On Wed, Mar 04, 2020 at 08:47:44AM +0100, Toke Høiland-Jørgensen wrote:

quoted

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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.

# 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

This is your new sysfs after ip netns exec.

netns has nothing do with bpffs despite your claims.

Try this instead:
# mkdir /tmp/bpf
# mount -t bpf bpf /tmp/bpf
# stat /tmp/bpf|grep Device
Device: 1eh/30d	Inode: 1           Links: 2
# stat -f /tmp/bpf|grep Type
    ID: 0        Namelen: 255     Type: bpf_fs
# mkdir /tmp/bpf/test
# ip netns add my
# ip netns exec my stat /tmp/bpf|grep Device
Device: 1eh/30d	Inode: 1           Links: 3
# ip netns exec my stat -f /tmp/bpf|grep Type
    ID: 0        Namelen: 255     Type: bpf_fs
# ip netns exec my ls /tmp/bpf/
test

Having said that we do allow remounting bpffs on top of existing one:
# mount -t bpf bpf /var/aa
# mkdir /var/aa/bb
# stat -f /var/aa/bb|grep Type
    ID: 0        Namelen: 255     Type: bpf_fs
# mount -t bpf bpf /var/aa
# stat -f /var/aa/bb|grep Type
stat: cannot read file system information for '/var/aa/bb': No such file or directory
# umount /var/aa
# stat -f /var/aa/bb|grep Type
    ID: 0        Namelen: 255     Type: bpf_fs

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