Re: [PATCH next] smb: client: Fix NULL vs ERR_PTR() returns in cifs_get_tcon_super()

[Wang Zhaolong]

> Since Paulo pointed out a problem with v4 of this patch, an obvious
> question is Dan's patch "independent enough" to take or would it make
> the v5 of your patch harder.  Let me know when there is a v5 of the
> patch so we can do more testing and review
>
> On Mon, Aug 18, 2025 at 8:30 AM Wang Zhaolong
> <wangzhaolong@xxxxxxxxxxxxxxx> wrote:
> >
Hi Steve, Paulo, and Dan,

Thank you very much for your attention to this patch.

The NULL pointer issue has already been addressed in the V4
version of the patch. I have considered submitting a V5 version
of the patch, but I have not yet decided whether to proceed with it.

While further analyzing the code, I discovered the following
potential issues in the implementation of `cifs_tree_connect()` in
`dfs.c`, which calls `cifs_get_dfs_tcon_super()`:

1. `cifs_get_dfs_tcon_super()` only returns a single superblock reference.
   Using this superblock, it retrieves the associated `cifs_sb`. However,
   a single tcon is supposed to be associated with multiple `cifs_sb`
   instances. I am unsure whether there is a mechanism that guarantees a
   DFS tcon (where `tcon->origin_fullpath` is non-NULL) has exactly one
   associated `cifs_sb`. My current patch series only retrieves the first
   `cifs_sb` that satisfies the condition, which may not be sufficient.

2. The only purpose of retrieving the `cifs_sb` is to update the `prepath`
   field of the `cifs_sb` in `tree_connect_dfs_target()` by calling
   `cifs_update_super_prepath()`. No other information from the VFS-layer
   superblock is used. However, retrieving the superblock is also meant to
   pin it in memory to prevent it from being released. Therefore, I am
   considering whether to simplify the process of updating the prepath.

If I decide to submit a V5 version of the patch, I will need to fully investigate:
1. Whether a DFS tcon can be associated with multiple `cifs_sb` instances.
2. If multiple `cifs_sb` instances are associated with a DFS tcon, whether
   the `prepath` field of all associated `cifs_sb` instances needs to be
    updated.

Only after thoroughly understanding these two points can I come up with a
better implementation for the V5 patch.

Resolving these questions will take some time. If anyone can provide
guidance or assistance, I would greatly appreciate it.

Best regards,
Wang Zhaolong
Wang Zhaolong