On Wed, Jul 06, 2005 at 09:33:13PM -0500, David Masover wrote:
> And speaking of which, the only doomsday scenario (running out of RAM)
> that I can think of with this scheme is if we have a ton of hardlinks to
> the same file and we try to move one of them. But this scales linearly
> with the number of hardlinks, I think. Maybe not quite, but certainly
> not exponentially.
>
> The only other doomsday scenario is if we have a ludicrously deep tree.
rename(a/b, c/b), if a and c are identical.
End result, either you deadlock trying to lock the same object twice, or
you end up removing b since the target of a rename is unlinked.
The VFS uses dentries, there is one per hardlinked object, and they have
a single parent only. So a/b and c/b are represented by the same inode,
but they have completely different dentry-aliases associated with them.
Similar things for removal, we can safely remove a file, but not a
directory that still has children, if you have 'meta-files' hanging of a
file, you'd have to get rid of the metadata objects first.
If you want to use a file as a directory, it probably will need the same
restrictions as a directory if you expect the dcache and VFS locking to
work correctly. So that means, _no hardlinks to files_ (the file system
could internally implement copy-on-write type links, or use a content
addressable storage to deal with diskspace issues) and the file system
probably has to d_unhash/destroy metadata objects before it can unlink
the file object, etc.
> To make this work in real usage, not DOS testing, we really need both of
> those, and even then I'm not sure it can work. What's the maximum
> number of hardlinks supported to a single file?
I believe nlink is a 16-bit value, so that would be either 32K or 64K
depending on signedness.
> What's the maximum tree depth?
Last time I checked, PATH_MAX is 4096, so that would be about 2048
single character directory names.
> Can these be limited to prevent actual DOS attempts?
Is that the 'nobody needs more than 64KB of memory' kind of DOS?
Jan
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