Revert "fs/9p: simplify iget to remove unnecessary paths"

This reverts commit 724a08450f74b02bd89078a596fd24857827c012.

This code simplification introduced significant regressions on servers
that do not remap inode numbers when exporting multiple underlying
filesystems with colliding inodes, as can be illustrated with simple
tmpfs exports in qemu with remapping disabled:
```
# host side
cd /tmp/linux-test
mkdir m1 m2
mount -t tmpfs tmpfs m1
mount -t tmpfs tmpfs m2
mkdir m1/dir m2/dir
echo foo > m1/dir/foo
echo bar > m2/dir/bar

# guest side
# started with -virtfs local,path=/tmp/linux-test,mount_tag=tmp,security_model=mapped-file
mount -t 9p -o trans=virtio,debug=1 tmp /mnt/t

ls /mnt/t/m1/dir
# foo
ls /mnt/t/m2/dir
# bar (works ok if directry isn't open)

# cd to keep first dir's inode alive
cd /mnt/t/m1/dir
ls /mnt/t/m2/dir
# foo (should be bar)
```
Other examples can be crafted with regular files with fscache enabled,
in which case I/Os just happen to the wrong file leading to
corruptions, or guest failing to boot with:
  | VFS: Lookup of 'com.android.runtime' in 9p 9p would have caused loop

In theory, we'd want the servers to be smart enough and ensure they
never send us two different files with the same 'qid.path', but while
qemu has an option to remap that is recommended (and qemu prints a
warning if this case happens), there are many other servers which do
not (kvmtool, nfs-ganesha, probably diod...), we should at least ensure
we don't cause regressions on this:
- assume servers can't be trusted and operations that should get a 'new'
inode properly do so. commit d05dcfdf5e16 (" fs/9p: mitigate inode
collisions") attempted to do this, but v9fs_fid_iget_dotl() was not
called so some higher level of caching got in the way; this needs to be
fixed properly before we can re-apply the patches.
- if we ever want to really simplify this code, we will need to add some
negotiation with the server at mount time where the server could claim
they handle this properly, at which point we could optimize this out.
(but that might not be needed at all if we properly handle the 'new'
check?)

Fixes: 724a08450f74 ("fs/9p: simplify iget to remove unnecessary paths")
Reported-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/all/20240408141436.GA17022@redhat.com/
Link: https://lkml.kernel.org/r/20240923100508.GA32066@willie-the-truck
Cc: stable@vger.kernel.org # v6.9+
Message-ID: <20241024-revert_iget-v1-4-4cac63d25f72@codewreck.org>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

0 files changed, 0 insertions, 0 deletions