On Tuesday February 20, zippel@fh-brandenburg.de wrote:
> Hi,
>
> On Tue, 20 Feb 2001, Neil Brown wrote:
>
> > 2/ lookup("..").
>
> A small question:
> Why exactly is this needed?
>
> bye, Roman
Having read the subsequent posts, I now see what you are thinking and
know how to answer this.
The problem that I see is indeed related to rename, but not in the way
that Trond mentions. (Tronds issue is a real, though very different,
issue).
Suppose I have a directory path
/a/b/c/d/e/f/g
Where filehandle X refers to /a/b and filehande Y refers to
/a/b/c/d/e/f/g
Then an NFS request comes in saying
RENAME X/c to Y/h
If this was performed, then you would end up with a directory path
/a/b
and a disconnected loop:
d/e/f/g/h
^ v
+-------+
which is not good.
This possibility is protected against by the VFS layer. It serialises
all directory renames using s_vfs_rename_sem and then checks that the
source of the rename isn't an ancestor of the target.
For this check to be reliable, each dentry for a directory must be
properly connected to the root.
Now if you have a filesystem that:
- cannot do ".." lookups efficiently, or doesn't want to and
- can protect against this sort of loop (and any other issues that
the VFS usually protects against) itself
then it can (with my patch) simply define decode_fh and encode_fh and
do whatever it likes, such as create a dentry that isn't properly
connected.
get_parent is only called by nfsd_find_fh_dentry which is a helper
routine which is normally called by the decode_fh function (and is
called by the default decode_fh function).
If a filesystem provides it's own decode_fh which doesn't call
nfsd_find_fh_dentry, then get_parent won't be called and isn't needed.
NeilBrown
-
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This archive was generated by hypermail 2b29 : Fri Feb 23 2001 - 21:00:24 EST