On Thu, 16 Sep 2010 16:47:29 +0200 Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > On Mon, 06 Sep 2010, NeilBrown wrote: > > Add dentry_revalidate method and fail validation of either the > > upper or lower dentry has been renamed or unlinked directly in the > > otherlying filesystem. > > This allows such changes to appear promptly in the overlay providing > > the file isn't currently in use. > > I fixed up some things in the revalidation logic and tested it out. > There are some unexpected effects, but they boil down to the fact that > busy directories can't be invalidated. Mostly it works as expected. > > However, the "rearange directories so that a/b becomes b/a" trick > still strikes in evil ways. Consider the following script: > > mkdir /upper/a/b > cd /mnt/overlay/a > while true; do > cd b > mv /upper/a/b /upper/b > mv /upper/a /upper/b/a > cd a > mv /upper/b/a /upper/a > mv /upper/b /upper/a/b > done > > It will create an ever deeper directory tree on the overlay. > > Can this be prevented? Probably, e.g. lookup should make sure that > each new directory gets a *unique* set of lower and upper dentries > (e.g. by creating hash tables indexed by lower and upper dentries). > > Is it worth the trouble? The only real problem here is that an unprivileged user with direct access to 'upper' could consume unlimited kernel memory without using a corresponding mount of storage space (so quotas etc wouldn't stop them). Maybe the easiest counter-measure to that is to keep track of a 'depth' of each dentry and disallow lookups that go much beyond PATH_MAX??? Either that or rely in the sysadmin to hide the upper/lower filesystems from untrusted users. I particularly see value in sysadmins being able to modify these filesystem - not so much for 'regular users'. NeilBrown > > Any other ideas? > > Thanks, > Miklos > > -- 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
