macvlan: fix allmulti reference counting in passthru mode

Enabling allmulticast on a down passthru macvlan, bringing it up,
then toggling allmulticast off and on can underflow the lower device's
allmulti reference count.

macvlan_change_rx_flags() only propagates IFF_ALLMULTI changes while
the macvlan is up. However, the passthru path in macvlan_open() skips
taking an allmulti reference when IFF_ALLMULTI is already set.
Clearing allmulticast after opening the device therefore decrements
a reference that was never taken. With no other allmulti users, the
unsigned counter wraps from zero to UINT_MAX. The next increment
fails with -EOVERFLOW and reports "allmulti touches roof".

The issue was observed during development testing with 5.10 and 6.12
kernels. It was reproduced on Linux 6.12.68 by enabling allmulticast
on a down passthru macvlan, bringing it up, and toggling allmulticast
off and on. This triggered the “allmulti touches roof” warning.

Signed-off-by: Prasad Koya <prasad@arista.com>
Signed-off-by: Dave Miller <davem@davem-thinkpadp1gen8.ibmlond.csb>

0 files changed, 0 insertions, 0 deletions