On Fri, 2010-02-12 at 22:30 -0800, jdow wrote: > From: "Marko Vojinovic" <vvmarko@xxxxxxxxx> > Sent: Friday, 2010/February/12 20:12 > > > > On Saturday 13 February 2010 00:09:38 Suvayu Ali wrote: > >> On Friday 12 February 2010 06:06 AM, Marko Vojinovic wrote: > >> > Now, hard links are not allowed for directories since they would allow > >> > for creation of loops (a directory containing itself), which is a Bad > >> > Idea, since it breaks recursion. The filesystem needs to be a *tree* if > >> > recursion is to function, so loops are forbidden (how would you delete > >> > a > >> > directory which contains itself?). > >> > >> I don't quite follow you here, don't you mean hardlinking directories > >> have the risk of introducing recursion? > > > > No, I am saying that hard linking directories allows creation of loop > > structures. These structures break any recursive algorithm that tries to > > go > > "downwards" in some directory structure. > > > > Deleting directories is a textbook example. In order to delete a > > directory, > > you first have to delete all files and subdirectories that it contains, > > and once > > it is empty, delete the directory itself. So deletion goes on via a > > recursive > > algorithm: > > > > 1) check whether there are files or dirs in target dir > > 2) delete any files present > > 3) execute yourself (from step 1) for every subdir as target dir > > 4) target dir is now empty, unlink it > > 5) done > > > > Now consider the directory structure of the form where inside dir a/ you > > have > > dir b/, and inside it dir c/ which is a hard link back to a/ (this is a > > simplest loop situation, much more complicated are possible). IOW, the > > directory a/ contains itself as a subdirectory two levels down. Now > > execute > > the above algorithm in order to delete a/ --- the algorithm will try to > > delete > > all subdirectories, namely b/, and execute itself over b/ in step 3. But > > to > > delete b/, it needs to execute itself over c/ which is actually a/. But to > > delete a/ it needs to execute itself over b/... > > It's even worse than that, Marko. > > You have a directory tree /a/b/c/d. You create a hard link to directory > /a/b inside of d. You get /a/b/c/d/b/c/d/b/c/d.... > > NOW you unlink /a/b. Now when you do an ls on /a you get . and .. only. > Now, I challenge you to delete the /b/c/d loop and everything it contains > without reformatting the disk or using low level disk edit functions. This is what I referred to in my original answer. Permitting arbitrary links to directories allows us to create structures which are disconnected from the root of the tree, which is of course Bad. poc -- users mailing list users@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe or change subscription options: https://admin.fedoraproject.org/mailman/listinfo/users Guidelines: http://fedoraproject.org/wiki/Communicate/MailingListGuidelines
