Re: [PATCH bpf-next 1/2] bpf, libbpf: add a new API bpf_object__reuse_maps()

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

On Fri, Jul 5, 2019 at 2:53 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 07/05/2019 10:44 PM, Anton Protopopov wrote:
> > Add a new API bpf_object__reuse_maps() which can be used to replace all maps in
> > an object by maps pinned to a directory provided in the path argument.  Namely,
> > each map M in the object will be replaced by a map pinned to path/M.name.
> >
> > Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
> > ---
> >  tools/lib/bpf/libbpf.c   | 34 ++++++++++++++++++++++++++++++++++
> >  tools/lib/bpf/libbpf.h   |  2 ++
> >  tools/lib/bpf/libbpf.map |  1 +
> >  3 files changed, 37 insertions(+)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 4907997289e9..84c9e8f7bfd3 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -3144,6 +3144,40 @@ int bpf_object__unpin_maps(struct bpf_object *obj, const char *path)
> >       return 0;
> >  }
> >
> > +int bpf_object__reuse_maps(struct bpf_object *obj, const char *path)

As is, bpf_object__reuse_maps() can be easily implemented by user
applications, as it's only using public libbpf APIs, so I'm not 100%
sure we need to add method like that to libbpf.

> > +{
> > +     struct bpf_map *map;
> > +
> > +     if (!obj)
> > +             return -ENOENT;
> > +
> > +     if (!path)
> > +             return -EINVAL;
> > +
> > +     bpf_object__for_each_map(map, obj) {
> > +             int len, err;
> > +             int pinned_map_fd;
> > +             char buf[PATH_MAX];
>
> We'd need to skip the case of bpf_map__is_internal(map) since they are always
> recreated for the given object.
>
> > +             len = snprintf(buf, PATH_MAX, "%s/%s", path, bpf_map__name(map));
> > +             if (len < 0) {
> > +                     return -EINVAL;
> > +             } else if (len >= PATH_MAX) {
> > +                     return -ENAMETOOLONG;
> > +             }
> > +
> > +             pinned_map_fd = bpf_obj_get(buf);
> > +             if (pinned_map_fd < 0)
> > +                     return pinned_map_fd;
>
> Should we rather have a new map definition attribute that tells to reuse
> the map if it's pinned in bpf fs, and if not, we create it and later on
> pin it? This is what iproute2 is doing and which we're making use of heavily.

I'd like something like that as well. This would play nicely with
recently added BTF-defined maps as well.

I think it should be not just pin/don't pin flag, but rather pinning
strategy, to accommodate various typical strategies of handling maps
that are already pinned. So something like this:

1. BPF_PIN_NOTHING - default, don't pin;
2. BPF_PIN_EXCLUSIVE - pin, but if map is already pinned - fail;
3. BPF_PIN_SET - pin; if existing map exists, reset its state to be
exact state of object's map;
4. BPF_PIN_MERGE - pin, if map exists, fill in NULL entries only (this
is how Cilium is pinning PROG_ARRAY maps, if I understand correctly);
5. BPF_PIN_MERGE_OVERWRITE - pin, if map exists, overwrite non-NULL values.

This list is only for illustrative purposes, ideally people that have
a lot of experience using pinning for real-world use cases would chime
in on what strategies are useful and make sense.

> In bpf_object__reuse_maps() bailing out if bpf_obj_get() fails is perhaps
> too limiting for a generic API as new version of an object file may contain
> new maps which are not yet present in bpf fs at that point.
>
> > +             err = bpf_map__reuse_fd(map, pinned_map_fd);
> > +             if (err)
> > +                     return err;
> > +     }
> > +
> > +     return 0;
> > +}
> > +
> >  int bpf_object__pin_programs(struct bpf_object *obj, const char *path)
> >  {
> >       struct bpf_program *prog;
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index d639f47e3110..7fe465a1be76 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -82,6 +82,8 @@ int bpf_object__variable_offset(const struct bpf_object *obj, const char *name,
> >  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,
> >                                     const char *path);
> > +LIBBPF_API int bpf_object__reuse_maps(struct bpf_object *obj,
> > +                                   const char *path);
> >  LIBBPF_API int bpf_object__pin_programs(struct bpf_object *obj,
> >                                       const char *path);
> >  LIBBPF_API int bpf_object__unpin_programs(struct bpf_object *obj,
> > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > index 2c6d835620d2..66a30be6696c 100644
> > --- a/tools/lib/bpf/libbpf.map
> > +++ b/tools/lib/bpf/libbpf.map
> > @@ -172,5 +172,6 @@ LIBBPF_0.0.4 {
> >               btf_dump__new;
> >               btf__parse_elf;
> >               bpf_object__load_xattr;
> > +             bpf_object__reuse_maps;
> >               libbpf_num_possible_cpus;
> >  } LIBBPF_0.0.3;
> >
>