Re: [PATCH net] ipv4/ipv6: preserve multicast filters after a failed source leave

[Ido Schimmel]

On Fri, Oct 30, 2026 at 02:36:03PM +0800, Xia Yan wrote:
> After joining a group with IP_ADD_MEMBERSHIP or IPV6_JOIN_GROUP,
> MCAST_LEAVE_SOURCE_GROUP for an unjoined source returns EADDRNOTAVAIL
> but switches the socket's filter mode, so the existing membership stops
> receiving. Leaving and rejoining the group restores reception.
> IP_DROP_SOURCE_MEMBERSHIP has the same problem.
> 
> ip_mc_source() and ip6_mc_source() allow an empty-filter mode switch before
> checking whether a source list exists for deletion. The failed request
> changes EXCLUDE {} to INCLUDE {}, so the socket no longer accepts packets
> from any source.
> 
> Check for a missing source list before changing the filter mode. Preserve
> EADDRNOTAVAIL for the rejected deletion and keep mode switches for valid
> source additions.
> 
> Found while reviewing the error paths of ip_mc_source() and
> ip6_mc_source().
> The reproducer uses socket calls without kernel changes or fault injection.
> 
> Tested with a UDP socket reproducer under UML using the IPv4 generic and
> legacy APIs and the IPv6 generic API. All three paths lose reception
> before this change and keep receiving with the fix. Source joins and
> leaves, block/unblock, last-source leave, and ordinary leave/rejoin also
> pass.
> 
> W=1 x86_64 builds with derived allmodconfig and allyesconfig configurations
> also pass, with no changes to the warning sets.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Xia Yan <yanxia@xxxxxxxxxxxxxxxxxxxx>

Same comment as:

https://lore.kernel.org/netdev/20261030151629.GA85001@shredder/

Not a regression and not a security issue / crash. Please target
net-next and drop the Fixes and stable tags.