netdev - [PATCH bpf-next v2] libbpf: Add getter for pointer to data area for internal maps

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Date:   Fri, 27 Mar 2020 13:58:18 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     daniel@...earbox.net, ast@...com
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        bpf@...r.kernel.org, netdev@...r.kernel.org,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: [PATCH bpf-next v2] libbpf: Add getter for pointer to data area for internal maps

For internal maps (most notably the maps backing global variables), libbpf
uses an internal mmaped area to store the data after opening the object.
This data is subsequently copied into the kernel map when the object is
loaded.

This adds a getter for the pointer to that internal data store. This can be
used to modify the data before it is loaded into the kernel, which is
especially relevant for RODATA, which is frozen on load. This same pointer
is already exposed to the auto-generated skeletons, so access to it is
already API; this just adds a way to get at it without pulling in the full
skeleton infrastructure.

Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
v2:
  - Add per-map getter for data area instead of a global rodata getter for bpf_obj

tools/lib/bpf/libbpf.c   | 9 +++++++++
 tools/lib/bpf/libbpf.h   | 1 +
 tools/lib/bpf/libbpf.map | 1 +
 3 files changed, 11 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 085e41f9b68e..a0055f8908fd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -6756,6 +6756,15 @@ void *bpf_map__priv(const struct bpf_map *map)
 	return map ? map->priv : ERR_PTR(-EINVAL);
 }
 
+void *bpf_map__data_area(const struct bpf_map *map, size_t *size)
+{
+	if (map->mmaped && map->libbpf_type != LIBBPF_MAP_KCONFIG) {
+		*size = map->def.value_size;
+		return map->mmaped;
+	}
+	return NULL;
+}
+
 bool bpf_map__is_offload_neutral(const struct bpf_map *map)
 {
 	return map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY;
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index d38d7a629417..baef0d2f3205 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -407,6 +407,7 @@ typedef void (*bpf_map_clear_priv_t)(struct bpf_map *, void *);
 LIBBPF_API int bpf_map__set_priv(struct bpf_map *map, void *priv,
 				 bpf_map_clear_priv_t clear_priv);
 LIBBPF_API void *bpf_map__priv(const struct bpf_map *map);
+LIBBPF_API void *bpf_map__data_area(const struct bpf_map *map, size_t *size);
 LIBBPF_API int bpf_map__reuse_fd(struct bpf_map *map, int fd);
 LIBBPF_API int bpf_map__resize(struct bpf_map *map, __u32 max_entries);
 LIBBPF_API bool bpf_map__is_offload_neutral(const struct bpf_map *map);
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 5129283c0284..258528045a85 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -243,5 +243,6 @@ LIBBPF_0.0.8 {
 		bpf_link__pin;
 		bpf_link__pin_path;
 		bpf_link__unpin;
+		bpf_map__data_area;
 		bpf_program__set_attach_target;
 } LIBBPF_0.0.7;
-- 
2.26.0