Re: Mount failure due to restricted access to a point along the mount path

[Steve French]

On Tue, May 14, 2013 at 3:51 AM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Fri, 10 May 2013 10:27:54 -0400
> Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>
>> On Fri, 10 May 2013 16:13:30 +0200
>> Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>
>> > Hi,
>> >
>> > A while ago this was discussed:
>> >
>> >   http://thread.gmane.org/gmane.linux.kernel.cifs/7779
>> >
>> > This is essentially a regression introduced by the shared superblock
>> > changes in 3.0 and several SUSE customers are complaining about it.
>> > I've created a temporary fix which reverts 29 commits related to the
>> > shared superblock changes.  It works, but it's obviously not a
>> > permanent fix, especially since we definitely don't want to diverge
>> > from mainline.
>> >
>> > Is this issue being worked on?  Don't other distros have similar reports?
>> >
>> > Thanks,
>> > Miklos
>>
>> I don't know of anyone currently working on it. There are a couple of
>> possible approaches to fixing it, I think:
>>
>> 1) if the dentries to get down to the root of the mount don't already
>> exist, then attach some sort of "placeholder" inode that can be fleshed
>> out later if and when the dentry is accessed via other means.
>>
>> 2) do something like what NFS does (see commit 54ceac45). This becomes
>> a bit more complicated due to the fact that the server may not hand out
>> real inode numbers and we sometimes have to fake them up.
>>
>> #1 is probably simpler to implement, but I'll confess that I haven't
>> thought through all of the potential problems with it.
>>
>
> So, giving this some more thought, I think #2 is really the correct way
> to fix this. Here's the main problem though:
>
> Suppose someone mounts:
>
>     //server/share/foo/bar/baz
>
> We make the sb->s_root point to the top level share, and then create a
> disconnected dentry for "baz" to return from ->mount.
>
> Then, a little while later, //server/share gets mounted separately and
> a user walks down to /foo/bar/baz within the same share.
>
> How do we ensure that we don't end up with two "baz" dentries in this
> situation? With NFS, we can be reasonably sure that there's a 1:1
> correspondance of filehandle to inode.
>
> Under CIFS, it's possible that it's faking up inode numbers if the
> server doesn't provide them via a UniqueID field. The only real
> identifying info we have for the inode in that case is the pathname.

Since this (support for server generated inode numbers) is most common
case (especially with SMB2 and later) - I don't mind making dependency
on the server supporting UniqueID for this.
-- 
Thanks,

Steve
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