Re: [PATCH v3 2/3] fs: fuse: add backing_files control file

[Amir Goldstein]

On Fri, May 9, 2025 at 8:34 AM Chen Linxuan via B4 Relay
<devnull+chenlinxuan.uniontech.com@xxxxxxxxxx> wrote:
>
> From: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>
>
> Add a new FUSE control file "/sys/fs/fuse/connections/*/backing_files"
> that exposes the paths of all backing files currently being used in
> FUSE mount points. This is particularly valuable for tracking and
> debugging files used in FUSE passthrough mode.
>
> This approach is similar to how fixed files in io_uring expose their
> status through fdinfo, providing administrators with visibility into
> backing file usage. By making backing files visible through the FUSE
> control filesystem, administrators can monitor which files are being
> used for passthrough operations and can force-close them if needed by
> aborting the connection.
>
> This exposure of backing files information is an important step towards
> potentially relaxing CAP_SYS_ADMIN requirements for certain passthrough
> operations in the future, allowing for better security analysis of
> passthrough usage patterns.
>
> The control file is implemented using the seq_file interface for
> efficient handling of potentially large numbers of backing files.
> Access permissions are set to read-only (0400) as this is an
> informational interface.
>
> FUSE_CTL_NUM_DENTRIES has been increased from 5 to 6 to accommodate the
> additional control file.
>
> Some related discussions can be found at links below.
>
> Link: https://lore.kernel.org/all/4b64a41c-6167-4c02-8bae-3021270ca519@xxxxxxxxxxx/T/#mc73e04df56b8830b1d7b06b5d9f22e594fba423e
> Link: https://lore.kernel.org/linux-fsdevel/CAOQ4uxhAY1m7ubJ3p-A3rSufw_53WuDRMT1Zqe_OC0bP_Fb3Zw@xxxxxxxxxxxxxx/
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxxxxx>
> ---

Looks good!
With minor nits fixed, please feel free to add:
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
(instead of Cc:)

...

>  static const struct fs_context_operations fuse_ctl_context_ops = {
> -       .get_tree       = fuse_ctl_get_tree,
> +       .get_tree = fuse_ctl_get_tree,
>  };
>
>  static int fuse_ctl_init_fs_context(struct fs_context *fsc)
> @@ -358,10 +489,10 @@ static void fuse_ctl_kill_sb(struct super_block *sb)
>  }
>
>  static struct file_system_type fuse_ctl_fs_type = {
> -       .owner          = THIS_MODULE,
> -       .name           = "fusectl",
> +       .owner = THIS_MODULE,
> +       .name = "fusectl",
>         .init_fs_context = fuse_ctl_init_fs_context,
> -       .kill_sb        = fuse_ctl_kill_sb,
> +       .kill_sb = fuse_ctl_kill_sb,
>  };

Please undo these whitespace changes.

Thanks,
Amir.