Re: [PATCH bpf-next v3 3/4] libbpf: Add auto-pinning of maps when loading BPF objects

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Oct 29, 2019 at 2:30 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Sun, Oct 27, 2019 at 1:53 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> From: Toke Høiland-Jørgensen <toke@redhat.com>
> >>
> >> This adds support to libbpf for setting map pinning information as part of
> >> the BTF map declaration, to get automatic map pinning (and reuse) on load.
> >> The pinning type currently only supports a single PIN_BY_NAME mode, where
> >> each map will be pinned by its name in a path that can be overridden, but
> >> defaults to /sys/fs/bpf.
> >>
> >> Since auto-pinning only does something if any maps actually have a
> >> 'pinning' BTF attribute set, we default the new option to enabled, on the
> >> assumption that seamless pinning is what most callers want.
> >>
> >> When a map has a pin_path set at load time, libbpf will compare the map
> >> pinned at that location (if any), and if the attributes match, will re-use
> >> that map instead of creating a new one. If no existing map is found, the
> >> newly created map will instead be pinned at the location.
> >>
> >> Programs wanting to customise the pinning can override the pinning paths
> >> using bpf_map__set_pin_path() before calling bpf_object__load() (including
> >> setting it to NULL to disable pinning of a particular map).
> >>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> >> ---
> >>  tools/lib/bpf/bpf_helpers.h |    6 ++
> >>  tools/lib/bpf/libbpf.c      |  142 ++++++++++++++++++++++++++++++++++++++++++-
> >>  tools/lib/bpf/libbpf.h      |   11 +++
> >>  3 files changed, 154 insertions(+), 5 deletions(-)
> >>
> >
> > [...]
> >
> >>
> >> -static int bpf_object__init_maps(struct bpf_object *obj, bool relaxed_maps)
> >> +static int bpf_object__build_map_pin_paths(struct bpf_object *obj,
> >> +                                          const char *path)
> >> +{
> >> +       struct bpf_map *map;
> >> +
> >> +       if (!path)
> >> +               path = "/sys/fs/bpf";
> >> +
> >> +       bpf_object__for_each_map(map, obj) {
> >> +               char buf[PATH_MAX];
> >> +               int err, len;
> >> +
> >> +               if (map->pinning != LIBBPF_PIN_BY_NAME)
> >> +                       continue;
> >
> > still think it's better be done from map definition parsing code
> > instead of a separate path, which will ignore most of maps anyways (of
> > course by extracting this whole buffer creation logic into a
> > function).
>
> Hmm, okay, can do that. I think we should still store the actual value
> of the 'pinning' attribute, though; and even have a getter for it. The
> app may want to do something with that information instead of having to
> infer it from map->pin_path. Certainly when we add other values of the
> pinning attribute, but we may as well add the API to get the value
> now...

Let's now expose more stuff than what we need to expose. If we really
will have a need for that, it's really easy to add. Right now you
won't even need to store pinning attribute in bpf_map, because you'll
be just setting proper pin_path in init_user_maps(), as suggested
above.

>
> >> +
> >> +               len = snprintf(buf, PATH_MAX, "%s/%s", path, bpf_map__name(map));
> >> +               if (len < 0)
> >> +                       return -EINVAL;
> >> +               else if (len >= PATH_MAX)
> >
> > [...]
> >
> >>         return 0;
> >>  }
> >>
> >> +static bool map_is_reuse_compat(const struct bpf_map *map,
> >> +                               int map_fd)
> >
> > nit: this should fit on single line?
> >
> >> +{
> >> +       struct bpf_map_info map_info = {};
> >> +       char msg[STRERR_BUFSIZE];
> >> +       __u32 map_info_len;
> >> +
> >> +       map_info_len = sizeof(map_info);
> >> +
> >> +       if (bpf_obj_get_info_by_fd(map_fd, &map_info, &map_info_len)) {
> >> +               pr_warn("failed to get map info for map FD %d: %s\n",
> >> +                       map_fd, libbpf_strerror_r(errno, msg, sizeof(msg)));
> >> +               return false;
> >> +       }
> >> +
> >> +       return (map_info.type == map->def.type &&
> >> +               map_info.key_size == map->def.key_size &&
> >> +               map_info.value_size == map->def.value_size &&
> >> +               map_info.max_entries == map->def.max_entries &&
> >> +               map_info.map_flags == map->def.map_flags &&
> >> +               map_info.btf_key_type_id == map->btf_key_type_id &&
> >> +               map_info.btf_value_type_id == map->btf_value_type_id);
> >
> > If map was pinned by older version of the same app, key and value type
> > id are probably gonna be different, even if the type definition itself
> > it correct. We probably shouldn't check that?
>
> Oh, I thought the type IDs would stay relatively stable. If not then I
> agree that we shouldn't be checking them here. Will fix.

