RE: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error

["misono.tomohiro@fujitsu.com" <misono.tomohiro@fujitsu.com>]

> Subject: [PATCH v2 3/3] virtiofsd: probe unshare(CLONE_FS) and print an error
> 
> An assertion failure is raised during request processing if
> unshare(CLONE_FS) fails. Implement a probe at startup so the problem can
> be detected right away.
> 
> Unfortunately Docker/Moby does not include unshare in the seccomp.json
> list unless CAP_SYS_ADMIN is given. Other seccomp.json lists always
> include unshare (e.g. podman is unaffected):
> https://raw.githubusercontent.com/seccomp/containers-golang/master/seccomp.json
> 
> Use "docker run --security-opt seccomp=path/to/seccomp.json ..." if the
> default seccomp.json is missing unshare.

Hi, sorry for a bit late.

unshare() was added to fix xattr problem: 
  https://github.com/qemu/qemu/commit/bdfd66788349acc43cd3f1298718ad491663cfcc#
In theory we don't need to call unshare if xattr is disabled, but it is hard to get to know
if xattr is enabled or disabled in fv_queue_worker(), right?

So, it looks good to me.
Reviewed-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>

Regards,
Misono

> 
> Cc: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  tools/virtiofsd/fuse_virtio.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/tools/virtiofsd/fuse_virtio.c b/tools/virtiofsd/fuse_virtio.c
> index 3b6d16a041..9e5537506c 100644
> --- a/tools/virtiofsd/fuse_virtio.c
> +++ b/tools/virtiofsd/fuse_virtio.c
> @@ -949,6 +949,22 @@ int virtio_session_mount(struct fuse_session *se)
>  {
>      int ret;
> 
> +    /*
> +     * Test that unshare(CLONE_FS) works. fv_queue_worker() will need it. It's
> +     * an unprivileged system call but some Docker/Moby versions are known to
> +     * reject it via seccomp when CAP_SYS_ADMIN is not given.
> +     *
> +     * Note that the program is single-threaded here so this syscall has no
> +     * visible effect and is safe to make.
> +     */
> +    ret = unshare(CLONE_FS);
> +    if (ret == -1 && errno == EPERM) {
> +        fuse_log(FUSE_LOG_ERR, "unshare(CLONE_FS) failed with EPERM. If "
> +                "running in a container please check that the container "
> +                "runtime seccomp policy allows unshare.\n");
> +        return -1;
> +    }
> +
>      ret = fv_create_listen_socket(se);
>      if (ret < 0) {
>          return ret;
> --
> 2.26.2