = MTU;
> +	return ioctl(ctl, SIOCSIFMTU, ifr);
> +}
> +
> +/* bring up interface */
> +static int interface_up(int ctl, 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(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;
> +}
> +
> +static int setup(void)
> +{
> +	struct ifreq ifr = {0};
> +	char ifname[IFNAMSIZ];
> +	int fd = -1;
> +	int ctl;
> +
> +	/* we need to set MTU, so do this in a namespace to play nicely */
> +	if (unshare(CLONE_NEWNET) == -1)
> +		return -1;
> +
> +	ctl = socket(AF_LOCAL, SOCK_STREAM, 0);
> +	if (ctl == -1)
> +		return -1;
> +
> +	memset(ifname, 0, sizeof(ifname));
> +	fd = create_interface(ifname, &ifr);
> +	if (fd == -1)
> +		goto err_close_ctl;
> +	if (disable_dad(ifname) == -1)
> +		goto err_close_fd;
> +	if (interface_up(ctl, &ifr) == -1)
> +		goto err_close_fd;
> +	if (set_mtu(ctl, &ifr) == -1)
> +		goto err_close_fd;
> +	goto err_close_ctl;
> +err_close_fd:
> +	close(fd);
> +	fd = -1;
> +err_close_ctl:
> +	close(ctl);
> +	return fd;
> +}
> +
> +int main(void)
> +{
> +	/* destination doesn't matter, use an IPv6 link-local multicast group */
> +	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 = 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();
> +	if (ns_fd == -1) {
> +		printf("[FAIL] test setup failed\n");
> +		return KSFT_FAIL;
> +	}
> +	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)
> +			goto send_again;

Here a usleep is in order instead of busy polling on sendmsg.

Responded to v1 earlier, sorry. Most points still hold afaik.


> +		printf("[FAIL] sendmsg: %s\n", strerror(errno));
> +		goto err_close_socket;
> +	} else if (rc != LARGER_THAN_MTU) {
> +		printf("[FAIL] sendmsg() returned %zi\n", rc);
> +		goto err_close_socket;
> +	}
> +	printf("[PASS] sendmsg() returned %zi\n", rc);
> +	err = KSFT_PASS;
> +
> +err_close_socket:
> +	close(s);
> +	close(ns_fd);
> +	return err;
> +}
> -- 
> 2.49.1
> 

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