> Josef Sipek wrote: > >On Wed, Jan 10, 2007 at 05:12:15PM +0100, Jan Kara wrote: > >> I see :). To me it just sounds as if you want to do remount-read-only > >>for source filesystems, which is operation we support perfectly fine, > >>and after that create union mount. But I agree you cannot do quite that > >>since you need to have write access later from your union mount. So > >>maybe it's not so easy as I thought. > >> On the other hand, there was some effort to support read-only > >> bind-mounts of > >>read-write filesystems (there were even some patches floating around but > >>I don't think they got merged) and that should be even closer to what > >>you'd need... > > > >Since the RO flag is per-mount point, how do you guarantee that no one is > >messing with the fs? (I haven't looked at the patches that do per mount > >ro flag, but this would require some over-arching ro flag - in the > >superblock most likely.) > > I thought about it, wrote an email, then cancelled it as it won't work. > > what I thought was that you could a limited unionfs case would be with X > layers read-only and the top layer read-write, and what you would do is > dynamically make read only bind mounts for the the X layers and since > you control the top layer hide it from the system. > > However, read only bind mounts are great if you want a limit a process > to accessing the files read-only, as they won't have access to the other > vfs_mounts, but it does nothing for the other vfs_mounts that are using > that same file system. hence, does us no good. Right, you'd need to remount read-only all the mountpoints of one filesystem. But if we had read-only bind-mounts, you could do such things from userspace. It won't be 100% reliable (as it would be racy) but as a basic protection against stupidity of admin it should work. And it would be 100% safe against malicious intentions of average user (who has no right to create new mountpoints). Honza -- Jan Kara <jack@xxxxxxx> SuSE CR Labs - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
