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Date:   Thu, 05 Mar 2020 11:37:11 +0100
From:   Toke Høiland-Jørgensen <>
To:     Alexei Starovoitov <>
Cc:     Alexei Starovoitov <>,
        Daniel Borkmann <>,
        Andrii Nakryiko <>,
        Andrii Nakryiko <>, bpf <>,
        Networking <>,
        Kernel Team <>
Subject: Re: [PATCH bpf-next 0/3] Introduce pinnable bpf_link kernel abstraction

Alexei Starovoitov <> writes:

> On Wed, Mar 04, 2020 at 08:47:44AM +0100, Toke Høiland-Jørgensen wrote:
>> >
>> >> And what about the case where the link fd is pinned on a bpffs that is
>> >> no longer available? I.e., if a netdevice with an XDP program moves
>> >> namespaces and no longer has access to the original bpffs, that XDP
>> >> program would essentially become immutable?
>> >
>> > 'immutable' will not be possible.
>> > I'm not clear to me how bpffs is going to disappear. What do you mean
>> > exactly?
>> # stat /sys/fs/bpf | grep Device
>> Device: 1fh/31d	Inode: 1013963     Links: 2
>> # mkdir /sys/fs/bpf/test; ls /sys/fs/bpf
>> test
>> # ip netns add test
>> # ip netns exec test stat /sys/fs/bpf/test
>> stat: cannot stat '/sys/fs/bpf/test': No such file or directory
>> # ip netns exec test stat /sys/fs/bpf | grep Device
>> Device: 3fh/63d	Inode: 12242       Links: 2
>> It's a different bpffs instance inside the netns, so it won't have
>> access to anything pinned in the outer one...
> Toke, please get your facts straight.
>> # stat /sys/fs/bpf | grep Device
>> Device: 1fh/31d	Inode: 1013963     Links: 2
> Inode != 1 means that this is not bpffs.
> I guess this is still sysfs.

Yes, my bad; I was confused because I was misremembering when 'ip'
mounts a new bpffs: I thought it was on every ns change, but it's only
when loading a BPF program, and I was in a hurry so I didn't check
properly; sorry about that.

Anyway, what I was trying to express:

> Still that doesn't mean that pinned link is 'immutable'.

I don't mean 'immutable' in the sense that it cannot be removed ever.
Just that we may end up in a situation where an application can see a
netdev with an XDP program attached, has the right privileges to modify
it, but can't because it can't find the pinned bpf_link. Right? Or am I
misunderstanding your proposal?

Amending my example from before, this could happen by:

1. Someone attaches a program to eth0, and pins the bpf_link to

2. eth0 is moved to a different namespace which mounts a new sysfs at

3. Inside that namespace, /sys/fs/bpf/myprog is no longer accessible, so
   xdp-loader can't get access to the original bpf_link; but the XDP
   program is still attached to eth0.

If this happens, and the bpf_link locks the program in place, doesn't
that mean that anything inside executed inside that namespace will be
unable to remove the XDP program? Thus making it 'immutable' from their


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