Re: Regression in 5.3 for some FS_USERNS_MOUNT (aka user-namespace-mountable) filesystems

[Al Viro <viro@zeniv.linux.org.uk>]

On Sat, Jul 27, 2019 at 01:37:05PM +0100, Al Viro wrote:

> > So yes I agree the function of interest is always capable in some form,
> > we just need the filesystem specific logic to check to see if we will
> > have capable over the filesystem that will be mounted.
> > 
> > I don't doubt that the new mount api has added a few new complexities.
> 
> So far it looks like *in this particular case* complexities would be
> reduced - with one exception all your ->permission() instances become
> identical.
> 
> Moreover, even in that case we still get the right overall behaviour
> with the same instance...

PS: For the record

	* I obviously agree with your reasoning behind making those checks
fs-dependent (they have to) and with putting them (back then) into
->mount() instances (since that was the first method called)
	* I agree (violently) with not liking them done inside ->mount().
	* in principle I agree that the stuff like "can that thing
be mounted in non-initial userns" might better off as a method rather
than a flag.

However
	* these days filesystem *can* have "which userns should the
capabilities be checked for?" handled outside ->mount().  Setting
fc->user_ns in ->init_fs_context() does just that; the thing is
called first in all cases.
	* with that done we get the same logics for all FS_USERNS_MOUNT
filesystems.  IOW, all your ->permission() methods would either become
NULL (for !FS_USERNS_MOUNT) or, for all non-NULL, identical to each other.
All variability between them is already taken care of when we set fc->user_ns.

The last one is what makes me somewhat dubious re having that method -
it's literally one bit of information encoded into a function pointer.
Do you anticipate any cases where the thing would *NOT* be of the same
form?  I.e. when something is userns-mountable, but the check is not
ns_capable(some userns, CAP_SYS_ADMIN)?

While we are at it, kobj_ns_...() look like preparations to something
that has never fully materialized.  What would sysfs mount checks be
supposed to do if we'd ever grown more than one struct kobj_ns_type_operations
instance?  Because that looks like the most plausible case of "we might
need trickier ->permission()"...