netdev - Re: [PATCH bpf-next 3/3] libbpf: Add pin option to automount BPF filesystem before pinning

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Date:   Tue, 22 Oct 2019 11:28:45 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 3/3] libbpf: Add pin option to automount BPF
 filesystem before pinning

On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@...hat.com>
>
> While the current map pinning functions will check whether the pin path is
> contained on a BPF filesystem, it does not offer any options to mount the
> file system if it doesn't exist. Since we now have pinning options, add a
> new one to automount a BPF filesystem at the pinning path if that is not

Next thing we'll be adding extra options to mount BPF FS... Can we
leave the task of auto-mounting BPF FS to tools/applications?

> already pointing at a bpffs.
>
> The mounting logic itself is copied from the iproute2 BPF helper functions.
>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  tools/lib/bpf/libbpf.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf.h |    5 ++++-
>  2 files changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index aea3916de341..f527224bb211 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -37,6 +37,7 @@
>  #include <sys/epoll.h>
>  #include <sys/ioctl.h>
>  #include <sys/mman.h>
> +#include <sys/mount.h>
>  #include <sys/stat.h>
>  #include <sys/types.h>
>  #include <sys/vfs.h>
> @@ -4072,6 +4073,35 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
>         return 0;
>  }
>
> +static int mount_bpf_fs(const char *target)
> +{
> +       bool bind_done = false;
> +
> +       while (mount("", target, "none", MS_PRIVATE | MS_REC, NULL)) {

what does this loop do? we need some comments explaining what's going
on here (or better yet just drop this entirely and let
bpftool/iproute2 do the mounting).

> +               if (errno != EINVAL || bind_done) {
> +                       pr_warning("mount --make-private %s failed: %s\n",
> +                                  target, strerror(errno));
> +                       return -1;
> +               }
> +
> +               if (mount(target, target, "none", MS_BIND, NULL)) {
> +                       pr_warning("mount --bind %s %s failed: %s\n",
> +                                  target, target, strerror(errno));
> +                       return -1;
> +               }
> +
> +               bind_done = true;
> +       }
> +
> +       if (mount("bpf", target, "bpf", 0, "mode=0700")) {
> +               fprintf(stderr, "mount -t bpf bpf %s failed: %s\n",
> +                       target, strerror(errno));
> +               return -1;
> +       }
> +
> +       return 0;
> +}
> +
>  static int get_pin_path(char *buf, size_t buf_len,
>                         struct bpf_map *map, struct bpf_object_pin_opts *opts,
>                         bool mkdir)
> @@ -4102,6 +4132,23 @@ static int get_pin_path(char *buf, size_t buf_len,

Nothing in `get_pin_path` indicates that it's going to do an entire FS
mount, please split this out of get_pin_path.

>                 err = make_dir(path);
>                 if (err)
>                         return err;
> +
> +               if (OPTS_GET(opts, mount_bpf_fs, false)) {
> +                       struct statfs st_fs;
> +                       char *cp;
> +
> +                       if (statfs(path, &st_fs)) {
> +                               char errmsg[STRERR_BUFSIZE];
> +
> +                               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> +                               pr_warning("failed to statfs %s: %s\n", path, cp);
> +                               return -errno;
> +                       }
> +                       if (st_fs.f_type != BPF_FS_MAGIC &&
> +                           mount_bpf_fs(path)) {
> +                               return -EINVAL;
> +                       }
> +               }
>         }
>
>         len = snprintf(buf, buf_len, "%s/%s", path, bpf_map__name(map));
> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> index 2131eeafb18d..76b9a6cc7063 100644
> --- a/tools/lib/bpf/libbpf.h
> +++ b/tools/lib/bpf/libbpf.h
> @@ -143,8 +143,11 @@ struct bpf_object_pin_opts {
>          * and this type used for all maps instead.
>          */
>         enum libbpf_pin_type override_type;
> +
> +       /* Whether to attempt to mount a BPF FS if it's not already mounted */
> +       bool mount_bpf_fs;
>  };
> -#define bpf_object_pin_opts__last_field override_type
> +#define bpf_object_pin_opts__last_field mount_bpf_fs
>
>  LIBBPF_API int bpf_object__pin_maps(struct bpf_object *obj, const char *path);
>  LIBBPF_API int bpf_object__unpin_maps(struct bpf_object *obj,
>