Re: [PATCH 00/13] overlay filesystem: request for inclusion (v16)

From: Al Viro
Date: Wed Mar 20 2013 - 17:32:12 EST

On Wed, Mar 20, 2013 at 03:41:08PM -0400, George Spelvin wrote:
> Sorry for being so very late to the party, but rather than messing
> with xattrs, why not just have a specific file (say, default /.whiteout,
> but selectable via a mount option) and links to it are counted as
> whiteout entries?
> All you need to do is resolve the link (it's probably a good idea
> to allow symlinks to avoid hard-link count limits) and compare the
> fs and inode number.
> It's kind of hackish, but it could be done on pretty much *any* Unix
> file system. VFAT would be SOL, but that's probably acceptable.
> (Any security options on mount point crossing or consistent ownership
> of symlinks that you want to impose would probably be of general VFS
> use, and again, not FS-specific.)

Yeah... Now, think what rm -rf foo/ would be doing. We read the
underlying directory. For each file in it we create a link to that
magical file of yours in covering one. _Then_ we do rmdir(), and what a joy
it turns out to be - we
* lock the covering directory
* read the covering directory and stat everything in it, checking that
it's a link to your file; we also read the underlying directory and verify
that everything in it has a matching whiteout in the covering one.
* once we are through, we read it *again*, this time doing unlinks
and praying real hard we won't crash in the meanwhile
* once that is finished, we finally can rmdir the fucker and unlock
its inode.

I'm sorry, but this is insane.
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