On Fri, Aug 10, 2018 at 04:53:58PM +0100, David Howells wrote: > Theodore Y. Ts'o <tytso@xxxxxxx> wrote: > > > Even *with* file system support, there's no way today for the VFS to > > keep track of whether a pathname resolution came through one > > mountpoint or another, so I can't do something like this: > > Ummm... Isn't that encoded in the vfsmount pointer in struct path? Well, yes, and we do use this as a hack to make read-only bind mounts work. But that's done as a special case, and it's for permissions checking only. The big problem is that there is single dentry cache object regardless of which mount point was used to access it. So that makes it impossible to support case folding as a mount-pointism. > > However, the case folding stuff - is that a superblockism of a mountpointism? It's a superblock-ism. As far as I know the *only* thing that we can support as a mount-pointism is the ro flag, and that's handled as a special case, and only if the original superblock was mounted read/write. ey That was my point; aside from the ro flag, we can't support any other mount options as a per-mount point thing, so the only thing we can do is to fail the mount if there are conflicting mount options. And I'm not really sure it helps the container use case, since the whole point is they want their "guest" to be able to blithely run "mount /dev/sda1 -o noxattr /mnt" and not worry about the fact that in some other container, someone had run "mount /dev/sda1 -o xattr /mnt". But having the second mount fail because of conflicting mount option breaks the illusion that containers are functionally as rich as VM's. So before you put in lots of work to support rejecting the attmpted mount if the mount options conflict, are we sure people will actually find this to be useful? Because it's not only fsopen() work for you, but each file system is going to have to implement new functions to answer the question "are these mount options conflicting or not?". Are we sure it's worth the effort? - Ted
