Message-ID: <CAEf4Bzb_FdXRo-LcgpjDqPe78hZoUkQsKZZET3HM-vZWc5SYZg@mail.gmail.com>
Date:   Tue, 29 Oct 2019 11:02:42 -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 v3 2/4] libbpf: Store map pin path and status in
 struct bpf_map

On Tue, Oct 29, 2019 at 2:01 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Sun, Oct 27, 2019 at 1:53 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> From: Toke Høiland-Jørgensen <toke@...hat.com>
> >>
> >> Support storing and setting a pin path in struct bpf_map, which can be used
> >> for automatic pinning. Also store the pin status so we can avoid attempts
> >> to re-pin a map that has already been pinned (or reused from a previous
> >> pinning).
> >>
> >> Acked-by: Andrii Nakryiko <andriin@...com>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> >> ---
> >>  tools/lib/bpf/libbpf.c   |  115 ++++++++++++++++++++++++++++++++++++----------
> >>  tools/lib/bpf/libbpf.h   |    3 +
> >>  tools/lib/bpf/libbpf.map |    3 +
> >>  3 files changed, 97 insertions(+), 24 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index ce5ef3ddd263..eb1c5e6ad4a3 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -226,6 +226,8 @@ struct bpf_map {
> >>         void *priv;
> >>         bpf_map_clear_priv_t clear_priv;
> >>         enum libbpf_map_type libbpf_type;
> >> +       char *pin_path;
> >> +       bool pinned;
> >>  };
> >>
> >>  struct bpf_secdata {
> >> @@ -4025,47 +4027,118 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
> >>         char *cp, errmsg[STRERR_BUFSIZE];
> >>         int err;
> >>
> >> -       err = check_path(path);
> >> -       if (err)
> >> -               return err;
> >> -
> >>         if (map == NULL) {
> >>                 pr_warn("invalid map pointer\n");
> >>                 return -EINVAL;
> >>         }
> >>
> >> -       if (bpf_obj_pin(map->fd, path)) {
> >> -               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> >> -               pr_warn("failed to pin map: %s\n", cp);
> >> -               return -errno;
> >> +       if (map->pinned) {
> >> +               pr_warn("map already pinned\n");
> >
> > it would be helpful to print the name of the map, otherwise user will
> > have to guess
>
> Well, the existing error message didn't include the map name, so I was
> just being consistent. But sure I can change it (and the old message as
> well).
>
> >> +               return -EEXIST;
> >> +       }
> >> +
> >> +       if (path && map->pin_path && strcmp(path, map->pin_path)) {
> >> +               pr_warn("map already has pin path '%s' different from '%s'\n",
> >> +                       map->pin_path, path);
> >
> > here pin_path probably would be unique enough, but for consistency we
> > might want to print map name as well
> >
> >> +               return -EINVAL;
> >> +       }
> >> +
> >> +       if (!map->pin_path && !path) {
> >> +               pr_warn("missing pin path\n");
> >
> > and here?
> >
> >> +               return -EINVAL;
> >>         }
> >>
> >> -       pr_debug("pinned map '%s'\n", path);
> >> +       if (!map->pin_path) {
> >> +               map->pin_path = strdup(path);
> >> +               if (!map->pin_path) {
> >> +                       err = -errno;
> >> +                       goto out_err;
> >> +               }
> >> +       }
> >
> > There is a bit of repetition of if conditions, based on whether we
> > have map->pin_path set (which is the most critical piece we care
> > about), so that makes it a bit harder to follow what's going on. How
> > about this structure, would it make a bit clearer what the error
> > conditions are? Not insisting, though.
> >
> > if (map->pin_path) {
> >   if (path && strcmp(...))
> >     bad, exit
> > else { /* no pin_path */
> >   if (!path)
> >     very bad, exit
> >   map->pin_path = strdup(..)
> >   if (!map->pin_path)
> >     also bad, exit
> > }
>
> Hmm, yeah, this may be better...
>
> >> +
> >> +       err = check_path(map->pin_path);
> >> +       if (err)
> >> +               return err;
> >> +
> >
> > [...]
> >
> >>
> >> +int bpf_map__set_pin_path(struct bpf_map *map, const char *path)
> >> +{
> >> +       char *old = map->pin_path, *new;
> >> +
> >> +       if (path) {
> >> +               new = strdup(path);
> >> +               if (!new)
> >> +                       return -errno;
> >> +       } else {
> >> +               new = NULL;
> >> +       }
> >> +
> >> +       map->pin_path = new;
> >> +       if (old)
> >> +               free(old);
> >
> > you don't really need old, just free map->pin_path before setting it
> > to new. Also assigning new = NULL will simplify if above.
>
> Right, will fix.
>
> >> +
> >> +       return 0;
> >> +}
> >> +
> >> +const char *bpf_map__get_pin_path(struct bpf_map *map)
> >> +{
> >> +       return map->pin_path;
> >> +}
> >> +
> >> +bool bpf_map__is_pinned(struct bpf_map *map)
> >> +{
> >> +       return map->pinned;
> >> +}
> >> +
> >>  int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
> >>  {
> >>         struct bpf_map *map;
> >> @@ -4106,17 +4179,10 @@ int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
> >
> > I might have missed something the change in some other patch, but
> > shouldn't pin_maps ignore already pinned maps? Otherwise we'll be
> > generating unnecessary warnings?
>
> Well, in the previous version this was in one of the options you didn't
> like. If I just change pin_maps() unconditionally, that will be a change
> in behaviour in an existing API. So I figured it would be better to
> leave this as-is. I don't think this function is really useful along
> with the auto-pinning anyway. If you're pinning all maps, why use
> auto-pinning? And if you want to do something custom to all the
> non-pinned maps you'd probably iterate through them yourself anyway and
> can react appropriately?

