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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jun 17, 2025 at 02:25:02PM -0400, Willem de Bruijn wrote:
> On Tue, Jun 17, 2025 at 1:29 PM Brett Sheffield <brett@librecast.net> wrote:
> >
> > Hi Greg,
> >
> > On 2025-06-17 15:47, Greg KH wrote:
> > > On Wed, Jun 11, 2025 at 01:04:29PM +0200, Brett Sheffield wrote:
> > > > 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.
> 
> Thanks for the analysis. I had received a report and was looking into
> it, but had not yet figured out this root cause.
> 
> > >
> > > Should we backport thse two to all of the other branches as well?
> >
> > I have confirmed the regression is present in all of those older kernels (except
> > 5.15.185 as that didn't boot on my test hardware - will look at that later).
> >
> > The patch appears to have been autoselected for applying to the stable tree:
> >
> > https://lore.kernel.org/all/?q=a18dfa9925b9ef6107ea3aa5814ca3c704d34a8a
> >
> > The patch follows on from a whole series of patches by Willem de Bruijn (CC), the
> > rest of which were not applied.
> >
> > Unless there is a good reason for applying this patch in isolation, the quickest
> > fix is simply to revert that commit in stable and this fixes the regression.
> >
> > Alternatives are:
> >
> > 1) apply a small fix for the regression (patch attached). This leaves a footgun
> > if you later decide to backport more of the series.
> >
> > 2) to backport and test the whole series of patches. See merge commit
> > aefd232de5eb2e77e3fc58c56486c7fe7426a228
> >
> > 3) In the case of 6.12.33, the two patches I referenced apply cleanly and are enough
> > to fix the problem.  There are conflicts on the other branches.
> >
> > Unless there is a specific reason to have backported
> > a18dfa9925b9ef6107ea3aa5814ca3c704d34a8a to stable I'd suggest just reverting
> > it.
> 
> FWIW, I did not originally intend for these changes to make it to stable.
> 
> The simplest short term solution is to revert this patch out of the
> stable trees. But long term that may give more conflicts as later
> patches need to be backported? Not sure what is wiser in such cases.

For now, I've applied the above 2 to the 6.12.y tree.  They do not apply
any older.  If I should drop the change from the older stable trees,
please let me know.

thanks,

greg k-h