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()"...
