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[34.145.139.141]) by smtp.gmail.com with UTF8SMTPSA id 6a1803df08f44-720b475a60esm12707306d6.34.2025.09.02.07.48.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 02 Sep 2025 07:48:57 -0700 (PDT) Date: Tue, 02 Sep 2025 10:48:57 -0400 From: Willem de Bruijn To: Brett A C Sheffield , willemdebruijn.kernel@gmail.com Cc: bacs@librecast.net, davem@davemloft.net, edumazet@google.com, gregkh@linuxfoundation.org, horms@kernel.org, kuba@kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, shuah@kernel.org, willemb@google.com Message-ID: In-Reply-To: <20250902142502.27278-1-bacs@librecast.net> References: <20250902142502.27278-1-bacs@librecast.net> Subject: Re: [PATCH net-next v5] selftests: net: add test for ipv6 fragmentation Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 > Reviewed-by: Willem de Bruijn > --- > 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 > + * > + * 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 > +#include > +#include > +#include > +#include > +#include > +#include > +#include > +#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 >