COW Links
Posted Apr 8, 2004 13:33 UTC (Thu) by
joern_engel (guest, #4663)
Parent article:
COW Links
Nice to see that LWN has picked up on my ugly patch. Yes, it is far from being complete and should be used with care. But as bad as it is, I already use it daily on my machine and nothing serious broke yet. :)
To answer some of the comments:
Inode numbers:
Contrary to Linus' comment, there are good arguments against using the same inode for cowlinks. People want to change the permissions, ownership and such independently for each copy. If you use my patch, be prepared for incompatible changes.
POSIX compliance:
It appears, as if complete POSIX compliance would be impossible anyway. The real copy is deferred until later, possible never. Whenever it does happen, -ENOSPC is possible, which is somewhat unexpected during open. Erik Biederman claims that no new problems arise that don't exist with sparse files already, but there may be more corner cases hidden somewhere.
Unionfs/Unionmount:
Unionmount can in principle do the same, unless you actually try do use it. Having to mount a new filesystem for each copy is a) clumsy, b) dangerous because you have to remember the exact order in which to remount everything after reboot and c) prevents anyone but root to use this feature. Bad idea.
On the other hand, it is possible to implement union mount on to of cowlinks, which may or may not be a sane design. I'll try to work closely with some of the people planning union mounts for 2.8.
"Messy" concept:
"Messy" is a description for both cowlinks and union mounts, until you start to think things through and define sane semantics. Think of the mess that can happen when you have hardlinks to symlinks. Obviously that is already forbidden and for the curious, this is how the cowlinks fit into the picture:
o Symlinks can point to hardlinks or cowlinks or regular files.
o Hardlinks can point to cowlinks or regular files.
o Cowlinks can point to invisible files.
o Invisible files cannot be accessed, except through a cowlink.
Security issues:
It appears, as if no new problems arise from the new design. Famous last words.
The current patch can definitely create a ton of new problems. Or rather, it can make existing problems exploitable. If all software was perfect, the existing patch was harmless.
(
Log in to post comments)
