On Fri, Mar 06, 2026 at 02:42:05AM +0000, Jiayuan Chen wrote:
> 2026/3/6 05:03, "Jay Vosburgh" <[email protected]
> mailto:[email protected]?to=%22Jay%20Vosburgh%22%20%3Cjv%40jvosburgh.net%3E
> > wrote:
>
>
> >
> > Nikolay Aleksandrov <[email protected]> wrote:
> >
> > >
> > > On Wed, Mar 04, 2026 at 09:27:28AM -0800, Jay Vosburgh wrote:
> > >
> > > >
> > > > Nikolay Aleksandrov <[email protected]> wrote:
> > > >
> > > > >On Wed, Mar 04, 2026 at 03:42:57PM +0800, Jiayuan Chen wrote:
> > > > >> From: Jiayuan Chen <[email protected]>
> > > > >>
> > > > >> bond_rr_gen_slave_id() dereferences bond->rr_tx_counter without a
> > > > NULL
> > > > >> check. rr_tx_counter is a per-CPU counter only allocated in
> > > > bond_open()
> > > > >> when the bond mode is round-robin. If the bond device was never
> > > > brought
> > > > >> up, rr_tx_counter remains NULL, causing a null-ptr-deref.
> > > > >>
> > > > >> The XDP redirect path can reach this code even when the bond is not
> > > > up:
> > > > >> bpf_master_redirect_enabled_key is a global static key, so when any
> > > > bond
> > > > >> device has native XDP attached, the XDP_TX -> xdp_master_redirect()
> > > > >> interception is enabled for all bond slaves system-wide. This
> > > > allows the
> > > > >> path xdp_master_redirect() -> bond_xdp_get_xmit_slave() ->
> > > > >> bond_xdp_xmit_roundrobin_slave_get() -> bond_rr_gen_slave_id() to be
> > > > >> reached on a bond that was never opened.
> > > > >>
> > > > >> Fix this by allocating rr_tx_counter unconditionally in bond_init()
> > > > >> (ndo_init), which is called by register_netdevice() and covers both
> > > > >> device creation paths (bond_create() and bond_newlink()). This also
> > > > >> handles the case where bond mode is changed to round-robin after
> > > > device
> > > > >> creation. The conditional allocation in bond_open() is removed.
> > > > Since
> > > > >> bond_destructor() already unconditionally calls
> > > > >> free_percpu(bond->rr_tx_counter), the lifecycle is clean: allocate
> > > > at
> > > > >> ndo_init, free at destructor.
> > > > >>
> > > > >> Note: rr_tx_counter is only used by round-robin mode, so this
> > > > >> deliberately allocates a per-cpu u32 that goes unused for other
> > > > modes.
> > > > >> Conditional allocation (e.g., in bond_option_mode_set) was
> > > > considered
> > > > >> but rejected: the XDP path can race with mode changes on a downed
> > > > bond,
> > > > >> and adding memory barriers to the XDP hot path is not justified for
> > > > >> saving 4 bytes per CPU.
> > > > >>
> > > > >> Fixes: 879af96ffd72 ("net, core: Add support for XDP redirection to
> > > > slave device")
> > > > >> Reported-by: [email protected]
> > > > >> Closes:
> > > > https://lore.kernel.org/all/[email protected]/T/
> > > > >> Signed-off-by: Jiayuan Chen <[email protected]>
> > > > >> ---
> > > > >> drivers/net/bonding/bond_main.c | 19 +++++++++++++------
> > > > >> 1 file changed, 13 insertions(+), 6 deletions(-)
> > > > >>
> > > > >
> > > > >IMO it's not worth it to waste memory in all modes, for an unpopular
> > > > mode.
> > > > >I think it'd be better to add a null check in bond_rr_gen_slave_id(),
> > > > >READ/WRITE_ONCE() should be enough since it is allocated only once,
> > > > and
> > > > >freed when the xmit code cannot be reachable anymore (otherwise we'd
> > > > have
> > > > >more bugs now). The branch will be successfully predicted practically
> > > > always,
> > > > >and you can also mark the ptr being null as unlikely. That way only
> > > > RR takes
> > > > >a very minimal hit, if any.
> > > >
> > > > Is what you're suggesting different from Jiayuan's proposal[0],
> > > > in the sense of needing barriers in the XDP hot path to insure
> > > > ordering?
> > > >
> > > > If I understand correctly, your suggestion is something like
> > > > (totally untested):
> > > >
> > > Basically yes, that is what I'm proposing + an unlikely() around that
> > > null check since it is really unlikely and will be always predicted
> > > correctly, this way it's only for RR mode.
> > >
> > Jiayuan,
> >
> > Do you agree that the patch below (including Nikolay's
> > suggestion to add "unlikely") resolves the original issue without memory
> > waste, and without introducing performance issues (barriers) into the
> > XDP path?
>
>
> Sure, it's basically similar to what my v1 did, but the patch below can be
> more generic.
>
> https://lore.kernel.org/netdev/[email protected]/T/#m08e3e53a8aa8d837ddc9242f4b14f2651a2b00aa
>
IMO that is worse, you'll add 2 new tests and potentially 1 more cache line
for everyone in a hot path that is used a lot and must be kept as fast as
possible.
> > -J
> >
> > >
> > > >
> > > > diff --git a/drivers/net/bonding/bond_main.c
> > > > b/drivers/net/bonding/bond_main.c
> > > > index eb27cacc26d7..ac2a4fc0aad0 100644
> > > > --- a/drivers/net/bonding/bond_main.c
> > > > +++ b/drivers/net/bonding/bond_main.c
> > > > @@ -4273,13 +4273,17 @@ void bond_work_cancel_all(struct bonding *bond)
> > > > static int bond_open(struct net_device *bond_dev)
> > > > {
> > > > struct bonding *bond = netdev_priv(bond_dev);
> > > > + u32 __percpu *rr_tx_tmp;
> > > > struct list_head *iter;
> > > > struct slave *slave;
> > > >
> > > > - if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN && !bond->rr_tx_counter)
> > > > {
> > > > - bond->rr_tx_counter = alloc_percpu(u32);
> > > > - if (!bond->rr_tx_counter)
> > > > + if (BOND_MODE(bond) == BOND_MODE_ROUNDROBIN &&
> > > > + !READ_ONCE(bond->rr_tx_counter)) {
> > > > + rr_tx_tmp = alloc_percpu(u32);
> > > > + if (!rr_tx_tmp)
> > > > return -ENOMEM;
> > > > + WRITE_ONCE(bond->rr_tx_counter, rr_tx_tmp);
> > > > +
> > > > }
> > > >
> > > > /* reset slave->backup and slave->inactive */
> > > > @@ -4866,6 +4870,9 @@ static u32 bond_rr_gen_slave_id(struct bonding
> > > > *bond)
> > > > struct reciprocal_value reciprocal_packets_per_slave;
> > > > int packets_per_slave = bond->params.packets_per_slave;
> > > >
> > > > + if (!READ_ONCE(bond->rr_tx_counter))
> > > > + packets_per_slave = 0;
> > > > +
> > > > switch (packets_per_slave) {
> > > > case 0:
> > > > slave_id = get_random_u32();
> > > >
> > > > -J
> > > >
> > > >
> > > > [0]
> > > > https://lore.kernel.org/netdev/[email protected]/
> > > >
> > >
> > ---
> > -Jay Vosburgh, [email protected]
> >