type IDs are just an ordered index of a type, as generated by Clang.
No stability guarantees. Just adding extra typedef somewhere in
unrelated type might shift all the type IDs around.

>
> >> +}
> >> +
> >> +static int
> >> +bpf_object__reuse_map(struct bpf_map *map)
> >> +{
> >> +       char *cp, errmsg[STRERR_BUFSIZE];
> >> +       int err, pin_fd;
> >> +
> >> +       pin_fd = bpf_obj_get(map->pin_path);
> >> +       if (pin_fd < 0) {
> >> +               if (errno == ENOENT) {
> >> +                       pr_debug("found no pinned map to reuse at '%s'\n",
> >> +                                map->pin_path);
> >> +                       return 0;
> >> +               }
> >> +
> >> +               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
> >> +               pr_warn("couldn't retrieve pinned map '%s': %s\n",
> >> +                       map->pin_path, cp);
> >> +               return -errno;
> >
> > store errno locally
>
> *shrugs* okay, if you insist...

I guess I do insist on correct handling of errno, instead of
potentially returning garbage value from some unrelated syscall from
inside of pr_warn's user-provided callback.

Even libbpf_strerror_r can garble errno (e.g., through its strerror_r
call), so make sure you store it before passing into
libbpf_strerror_r().

>
> >> +       }
> >> +
> >> +       if (!map_is_reuse_compat(map, pin_fd)) {
> >> +               pr_warn("couldn't reuse pinned map at '%s': "
> >> +                       "parameter mismatch\n", map->pin_path);
> >> +               close(pin_fd);
> >> +               return -EINVAL;
> >> +       }
> >> +
> >> +       err = bpf_map__reuse_fd(map, pin_fd);
> >> +       if (err) {
> >> +               close(pin_fd);
> >> +               return err;
> >> +       }
> >> +       map->pinned = true;
> >> +       pr_debug("reused pinned map at '%s'\n", map->pin_path);
> >> +
> >> +       return 0;
> >> +}
> >> +
> >
> > [...]
> >
> >> +enum libbpf_pin_type {
> >> +       LIBBPF_PIN_NONE,
> >> +       /* PIN_BY_NAME: pin maps by name (in /sys/fs/bpf by default) */
> >> +       LIBBPF_PIN_BY_NAME,
> >> +};
> >> +
> >>  LIBBPF_API int bpf_object__pin_maps(struct bpf_object *obj, const char *path);
> >
> > pin_maps should take into account opts->auto_pin_path, shouldn't it?
> >
> > Which is why I also think that auto_pin_path is bad name, because it's
> > not only for auto-pinning, it's a pinning root path, so something like
> > pin_root_path or just pin_root is better and less misleading name.
>
> I view auto_pin_path as something that is used specifically for the
> automatic pinning based on the 'pinning' attribute. Any other use of
> pinning is for custom use and the user can pass a custom pin path to
> those functions.

What's the benefit of restricting it to just this use case? If app
wants to use something other than /sys/fs/bpf as a default root path,
why would that be restricted only to auto-pinned maps? It seems to me
that having set this on bpf_object__open() and then calling
bpf_object__pin_maps(NULL) should just take this overridden root path
into account. Isn't that a logical behavior?