On 2015-07-22 10:09, J. Bruce Fields wrote:
Except if the user can write to the filesystem's backing storage (be it a device or a file), and has sufficient knowledge of the on-disk structures, they can create all the cycles they want in the metadata. So unless the kernel builds the graph internally by parsing the metadata _and_ has some way to detect that the on-disk metadata has hit a cycle (which may not just involve 2 items), then you still have the potential for a DoS attack.On Wed, Jul 22, 2015 at 05:56:40PM +1000, Dave Chinner wrote:On Tue, Jul 21, 2015 at 01:37:21PM -0400, J. Bruce Fields wrote:On Fri, Jul 17, 2015 at 12:47:35PM +1000, Dave Chinner wrote: So, for example, a screwed up on-disk directory structure shouldn't result in creating a cycle in the dcache and then deadlocking.Therein lies the problem: how do you detect such structural defects without doing a full structure validation?You can prevent cycles in a graph if you can prevent adding an edge which would be part of a cycle.
Trust me, I've done this before (quite a while back when I was just starting out with programming on Linux) with hard-link cycles in an ext4 filesystem in a virtual machine just to see what would happen (IIRC, something deadlocked, I can't remember though if it was fsck or trying to access the file once the FS was mounted) (and in fact, I think I may try this again just to see if anything has changed).
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