[xdp-hints] Re: [PATCH bpf-next v3 03/12] bpf: XDP metadata RX kfuncs

[Martin KaFai Lau <martin.lau@linux.dev>]

On 12/8/22 11:07 AM, Stanislav Fomichev wrote:
>>> @@ -102,11 +112,25 @@ int bpf_prog_offload_init(struct bpf_prog *prog, union bpf_attr *attr)
>>>        if (err)
>>>                goto err_maybe_put;
>>>
>>> +     prog->aux->offload_requested = !(attr->prog_flags & BPF_F_XDP_HAS_METADATA);
>>> +
>>
>> If I read the set correctly, bpf prog can either use metadata kfunc or offload
>> but not both. It is fine to start with only supporting metadata kfunc when there
>> is no offload but will be useful to understand the reason. I assume an offloaded
>> bpf prog should still be able to call the bpf helpers like adjust_head/tail and
>> the same should go for any kfunc?
> 
> Yes, I'm assuming there should be some work on the offloaded device
> drivers to support metadata kfuncs.
> Offloaded kfuncs, in general, seem hard (how do we call kernel func
> from the device-offloaded prog?); so refusing kfuncs early for the
> offloaded case seems fair for now?

Ah, ok.  I was actually thinking the HW offloaded prog can just use the software 
ndo_* kfunc (like other bpf-helpers).  From skimming some 
bpf_prog_offload_ops:prepare implementation, I think you are right and it seems 
BPF_PSEUDO_KFUNC_CALL has not been recognized yet.

[ ... ]

>>> @@ -226,10 +263,17 @@ static void __bpf_prog_offload_destroy(struct bpf_prog *prog)
>>>
>>>    void bpf_prog_offload_destroy(struct bpf_prog *prog)
>>>    {
>>> +     struct net_device *netdev = NULL;
>>> +
>>>        down_write(&bpf_devs_lock);
>>> -     if (prog->aux->offload)
>>> +     if (prog->aux->offload) {
>>> +             netdev = prog->aux->offload->netdev;
>>>                __bpf_prog_offload_destroy(prog);
>>> +     }
>>>        up_write(&bpf_devs_lock);
>>> +
>>> +     if (netdev)
>>
>> May be I have missed a refcnt or lock somewhere.  Is it possible that netdev may
>> have been freed?
> 
> Yeah, with the offload framework, there are no refcnts. We put an
> "offloaded" device into a separate hashtable (protected by
> rtnl/semaphore).
> maybe_remove_bound_netdev will re-grab the locks (due to ordering:
> rtnl->bpf_devs_lock) and remove the device from the hashtable if it's
> still there.
> At least this is how, I think, it should work; LMK if something is
> still fishy here...
> 
> Or is the concern here that somebody might allocate new netdev reusing
> the same address? I think I have enough checks in
> maybe_remove_bound_netdev to guard against that. Or, at least, to make
> it safe :-)

Race is ok because ondev needs to be removed anyway when '!ondev->offdev && 
list_empty(&ondev->progs)'?  hmmm... tricky, please add a comment. :)

Why it cannot be done together in the bpf_devs_lock above?  The above cannot 
take an extra rtnl_lock before bpf_devs_lock?