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

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Date:   Tue, 22 Oct 2019 11:49:10 -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 1/3] libbpf: Store map pin path in struct bpf_map

On Tue, Oct 22, 2019 at 11:45 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
>
> > On Tue, Oct 22, 2019 at 11:13 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> Andrii Nakryiko <andrii.nakryiko@...il.com> writes:
> >>
> >> > On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >> >>
> >> >> From: Toke Høiland-Jørgensen <toke@...hat.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@...hat.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.
>
> So something like
>
> if (path && map->pin_path && strcmp(path, map->pin_path))

can we unpin not pinned map? sounds like an error condition?

so:

if (!map->pin_path)
    return -EWHATAREYOUDOING;
if (path && strcmp(path, map->pin_path))
    return -EHUH;
path = map->pin_path; /* or just use map->ping_path explicitly */

... proceed ...

>  return -EINVAL
> else if (!path)
>  path = map->pin_path;
>
> ?
>
> > Ideally, all this pinning will just be done declaratively and will
> > happen automatically, so users won't even have to know about this API
> > :)
>
> Yeah, that's where I'm hoping to get to. But, well, the pin/unpin
> functions already exist so we do need to keep them working...
>
> -Toke