Re: [PATCH bpf-next 2/3] libbpf: Support configurable pinning of maps from BTF annotations

Andrii Nakryiko [off-list ref] writes:

On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

From: Toke Høiland-Jørgensen <redacted>

This adds support to libbpf for setting map pinning information as part of
the BTF map declaration. We introduce a new pair of functions to pin and
unpin maps based on this setting, as well as a getter and setter function
for the pin information that callers can use after map load.

The pin_type supports two modes: LOCAL pinning, which requires the caller
to set a pin path using bpf_object_pin_opts, and a global mode, where the
path can still be overridden, but defaults to /sys/fs/bpf. This is inspired
by the two modes supported by the iproute2 map definitions. In particular,
it should be possible to express the current iproute2 operating mode in
terms of the options introduced here.

The new pin functions will skip any maps that do not have a pinning type
set, unless the 'override_type' option is set, in which case all maps will
be pinning using the pin type set in that option. This also makes it
possible to express the old pin_maps and unpin_maps functions in terms of
the new option-based functions.

Signed-off-by: Toke Høiland-Jørgensen <redacted>
---

So few high-level thoughts.

1. I'd start with just NONE and GLOBAL as two pinning modes. It might
be worth-while to name GLOBAL something different just to specify that
it is just pinning, either to default /sys/fs/bpf root or some other
user-provided root path.
1a. LOCAL seems to behave exactly like GLOBAL, just uses separate
option for a path. So we effectively have two GLOBAL modes, one with
default (but overrideable) /sys/fs/bpf, another with user-provided
mandatory path. The distinction seem rather small and arbitrary.
What's the use case?

Supporting iproute2, mostly :)

Don't terribly mind dropping LOCAL, though; I don't have any particular
use case in mind for it myself.

2. When is pin type override useful? Either specify it once
declaratively in map definition, or just do pinning programmatically?

Dunno if it's really useful, actually. 

3. I think we should make pinning path override into
bpf_object_open_opts and keep bpf_object__pin_maps simple. We are
probably going to make map pinning/sharing automatic anyway, so that
will need to happen as part of either open or load operation.

I actually started with just writing automatic map pinning logic for
open(), but found myself re-implementing most of the logic in map_pin().
So figured I might as well expose it to that as well.

For open/load I think the logic should be that we parse the pinning
attribute on open and set map->pin_path from that. Then load() looks at
pin_path and does the reuse/create dance. That way, an application can
set its own pin_paths between open and load to support legacy formats
(like iproute2 needs to).

4. Once pinned, map knows its pinned path, just use that, I don't see
any reasonable use case where you'd want to override path just for
unpinning.

Well, unpinning may need to re-construct the pin path. E.g.,
applications that exit after loading and are re-run after unloading,
such as iproute2, probably want to be able to unpin maps. Unfortunately
I don't think there is a way to get the pin path(s) of an object from
the kernel, though, is there? That would be kinda neat for implementing
something like `ip link set dev eth0 xdp off unpin`.

Does it make sense?

quoted

 tools/lib/bpf/bpf_helpers.h |    8 +++
 tools/lib/bpf/libbpf.c      |  123 ++++++++++++++++++++++++++++++++++++-------
 tools/lib/bpf/libbpf.h      |   33 ++++++++++++
 tools/lib/bpf/libbpf.map    |    4 +
 4 files changed, 148 insertions(+), 20 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 2203595f38c3..a23cf55d41b1 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h

@@ -38,4 +38,12 @@ struct bpf_map_def {
        unsigned int map_flags;
 };

+enum libbpf_pin_type {
+       LIBBPF_PIN_NONE,
+       /* PIN_LOCAL: pin maps by name in path specified by caller */
+       LIBBPF_PIN_LOCAL,

Daniel mentioned in previous discussions that LOCAL mode is never
used. I'd like to avoid supporting unnecessary stuff. Is it really
useful?

Oh, he did? In that case, let's definitely get rid of it :)

quoted

+       /* PIN_GLOBAL: pin maps by name in global path (/sys/fs/bpf by default) */
+       LIBBPF_PIN_GLOBAL,
+};
+
 #endif

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b4fdd8ee3bbd..aea3916de341 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;
+       enum libbpf_pin_type pinning;
        char *pin_path;
 };

@@ -1270,6 +1271,22 @@ static int bpf_object__init_user_btf_map(struct bpf_object *obj,
                        }
                        map->def.value_size = sz;
                        map->btf_value_type_id = t->type;
+               } else if (strcmp(name, "pinning") == 0) {
+                       __u32 val;
+
+                       if (!get_map_field_int(map_name, obj->btf, def, m,
+                                              &val))
+                               return -EINVAL;
+                       pr_debug("map '%s': found pinning = %u.\n",
+                                map_name, val);
+
+                       if (val && val != LIBBPF_PIN_LOCAL &&
+                           val != LIBBPF_PIN_GLOBAL) {

let's write out LIBBPF_PIN_NONE explicitly, instead of just `val`?

