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

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Message-ID: <87blu8odhf.fsf@toke.dk>
Date:   Tue, 22 Oct 2019 20:13:16 +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 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?

-Toke