Message-ID: <2609348a-a70f-4dba-853f-e5163c99e1a8@blackwall.org>
Date: Tue, 17 Sep 2024 10:14:50 +0300
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: 김지원 <jiwonaid0@...il.com>
Cc: jv@...sburgh.net, andy@...yhouse.net, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, ast@...nel.org,
 daniel@...earbox.net, hawk@...nel.org, john.fastabend@...il.com,
 joamaki@...il.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
 syzbot+c187823a52ed505b2257@...kaller.appspotmail.com
Subject: Re: [PATCH net] bondig: Add bond_xdp_check for bond_xdp_xmit in
 bond_main.c

On 17/09/2024 07:26, 김지원 wrote:
> On Mon, Sep 16, 2024 at 5:48 PM Nikolay Aleksandrov <razor@...ckwall.org> wrote:
>>
>> On 16/09/2024 08:50, Jiwon Kim wrote:
>>> Add bond_xdp_check to ensure the bond interface is in a valid state.
>>>
>>> syzbot reported WARNING in bond_xdp_get_xmit_slave.
>>> In bond_xdp_get_xmit_slave, the comment says
>>> /* Should never happen. Mode guarded by bond_xdp_check() */.
>>> However, it does not check the status when entering bond_xdp_xmit.
>>>
>>> Reported-by: syzbot+c187823a52ed505b2257@...kaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=c187823a52ed505b2257
>>> Fixes: 9e2ee5c7e7c3 ("net, bonding: Add XDP support to the bonding driver")
>>> Signed-off-by: Jiwon Kim <jiwonaid0@...il.com>
>>> ---
>>>  drivers/net/bonding/bond_main.c | 33 ++++++++++++++++++---------------
>>>  1 file changed, 18 insertions(+), 15 deletions(-)
>>>
>>
>> How did you figure the problem is there? Did you take any time to actually
>> understand it? This patch doesn't fix anything, the warning can be easily
>> triggered with it. The actual fix is to remove that WARN_ON() altogether
>> and downgrade the netdev_err() to a ratelimited version. The reason is that
>> we can always get to a state where at least 1 bond device has xdp program
>> installed which increases bpf_master_redirect_enabled_key and another bond
>> device which uses xdpgeneric, then install an ebpf program that simply
>> returns ACT_TX on xdpgeneric bond's slave and voila - you get the warning.
>>
>> setup is[1]:
>>  $ ip l add veth0 type veth peer veth1
>>  $ ip l add veth3 type veth peer veth4
>>  $ ip l add bond0 type bond mode 6 # <- transmit-alb mode, unsupported by xdp
>>  $ ip l add bond1 type bond # <- rr mode by default, supported by xdp
>>  $ ip l set veth0 master bond1
>>  $ ip l set bond1 up
>>  $ ip l set dev bond1 xdpdrv object tx_xdp.o section xdp_tx # <- we need xdpdrv program to increase the static key, more below
>>  $ ip l set veth3 master bond0
>>  $ ip l set bond0 up
>>  $ ip l set veth4 up
>>  $ ip l set veth3 xdpgeneric object tx_xdp.o section xdp_tx # <- now we'll hit the codepath we need after veth3 Rx's a packet
>>
>>
>> If you take the time to look at the call stack and the actual code, you'll
>> see it goes something like (for the xdpgeneric bond slave, veth3):
>> ...
>> bpf_prog_run_generic_xdp() for veth3
>>  -> bpf_prog_run_xdp()
>>    -> __bpf_prog_run() # return ACT_TX
>>      -> xdp_master_redirect() # called because we have ACT_TX && netif_is_bond_slave(xdp->rxq->dev)
>>        -> master->netdev_ops->ndo_xdp_get_xmit_slave(master, xdp); # and here we go, WARN_ON()
>>
>> I've had a patch for awhile now about this and have taken the time to look into it.
>> I guess it's time to dust it off and send it out for review. :)
>>
>> Thanks,
>>  Nik
> 
> Hi Nikolay,
> 
> Thank you for taking the time to provide a detailed setup and call
> stack analysis.
> Would you be handling the new patch? If you don't mind, may I revise
> this patch to
> 
> - Replace with net_ratelimit()
> - Remove the WARN_ON()
> - Update the comment appropriately
> 
> Thanks again for your insights and patience.
> 
> Sincerely,
> 
> Jiwon Kim

sure, I don't mind, change the patch and I'll review the new one

Cheers,
 Nik

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