OK.

quoted

+                               pr_warning("map '%s': invalid pinning value %u.\n",
+                                          map_name, val);
+                               return -EINVAL;
+                       }
+                       map->pinning = val;
                } else {
                        if (strict) {
                                pr_warning("map '%s': unknown field '%s'.\n",

@@ -4055,10 +4072,51 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
        return 0;
 }

-int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
+static int get_pin_path(char *buf, size_t buf_len,
+                       struct bpf_map *map, struct bpf_object_pin_opts *opts,
+                       bool mkdir)
+{
+       enum libbpf_pin_type type;
+       const char *path;
+       int err, len;
+
+       type = OPTS_GET(opts, override_type, 0) ?: map->pinning;
+
+       if (type == LIBBPF_PIN_GLOBAL) {
+               path = OPTS_GET(opts, path_global, NULL);
+               if (!path)
+                       path = "/sys/fs/bpf";
+       } else if (type == LIBBPF_PIN_LOCAL) {
+               path = OPTS_GET(opts, path_local, NULL);
+               if (!path) {
+                       pr_warning("map '%s' set pinning to PIN_LOCAL, "
+                                  "but no local path provided. Skipping.\n",
+                                  bpf_map__name(map));
+                       return 0;
+               }
+       } else {
+               return 0;
+       }
+
+       if (mkdir) {
+               err = make_dir(path);
+               if (err)
+                       return err;
+       }
+
+       len = snprintf(buf, buf_len, "%s/%s", path, bpf_map__name(map));
+       if (len < 0)
+               return -EINVAL;
+       else if (len >= buf_len)
+               return -ENAMETOOLONG;
+       return len;
+}
+
+int bpf_object__pin_maps_opts(struct bpf_object *obj,
+                             struct bpf_object_pin_opts *opts)
 {
        struct bpf_map *map;
-       int err;
+       int err, len;

        if (!obj)
                return -ENOENT;

@@ -4068,21 +4126,17 @@ int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
                return -ENOENT;
        }

-       err = make_dir(path);
-       if (err)
-               return err;
+       if (!OPTS_VALID(opts, bpf_object_pin_opts))
+               return -EINVAL;

        bpf_object__for_each_map(map, obj) {
                char buf[PATH_MAX];
-               int len;

-               len = snprintf(buf, PATH_MAX, "%s/%s", path,
-                              bpf_map__name(map));
-               if (len < 0) {
-                       err = -EINVAL;
-                       goto err_unpin_maps;
-               } else if (len >= PATH_MAX) {
-                       err = -ENAMETOOLONG;
+               len = get_pin_path(buf, PATH_MAX, map, opts, true);
+               if (len == 0) {
+                       continue;
+               } else if (len < 0) {
+                       err = len;
                        goto err_unpin_maps;
                }

@@ -4104,7 +4158,16 @@ int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
        return err;
 }

-int bpf_object__unpin_maps(struct bpf_object *obj, const char *path)
+int bpf_object__pin_maps(struct bpf_object *obj, const char *path)
+{
+       LIBBPF_OPTS(bpf_object_pin_opts, opts,
+                   .path_global = path,
+                   .override_type = LIBBPF_PIN_GLOBAL);

style nit: extra line between declaration and statements

quoted

+       return bpf_object__pin_maps_opts(obj, &opts);
+}
+
+int bpf_object__unpin_maps_opts(struct bpf_object *obj,
+                             struct bpf_object_pin_opts *opts)
 {
        struct bpf_map *map;
        int err;

@@ -4112,16 +4175,18 @@ int bpf_object__unpin_maps(struct bpf_object *obj, const char *path)
        if (!obj)
                return -ENOENT;

+       if (!OPTS_VALID(opts, bpf_object_pin_opts))
+               return -EINVAL;

specifying pin options for unpin operation looks cumbersome. We know
the pinned path, just use that and keep unpinning simple?

You are right, but see above re: recreating pin paths on re-run.

-Toke