Re: 6.12.y longterm regression - IPv6 UDP packet fragmentation

On Wed, Jun 11, 2025 at 01:04:29PM +0200, Brett Sheffield wrote:

Hello Stable Maintainers,

Longterm kernel 6.12.y backports commit:

• a18dfa9925b9ef6107ea3aa5814ca3c704d34a8a "ipv6: save dontfrag in cork"

It's also in older kernels: 5.10.238 5.15.185 6.1.141 6.6.93

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.

Should we backport thse two to all of the other branches as well?

thanks,

greg k-h