The commit message says:
Nothing is ever sent to such a group, so joining it has no use.
Does this hold for the group IDs that genetlink assigns as fixed values?
No, that sentence is too broad. An early join does get the messages once
a family registers. That is the problem this patch fixes.
Is this regression intended for privileged callers? I couldn't find
existing userspace that does this; dropwatch appears to resolve the
family first.
Yes. Before the family registers there are no group flags to check, so
the patch rejects every caller. I don't know of a program that joins
early either. A program that resolves the family first still joins.
The commit message mentions the getsockname() change and the setsockopt()
-ENOENT. Could it also say that early subscription to fixed IDs stops
working for privileged callers, and that NETLINK_LIST_MEMBERSHIPS output
changes as well?
I can add both, and a comment on the -ENOENT meaning on the bind member,
if a new version is needed. I won't send one only for this unless
someone asks.
Anil