Setting mount propagation type in new mount API
Hello David, Miklos, I've been looking at the new mount API (fsopen(), fsconfig(), fsmount(), move_mount(), etc.) and among the details that remain mysterious to me is this: how does one set the propagation type (private/shared/slave/unbindable) of a new mount and change the propagation type of an existing mount? I've looked at the kernel source for a bit, and did not see how this is possible. The draft manual pages sent out a few months ago provide little clue, with the only hint being in the draft fsopen(2) page, which says of fsmount(): fsmount() takes the file descriptor returned by fsopen() and cre‐ ates a mount object for the filesystem root specified there. The attributes of the mount object are set from the mount_attrs param‐ eter. The attributes specify the propagation and mount restric‐ tions to be applied to accesses through this mount. However, that text appears *not* to be true. The 'mount_attrs' argument of fsmount() does not seem to permit specification of propagation type, since in the kernel there is this check: if (attr_flags & ~(MOUNT_ATTR_RDONLY | MOUNT_ATTR_NOSUID | MOUNT_ATTR_NODEV | MOUNT_ATTR_NOEXEC | MOUNT_ATTR__ATIME | MOUNT_ATTR_NODIRATIME)) return -EINVAL; Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/
Re: Setting mount propagation type in new mount API
On Fri, May 15, 2020 at 1:40 PM Michael Kerrisk (man-pages) wrote: > > Hello David, Miklos, > > I've been looking at the new mount API (fsopen(), fsconfig(), > fsmount(), move_mount(), etc.) and among the details that remain > mysterious to me is this: how does one set the propagation type > (private/shared/slave/unbindable) of a new mount and change the > propagation type of an existing mount? Existing mount can be chaged with mount(NULL, path, NULL, MS_$(propflag), NULL). To do that with a detached mount created by fsmount(2) the "/proc/self/fd/$fd" trick can be used. The plan was to introduce a mount_setattr(2) syscall, but that hasn't happened yet... I'm not sure we should be adding propagation flags to fsmount(2), since that is a less generic mechanism than mount_setattr(2) or just plain mount(2) as shown above. Thanks, Miklos
Re: Setting mount propagation type in new mount API
On Fri, May 15, 2020 at 3:04 PM Miklos Szeredi wrote: > > On Fri, May 15, 2020 at 1:40 PM Michael Kerrisk (man-pages) > wrote: > > > > Hello David, Miklos, > > > > I've been looking at the new mount API (fsopen(), fsconfig(), > > fsmount(), move_mount(), etc.) and among the details that remain > > mysterious to me is this: how does one set the propagation type > > (private/shared/slave/unbindable) of a new mount and change the > > propagation type of an existing mount? > > Existing mount can be chaged with mount(NULL, path, NULL, MS_$(propflag), > NULL). > > To do that with a detached mount created by fsmount(2) the > "/proc/self/fd/$fd" trick can be used. > > The plan was to introduce a mount_setattr(2) syscall, but that hasn't > happened yet... I'm not sure we should be adding propagation flags to > fsmount(2), since that is a less generic mechanism than > mount_setattr(2) or just plain mount(2) as shown above. Also note that only setting MS_SHARED makes sense on a new mount returned by fsmount(2) because - MS_PRIVATE is a no op, due to mount already being private - same for MS_SLAVE, since it's only different from MS_PRIVATE on mounts receiving propagation, which a new mount by definition isn't - MS_UNBINDABLE just prevents move_mount(2) from working so that's not really useful, though at least it does something A more interesting issue is whether we'd want to control the propagation of the target when moving into a shared tree. I.e. should there be a MOVE_MOUNT_DONTPROPAGATE flag for move_mount(20 that prevents the new mount from being propagated... Thanks, Miklos
Re: Setting mount propagation type in new mount API
Michael Kerrisk (man-pages) wrote: > I've been looking at the new mount API (fsopen(), fsconfig(), > fsmount(), move_mount(), etc.) and among the details that remain > mysterious to me is this: how does one set the propagation type > (private/shared/slave/unbindable) of a new mount and change the > propagation type of an existing mount? Christian said he was going to have a go at writing mount_setattr(). It's not trivial as it has to be able to handle AT_RECURSIVE. David
Re: Setting mount propagation type in new mount API
On Mon, May 18, 2020 at 03:30:34PM +0100, David Howells wrote: > Michael Kerrisk (man-pages) wrote: > > > I've been looking at the new mount API (fsopen(), fsconfig(), > > fsmount(), move_mount(), etc.) and among the details that remain > > mysterious to me is this: how does one set the propagation type > > (private/shared/slave/unbindable) of a new mount and change the > > propagation type of an existing mount? > > Christian said he was going to have a go at writing mount_setattr(). It's not > trivial as it has to be able to handle AT_RECURSIVE. Right, I've put this on my roadmap now. It's becoming more urgent for us too since I've already switched over a few bits to the new mount api to make use of detached/anonymous mounts. I've planned to start working on a version early next week. Christian
