6.12.y longterm regression - IPv6 UDP packet fragmentation

[Brett Sheffield <bacs@librecast.net>]

[-- Attachment #1: Type: text/plain, Size: 939 bytes --]

Hello Stable Maintainers,

Longterm kernel 6.12.y backports commit:

- a18dfa9925b9ef6107ea3aa5814ca3c704d34a8a "ipv6: save dontfrag in cork"

but does not backport these related commits:

- 54580ccdd8a9c6821fd6f72171d435480867e4c3 "ipv6: remove leftover ip6 cookie initializer"
- 096208592b09c2f5fc0c1a174694efa41c04209d "ipv6: replace ipcm6_init calls with ipcm6_init_sk"

This causes a regression when sending IPv6 UDP packets by preventing
fragmentation and instead returning EMSGSIZE. I have attached a program which
demonstrates the issue.

sendmsg() returns correctly (8192) on a working kernel, and returns -1
(EMSGSIZE) when the regression is present.

The regression is not present in the mainline kernel.

Applying the two missing commits to 6.12.y fixes the regression.

Cheers,


Brett
-- 
Brett Sheffield (he/him)
Librecast - Decentralising the Internet with Multicast
https://librecast.net/
https://blog.brettsheffield.com/

[-- Attachment #2: 6.12.y-frag.c --]
[-- Type: text/plain, Size: 1629 bytes --]

/*
 * 6.12.y backports commit:
 * - a18dfa9925b9ef6107ea3aa5814ca3c704d34a8a "ipv6: save dontfrag in cork"
 * but does not backport:
 * - 54580ccdd8a9c6821fd6f72171d435480867e4c3 "ipv6: remove leftover ip6 cookie initializer"
 * - 096208592b09c2f5fc0c1a174694efa41c04209d "ipv6: replace ipcm6_init calls with ipcm6_init_sk"
 *
 * This causes a regression when sending IPv6 UDP packets by preventing
 * fragmentation and instead returning EMSGSIZE. This program demonstrates the
 * issue. sendmsg returns correctly (8192) on a working kernel, and returns -1
 * (EMSGSIZE) when the regression is present.
 *
 * The regression is not present in the mainline kernel.
 *
 * Applying the missing commits to 6.12.y fixes the regression.
 */

#include <netinet/in.h>
#include <sys/socket.h>
#include <stdio.h>

#define LARGER_THAN_MTU 8192

int main(void)
{
	/* address doesn't matter, use an IPv6 multicast address for simplicity */
	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 = 4242
	};
	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 = socket(AF_INET6, SOCK_DGRAM, 0);
	msg.msg_name = (struct sockaddr *)&sa;
	msg.msg_namelen = sizeof sa;
	rc = sendmsg(s, &msg, 0);
	if (rc == -1) {
		perror("send");
		return 1;
	}
	printf("send() returned %zi\n", rc);
	return 0;
}