Re: [PATCH bpf-next 1/3] libbpf: Store map pin path in struct bpf_map

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

On Tue, Oct 22, 2019 at 11:13 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> From: Toke Høiland-Jørgensen <toke@redhat.com>
> >>
> >> When pinning a map, store the pin path in struct bpf_map so it can be
> >> re-used later for un-pinning. This simplifies the later addition of per-map
> >> pin paths.
> >>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> >> ---
> >>  tools/lib/bpf/libbpf.c |   19 ++++++++++---------
> >>  1 file changed, 10 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index cccfd9355134..b4fdd8ee3bbd 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -226,6 +226,7 @@ struct bpf_map {
> >>         void *priv;
> >>         bpf_map_clear_priv_t clear_priv;
> >>         enum libbpf_map_type libbpf_type;
> >> +       char *pin_path;
> >>  };
> >>
> >>  struct bpf_secdata {
> >> @@ -1929,6 +1930,7 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
> >>         if (err)
> >>                 goto err_close_new_fd;
> >>         free(map->name);
> >> +       zfree(&map->pin_path);
> >>
> >
> > While you are touching this function, can you please also fix error
> > handling in it? We should store -errno locally on error, before we
> > call close() which might change errno.
>
> Didn't actually look much at the surrounding function, TBH. I do expect
> that I will need to go poke into this for the follow-on "automatic reuse
> of pinned maps" series anyway. But sure, I can do a bit of cleanup in a
> standalone patch first :)
>
> >>         map->fd = new_fd;
> >>         map->name = new_name;
> >> @@ -4022,6 +4024,7 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
> >>                 return -errno;
> >>         }
> >>
> >> +       map->pin_path = strdup(path);
> >
> > if (!map->pin_path) {
> >     err = -errno;
> >     goto err_close_new_fd;
> > }
>
> Right.
>
> >>         pr_debug("pinned map '%s'\n", path);
> >>
> >>         return 0;
> >> @@ -4031,6 +4034,9 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
> >>  {
> >>         int err;
> >>
> >> +       if (!path)
> >> +               path = map->pin_path;
> >
> > This semantics is kind of weird. Given we now remember pin_path,
> > should we instead check that user-provided path is actually correct
> > and matches what we stored? Alternatively, bpf_map__unpin() w/o path
> > argument looks like a cleaner API.
>
> Yeah, I guess the function without a path argument would make the most
> sense. However, we can't really change the API of bpf_map__unpin()
> (unless you're proposing a symbol-versioned new version?). Dunno if it's
> worth it to include a new, somewhat oddly-named, function to achieve
> this? For the internal libbpf uses at least it's easy enough for the
> caller to just go bpf_map__unpin(map, map->pin_path), so I could also
> just drop this change? WDYT?

I'd probably do strcmp(map->pin_path, path), if path is specified.
This will support existing use cases, will allow NULL if we don't want
to bother remembering pin_path, will prevent weird use case of pinning
to one path, but unpinning another one.

Ideally, all this pinning will just be done declaratively and will
happen automatically, so users won't even have to know about this API
:)

>
> -Toke