On Thu, May 03, 2018 at 02:43:17PM +0100, Al Viro wrote: > On Thu, May 03, 2018 at 03:04:36PM +0200, Christian Brauner wrote: > > > >From a userspace perspective we often run into the case where we simply > > want to know whether a given mountpoint is MS_SHARED or is MS_SLAVE. > > If it is we remount it as MS_PRIVATE to prevent any propagation from > > happening. We don't care about the peer relationship or how the > > propagation is exactly setup. We only want to prevent any propagation > > from happening. > > So what's to stop you from doing exactly that -- > mount(NULL, "/", NULL, MS_PRIVATE | MS_REC, NULL) > without bothering with statfs() in the first place? It's not like the > damn thing had been costly - it's O(mounts in your namespace) with not > to high constant, and in any case cheaper than allocating all of them > back when you did clone(2). Confused... That's the case for the whole rootfs. But there are cases where I want to leave the rootfs itself alone and only remount some paths. > > > The above case is what I see most often. A more specific use-case is to > > differentiate between MS_SLAVE and MS_SHARED mountpoints. > > Mountpoints that are MS_SLAVE are kept intact and mountpoints that are > > MS_SHARED are made MS_PRIVATE. > > > > For both cases the only way to do this right now is by parsing > > /proc/<pid>/mountinfo. Yes, it is doable but still it is somewhat costly > > and annoying as e.g. those mount propagation fields are optional. > > Umm... And how would you get the list of mountpoints to feed to statfs() > if not by parsing mountinfo? IDGI... There are cases where you have list of known-mountpoints or paths to check but you don't necessarily know whether they are MS_SHARED or MS_SLAVE. There's also a case where I send a list of fds via SCM_RIGHTS to an isolated process that calls fstatvfs() on these fds to determine their mount flags without changing them but cache this info. Christian
