Re: [PATCH RFC 38/41] fuse: refuse files on pseudo filesystems as passthrough backing files

[Amir Goldstein <amir73il@xxxxxxxxx>]

On Tue, Nov 10, 2026 at 10:14:54AM +0100, Christian Brauner wrote:
> The fuse server can register any regular file as a backing file. procfs
> and binfmt_misc files are already refused by the stacking depth check as
> both set s_stack_depth to FILESYSTEM_MAX_STACK_DEPTH. sysfs and the
> other pseudo filesystems aren't. Reads and writes of the fuse file would
> then be done on the pseudo file with the fuse server's credentials.
> Refuse files on pseudo filesystems just like loop does.
> 
> Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
> ---
>  fs/fuse/backing.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/fuse/backing.c b/fs/fuse/backing.c
> index 8c21a8a74efb..4e4fb810ddaa 100644
> --- a/fs/fuse/backing.c
> +++ b/fs/fuse/backing.c
> @@ -147,6 +147,9 @@ static int fuse_backing_open_file(struct fuse_conn *fc, 
> struct fuse_backing *fb,
>               if (inode->i_sb->s_stack_depth >= fc->max_stack_depth)
>                       return -ELOOP;
>  
> +             if (file_is_pseudo(file))
> +                     return -EINVAL;
> +
>               fb->type = FUSE_BACKING_PATH;
>               fb->path = file->f_path;
>               path_get(&fb->path);
> 
> -- 
> 2.53.0
> 

Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>