Re: Please backport bridge multicast exponential field encoding fix series to 6.1.y/6.6.y/6.12.y/6.18.y/7.0.y

On Thu, Jul 9, 2026 at 7:52=E2=80=AFPM Greg KH <[email protected]>=
 wrote:
>
> On Thu, Jul 09, 2026 at 06:35:04PM +0530, Ujjal Roy wrote:
> > On Thu, Jul 9, 2026 at 6:23=E2=80=AFPM Greg KH <gregkh@linuxfoundation.=
org> wrote:
> > >
> > > On Thu, Jul 09, 2026 at 06:12:40PM +0530, Ujjal Roy wrote:
> > > > On Thu, Jul 9, 2026 at 4:34=E2=80=AFPM Greg KH <gregkh@linuxfoundat=
ion.org> wrote:
> > > > >
> > > > > On Thu, Jul 09, 2026 at 10:13:27AM +0000, Ujjal Roy wrote:
> > > > > > Hi Greg,
> > > > > >
> > > > > > Please consider backporting the following bridge multicast fix =
series to 6.1.y, 6.6.y, 6.12.y, 6.18.y and 7.0.y.
> > > > > >
> > > > > > 726fa7da2d8c ("ipv4: igmp: get rid of IGMPV3_{QQIC,MRC} and sim=
plify calculation")
> > > > > > 12cfb4ecc471 ("ipv6: mld: rename mldv2_mrc() and add mldv2_qqi(=
)")
> > > > > > 95bfd196f0dc ("ipv4: igmp: encode multicast exponential fields"=
)
> > > > > > e51560f4220a ("ipv6: mld: encode multicast exponential fields")
> > > > > > 529dbe762de0 ("selftests: net: bridge: add MRC and QQIC field e=
ncoding tests")
> > > > >
> > > > > Why is any of this needed in older kernels?
> > > > >
> > > > > And 7.0.y is long end-of-life.
> > > > >
> > > > > And why, if this does fix issues, was it not tagged for stable to=
 start
> > > > > with?
> > > > >
> > > > > thanks,
> > > > >
> > > > > greg k-h
> > > >
> > > > I already explained this in the email thread, "Please backport brid=
ge
> > > > multicast exponential field encoding fix series to stable kernels".
> > >
> > > Sorry, but that's not here (remember, some of us get 1000+ emails a
> > > day.)
> > >
> > > Please explain why patches need to be backported when asking for them=
 to
> > > be backported.
> > >
> > > thanks,
> > >
> > > greg k-h
> >
> > Sorry for breaking the thread. I understand your point, I will
> > maintain this in the future.
> > How should I send the patchset that addresses the conflicts on 5.10.y
> > and 7.1.y? Shall I send the conflicts patchset as a series via a
> > different thread or how? I've never done this before, so I'm asking.
> >
> > Here is the explanation for why the patches need to be backported:
> >
> > History: The multicast stack currently supports decoding of IGMPv3 and
> > MLDv2 exponential timer field encodings, but lacks the corresponding
> > encoding logic when generating multicast query packets. As a result,
> > query intervals and response codes exceeding the linear encoding range
> > can be transmitted incorrectly. This can cause multicast queriers and
> > listeners to interpret different timing values, resulting in protocol
> > interoperability issues, membership timeouts, and premature multicast
> > group expiration.
> >
> > Testing: The series adds the missing encoding support for both IGMPv3
> > and MLDv2 and includes selftests that validate the behavior.
> > I backported the series to v6.6.123.2 and verified the accompanying
> > selftests. The selftests fail on the unpatched kernel and pass after
> > applying the series, demonstrating both the bug and the effectiveness
> > of the fix.
> >
> > Given that this is a protocol correctness issue affecting multicast
> > query generation, please consider backporting the complete series to
> > all applicable stable kernels.
> >
>
> But this really seems like a new feature being added, it's not fixing a
> regression of something that previously worked, right?
>
> WHy can't people just update to the latest kernel release to get this if
> they need it for their environments?
>
> thanks,
>
> greg k-h

This is a corner case when people set the query timer value higher
than 128. People usually use the default value and don't change it, so
they may not encounter this issue. But I found it when I changed the
value during some extensive validation of protocol timeouts.

If one host doesn't have this fix, clients will observe premature
multicast group expiration. For example, a set-top box channel might
disconnect early.

But we can ignore this until few more people request this fix for older ker=
nels.