Subject: Re: [PATCH bpf-next v3 2/4] libbpf: Store map pin path and status in struct bpf_map

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 29 Oct 2019 19:36:12 +0100

Andrii Nakryiko <[email-masked]> writes:

> On Tue, Oct 29, 2019 at 2:01 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
>>
>> Andrii Nakryiko <[email-masked]> writes:
>>
>> > On Sun, Oct 27, 2019 at 1:53 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>> >>
>> >> From: Toke Høiland-Jørgensen <[email-masked]>
>> >>
>> >> Support storing and setting a pin path in struct bpf_map, which can be used
>> >> for automatic pinning. Also store the pin status so we can avoid attempts
>> >> to re-pin a map that has already been pinned (or reused from a previous
>> >> pinning).
>> >>
>> >> Acked-by: Andrii Nakryiko <[email-masked]>
>> >> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
>> >> ---
>> >>  tools/lib/bpf/libbpf.c   |  115 ++++++++++++++++++++++++++++++++++++----------
>> >>  tools/lib/bpf/libbpf.h   |    3 +
>> >>  tools/lib/bpf/libbpf.map |    3 +
>> >>  3 files changed, 97 insertions(+), 24 deletions(-)
>> >>
>> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> >> index ce5ef3ddd263..eb1c5e6ad4a3 100644
>> >> --- a/tools/lib/bpf/libbpf.c
>> >> +++ b/tools/lib/bpf/libbpf.c
>> >> @@ -226,6 +226,8 @@ struct bpf_map {
>> >>         void *priv;
>> >>         bpf_map_clear_priv_t clear_priv;
>> >>         enum libbpf_map_type libbpf_type;
>> >> +       char *pin_path;
>> >> +       bool pinned;
>> >>  };
>> >>
>> >>  struct bpf_secdata {
>> >> @@ -4025,47 +4027,118 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
>> >>         char *cp, errmsg[STRERR_BUFSIZE];
>> >>         int err;
>> >>
>> >> -       err = check_path(path);
>> >> -       if (err)
>> >> -               return err;
>> >> -
>> >>         if (map == NULL) {
>> >>                 pr_warn("invalid map pointer\n");
>> >>                 return -EINVAL;
>> >>         }
>> >>
>> >> -       if (bpf_obj_pin(map->fd, path)) {
>> >> -               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> >> -               pr_warn("failed to pin map: %s\n", cp);
>> >> -               return -errno;
>> >> +       if (map->pinned) {
>> >> +               pr_warn("map already pinned\n");
>> >
>> > it would be helpful to print the name of the map, otherwise user will
>> > have to guess
>>
>> Well, the existing error message didn't include the map name, so I was
>> just being consistent. But sure I can change it (and the old message as
>> well).
>>
>> >> +               return -EEXIST;
>> >> +       }
>> >> +
>> >> +       if (path && map->pin_path && strcmp(path, map->pin_path)) {
>> >> +               pr_warn("map already has pin path '%s' different from '%s'\n",
>> >> +                       map->pin_path, path);
>> >
>> > here pin_path probably would be unique enough, but for consistency we
>> > might want to print map name as well
>> >
>> >> +               return -EINVAL;
>> >> +       }
>> >> +
>> >> +       if (!map->pin_path && !path) {
>> >> +               pr_warn("missing pin path\n");
>> >
>> > and here?
>> >
>> >> +               return -EINVAL;
>> >>         }
>> >>
>> >> -       pr_debug("pinned map '%s'\n", path);
>> >> +       if (!map->pin_path) {
>> >> +               map->pin_path = strdup(path);
>> >> +               if (!map->pin_path) {
>> >> +                       err = -errno;
>> >> +                       goto out_err;
>> >> +               }
>> >> +       }
>> >
>> > There is a bit of repetition of if conditions, based on whether we
>> > have map->pin_path set (which is the most critical piece we care
>> > about), so that makes it a bit harder to follow what's going on. How
>> > about this structure, would it make a bit clearer what the error
>> > conditions are? Not insisting, though.
>> >
>> > if (map->pin_path) {
>> >   if (path && strcmp(...))
>> >     bad, exit
>> > else { /* no pin_path */
>> >   if (!path)
>> >     very bad, exit
>> >   map->pin_path = strdup(..)
>> >   if (!map->pin_path)
>> >     also bad, exit
>> > }
>>
>> Hmm, yeah, this may be better...
>>
>> >> +
>> >> +       err = check_path(map->pin_path);
>> >> +       if (err)
>> >> +               return err;
>> >> +
>> >
>> > [...]
>> >
>> >>
>> >> +int bpf_map__set_pin_path(struct bpf_map *map, const char *path)
>> >> +{
>> >> +       char *old = map->pin_path, *new;
>> >> +
>> >> +       if (path) {
>> >> +               new = strdup(path);
>> >> +               if (!new)
>> >> +                       return -errno;
>> >> +       } else {
>> >> +               new = NULL;
>> >> +       }
>> >> +
>> >> +       map->pin_path = new;
>> >> +       if (old)
>> >> +               free(old);
>> >
>> > you don't really need old, just free map->pin_path before setting it
>> > to new. Also assigning new = NULL will simplify if above.
>>
>> Right, will fix.
>>
>> >> +
>> >> +       return 0;
>> >> +}
>> >> +
>> >> +const char *bpf_map__get_pin_path(struct bpf_map *map)
>> >> +{
>> >> +       return map->pin_path;
>> >> +}
>> >> +
>> >> +bool bpf_map__is_pinned(struct bpf_map *map)
>> >> +{
>> >> +       return map->pinned;
>> >> +}
>> >> +
>> >>  int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
>> >>  {
>> >>         struct bpf_map *map;
>> >> @@ -4106,17 +4179,10 @@ int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
>> >
>> > I might have missed something the change in some other patch, but
>> > shouldn't pin_maps ignore already pinned maps? Otherwise we'll be
>> > generating unnecessary warnings?
>>
>> Well, in the previous version this was in one of the options you didn't
>> like. If I just change pin_maps() unconditionally, that will be a change
>> in behaviour in an existing API. So I figured it would be better to
>> leave this as-is. I don't think this function is really useful along
>> with the auto-pinning anyway. If you're pinning all maps, why use
>> auto-pinning? And if you want to do something custom to all the
>> non-pinned maps you'd probably iterate through them yourself anyway and
>> can react appropriately?
>
> Auto-pinned maps didn't exist before, so interaction between
> auto-pinned and explicitly pinned maps is a new behavior.
>
> With current code using explicit pin_maps and auto-pinned maps is
> impossible, or am I missing something? While admittedly scenarios in
> which you'll have to use explicit bpf_object__pin_maps() while you
> have auto-pinned maps and bpf_map__set_pin_path() are quite exotic
> (e.g., auto-pin some maps at default path and pin all the rest at some
> other custom root), I think we should still try to make existing APIs
> combinable in some sane way.

Sure, I'm not objecting to making things play nicely with each other to
the largest extent possible. I'm just vary of changing existing
behaviour.

> The only downside of ignoring already pinned maps is that while
> previously calling pin_maps() twice in a row would fail fails second
> time, now the second pin_maps() will be a noop. I think that's benign
> and acceptable change in behavior? WDYT?

Changing something that would previously fail to just silently succeed
does make me a bit twitchy. However, I suppose that as long as we try to
make sure it really is a no-op (i.e., re-pinning a map *in the same
path* can "succeed" silently). Will try something to that effect...

-Toke