Auto-pinned maps didn't exist before, so interaction between
auto-pinned and explicitly pinned maps is a new behavior.

With current code using explicit pin_maps and auto-pinned maps is
impossible, or am I missing something? While admittedly scenarios in
which you'll have to use explicit bpf_object__pin_maps() while you
have auto-pinned maps and bpf_map__set_pin_path() are quite exotic
(e.g., auto-pin some maps at default path and pin all the rest at some
other custom root), I think we should still try to make existing APIs
combinable in some sane way.

The only downside of ignoring already pinned maps is that while
previously calling pin_maps() twice in a row would fail fails second
time, now the second pin_maps() will be a noop. I think that's benign
and acceptable change in behavior? WDYT?

>
> >>
> >>  err_unpin_maps:
> >>         while ((map = bpf_map__prev(map, obj))) {
> >> -               char buf[PATH_MAX];
> >> -               int len;
> >> -
> >> -               len = snprintf(buf, PATH_MAX, "%s/%s", path,
> >> -                              bpf_map__name(map));
> >> -               if (len < 0)
> >> -                       continue;
> >> -               else if (len >= PATH_MAX)
> >> +               if (!map->pin_path)
> >>                         continue;
> >>
> >> -               bpf_map__unpin(map, buf);
> >> +               bpf_map__unpin(map, NULL);
> >
> > so this will unpin auto-pinned maps (from BTF-defined maps). Is that
> > the desired behavior? I guess it might be ok (if you can't pin all of
> > your maps, you should probably clean all of them up?), but just
> > bringing it up.
>
> Yeah, I realise that. Not entirely sure it's the right thing to do, but
> there not really any way to disambiguate how the map was pinned; unless
> we want to add another state field just for that? So I guess it's either
> "don't do any cleanup" or just "unpin everything". And since I don't
> think it'll be terribly useful to combine the use of this function with
> auto-pinning anyway, I think it's probably fine to just unpin everything
> here?

Yeah, I think all-or-nothing regarding pinning is ok behavior. It
would be strange to have BPF application which is fine with only some
of maps to be pinned.

>
> -Toke

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