[Linux-cachefs] [PATCH v2 2/2] cachefiles: extend ro check to private mount

[Amir Goldstein]

On Wed, Apr 7, 2021 at 12:02 PM Christian Brauner <brauner at kernel.org> wrote:
>
> From: Christian Brauner <christian.brauner at ubuntu.com>
>
> So far cachefiles only verified that the superblock wasn't read-only but
> didn't check whether the mount was. This made sense when we did not use
> a private mount because the read-only state could change at any point.
>
> Now that we have a private mount and mount properties can't change
> behind our back extend the read-only check to include the vfsmount.
>
> The __mnt_is_readonly() helper will check both the mount and the
> superblock.  Note that before we checked root->d_sb and now we check
> mnt->mnt_sb but since we have a matching <vfsmount, dentry> pair here
> this is only syntactical change, not a semantic one.
>
> Here's how this works:
>
> mount -o ro --bind /var/cache/fscache/ /var/cache/fscache/
>
> systemctl start cachefilesd
>   Job for cachefilesd.service failed because the control process exited with error code.
>   See "systemctl status cachefilesd.service" and "journalctl -xe" for details.
>
> dmesg | grep CacheFiles
>   [    2.922514] CacheFiles: Loaded
>   [  272.206907] CacheFiles: Failed to register: -30
>
> errno 30
>   EROFS 30 Read-only file system
>
> Cc: David Howells <dhowells at redhat.com>
> Cc: linux-cachefs at redhat.com
> Cc: linux-fsdevel at vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
> ---
> /* v2 */
> patch introduced
> ---
>  fs/cachefiles/bind.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/cachefiles/bind.c b/fs/cachefiles/bind.c
> index bbace3e51f52..cb8dd9ecc090 100644
> --- a/fs/cachefiles/bind.c
> +++ b/fs/cachefiles/bind.c
> @@ -141,8 +141,13 @@ static int cachefiles_daemon_add_cache(struct cachefiles_cache *cache)
>             !root->d_sb->s_op->sync_fs)
>                 goto error_unsupported;
>
> +       /*
> +        * Verify our mount and superblock aren't read-only.
> +        * Note, while our private mount is guaranteed to not change anymore
> +        * the superblock may still go read-only later.
> +        */
>         ret = -EROFS;
> -       if (sb_rdonly(root->d_sb))
> +       if (__mnt_is_readonly(cache->mnt))
>                 goto error_unsupported;
>

I suppose ovl_get_upper() and ecryptfs_mount() could use a similar fix?
I can post the ovl fix myself.

Thanks,
Amir.