On Wed, Jan 17, 2018 at 10:53:36AM +0100, Miklos Szeredi wrote: > Documenting --make-private thing properly would also help. To me the > wording "make private" strongly implies "I want to make submounts > private to this instance". See for example rhbz#1432211. > > > If anything, "private bind on itself" would be a useful operation. > > Turning given location into a mountpoint, and having everything > > under it looking as it used to, but with no propagation at all. > > Without bothering anybody else, even if location currently happens > > to be on a shared/master mount. > > > > I can slap that together for mount(2), but I'm not sure what a sane > > combination of flags for that would look like ;-) For fsmount > > I think it would be very useful thing to have. > > Yes, I think such an operation would be pretty useful. Not sure if > it's the whole story, though. FWIW, there's a fun variant of the API: * fsopen(): string -> fsfd; takes fs type name, returns a file descriptor connected to fs driver. Subsequent read/write on it is used to pass options, authenticate, etc. - all you need to talk the driver into creating an active instance. * fspick(): location -> fsfd; fsfd connected to filesystem mounted at given place. After that you can talk to the driver to get superblock-level remount. * new_mount(): fsfd x string -> fd. Creates a vfsmount and gives a file descriptor for given relative pathname. * clone_mount(): location x bool -> fd. Copies a vfsmount or an entire subtree (depending upon the second parameter) and returns a file descriptor. Basically, bind or rbind sans attaching it anywhere. * change_flags(): fd x (propagation or vfsmount flags) x bool -> int fd should point to root of some vfsmount (O_PATH, or either of the previous two. Flag is "do we want it to affect the entire subtree"; the tricky question is what to do with vfsmount flags - for those we might want things like "here's the full set" or "change those flags thus". Hell knows - there might be two primitives there; the second one would be fd x mask x new_flags x bool -> int, as in "set the bits present in mask to values as in new_flags". Not sure. * move_mount(): fd x location x bool -> int. fd - what to move, location - where to put it, bool - do we want to suppress propagation. Potentially hacky part is that if fd is not attached to anything, we simply attach it; otherwise - move. Normal mount: fsopen, talk to driver, new_mount, move_mount, close descriptors mount --bind: fd = clone_mount(old, false); move_mount(fd, new, false); close mount --rbind: clone_mount(old, true); move_mount; close mount --make-shared et.al.: open(..., O_PATH); change_flags; close mount --move: open; mount_move; close vfsmount-level remount: open; change_flags (or change_mount_flags, if we keep it separate from topology ones); close sb-level remount: fspick; talk to driver; close make an arbitrary subtree really private (as discussed upthread): fd = clone_mount(old, true); change_flags (or change_propagation_flags); mount_move(fd, old, true); close(fd); The tricky part in terms of implementation is that we want a tree created by clone_mount() and never attached anywhere to be dissolved on the final close() of the result of clone_mount(). It's not quite O_PATH - we want file_operations for that sucker that would have ->release() doing that. It would do namespace_lock(), check ->f_path.mnt for a flag and do umount_tree() if not set, then namespace_unlock(). move_mount() would set the flag.
