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Message-ID: <willemdebruijn.kernel.e019291a1132@gmail.com>
Date: Tue, 02 Sep 2025 10:48:57 -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 v5] 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>
> Reviewed-by: Willem de Bruijn <willemb@...gle.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@...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 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
>
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