netdev - Re: netns_id in bpf_sk_lookup

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Date:   Mon, 19 Nov 2018 21:28:43 +0100
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     David Ahern <dsahern@...il.com>, Joe Stringer <joe@...d.net.nz>
Cc:     netdev <netdev@...r.kernel.org>, daniel@...earbox.net
Subject: Re: netns_id in bpf_sk_lookup_{tcp,udp}

Le 19/11/2018 à 20:54, David Ahern a écrit :
> On 11/19/18 12:47 PM, Joe Stringer wrote:
>> On Mon, 19 Nov 2018 at 10:39, David Ahern <dsahern@...il.com> wrote:
>>>
>>> On 11/19/18 11:36 AM, Joe Stringer wrote:
>>>> Hi David, thanks for pointing this out.
>>>>
>>>> This is more of an oversight through iterations, the runtime lookup
>>>> will fail to find a socket if the netns value is greater than the
>>>> range of a uint32 so I think it would actually make more sense to drop
>>>> the parameter size to u32 rather than u64 so that this would be
>>>> validated at load time rather than silently returning NULL because of
>>>> a bad parameter.
>>>
>>> ok. I was wondering if it was a u64 to handle nsid of 0 which as I
>>> understand it is a legal nsid. If you drop to u32, how do you know when
>>> nsid has been set?
>>
>> I was operating under the assumption that 0 represents the root netns
>> id, and cannot be assigned to another non-root netns.
>>
>> Looking at __peernet2id_alloc(), it seems to me like it attempts to
>> find a netns and if it cannot find one, returns 0, which then leads to
>> a scroll over the idr starting from 0 to INT_MAX to find a legitimate
>> id for the netns, so I think this is a fair assumption?
The NET_ID_ZERO trick is used to manage nsid 0 in net_eq_idr() (idr_for_each()
stops when the callback returns != 0).

>>
> 
> Maybe Nicolas can give a definitive answer; as I recall he added the
> NSID option. I have not had time to walk the code. But I do recall
> seeing an id of 0. e.g, on my dev box:
> $ ip netns
> vms (id: 0)
> 
> And include/uapi/linux/net_namespace.h shows -1 as not assigned.
Yes, 0 is a valid value and can be assigned to any netns.
nsid are signed 32 bit values. Note that -1 (NETNSA_NSID_NOT_ASSIGNED) is used
by the kernel to express that the nsid is not assigned. It can also be used by
the user to let the kernel chooses a nsid.

$ ip netns add foo
$ ip netns add bar
$ ip netns
bar
foo
$ ip netns set foo 0
$ ip netns set bar auto
$ ip netns
bar (id: 1)
foo (id: 0)


Regards,
Nicolas