Re: VFS regression with 9pfs ("Lookup would have caused loop")

[Christian Brauner <brauner@kernel.org>]

On Tue, Oct 15, 2024 at 08:07:10PM +0200, Thorsten Leemhuis wrote:
> Hi Will! Top-posting for once, to make this easily accessible to everyone.
> 
> Thx for bringing this to my attention. I had hoped that Eric might reply
> and waited a bit, but that did not happen. I kind of expected that, as
> he seems to be  somewhat afk, as the last mail from him on lore is from
> mid-September; and in the weeks before that he did not post much either.
> Hmmm. :-/
> 
> CCed Christian and Al, maybe they might be able to help directly or
> indirectly somehow. If not, we likely need to get Linus involved to
> decide if we want to at least temporarily revert the changes you mentioned.

Sorry, I'm just getting to this thread now as I'm still out with a
fscking case of the flu.
/me reads...

> 
> Ciao, Thorsten
> 
> On 09.10.24 17:34, Will Deacon wrote:
> > On Mon, Sep 23, 2024 at 11:05:08AM +0100, Will Deacon wrote:
> >>
> >> I'm trying to use kvmtool to run a simple guest under an Android host
> >> but, for v6.9+ guest kernels, 'init' reliably fails to run from a 9pfs
> >> mount because VFS emits this error:
> >>
> >>   | VFS: Lookup of 'com.android.runtime' in 9p 9p would have caused loop
> >>
> >> The host directory being shared is a little odd, as it has symlinks out
> >> to other mount points. In the guest, /apex is a symlink to /host/apex.
> >> On the host, /apex/com.android.runtime is a mounted loopback device:
> >>
> >> /dev/block/loop13 on /apex/com.android.runtime type ext4 (ro,dirsync,seclabel,nodev,noatime)
> >>
> >> This used to work prior to 724a08450f74 ("fs/9p: simplify iget to remove
> >> unnecessary paths") and it looks like Oleg ran into something similar
> >> before:
> >>
> >>   https://lore.kernel.org/all/20240408141436.GA17022@redhat.com/
> >>
> >> although he worked around it by driving QEMU with different options.
> >>
> >> I can confirm that reverting the following commits gets mainline guests
> >> working again for me:
> >>
> >> 	724a08450f74 "fs/9p: simplify iget to remove unnecessary paths"
> >> 	11763a8598f8 "fs/9p: fix uaf in in v9fs_stat2inode_dotl"
> >> 	10211b4a23cf "fs/9p: remove redundant pointer v9ses"
> >> 	d05dcfdf5e16 " fs/9p: mitigate inode collisions"
> >>
> >> Do you have any better ideas? I'm happy to test anything you might have,
> >> since this is 100% reproducible on my setup.
> > 
> > Adding the regression tracker as I've not heard anything back on this :(
> > #regzbot introduced: 724a08450f74