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

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Date:   Tue, 22 Oct 2019 21:06:32 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.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

Andrii Nakryiko <andrii.nakryiko@...il.com> writes:

> 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?

See my other comment about programs exiting. For an example, see the XDP
tutorial (just pretend for a moment that that TODO comment was actually
there :)):

https://github.com/xdp-project/xdp-tutorial/blob/master/basic04-pinning-maps/xdp_loader.c#L135

-Toke