bpf: Add bpf_for_each_map_elem() helper

The bpf_for_each_map_elem() helper is introduced which
iterates all map elements with a callback function. The
helper signature looks like
  long bpf_for_each_map_elem(map, callback_fn, callback_ctx, flags)
and for each map element, the callback_fn will be called. For example,
like hashmap, the callback signature may look like
  long callback_fn(map, key, val, callback_ctx)

There are two known use cases for this. One is from upstream ([1]) where
a for_each_map_elem helper may help implement a timeout mechanism
in a more generic way. Another is from our internal discussion
for a firewall use case where a map contains all the rules. The packet
data can be compared to all these rules to decide allow or deny
the packet.

For array maps, users can already use a bounded loop to traverse
elements. Using this helper can avoid using bounded loop. For other
type of maps (e.g., hash maps) where bounded loop is hard or
impossible to use, this helper provides a convenient way to
operate on all elements.

For callback_fn, besides map and map element, a callback_ctx,
allocated on caller stack, is also passed to the callback
function. This callback_ctx argument can provide additional
input and allow to write to caller stack for output.

If the callback_fn returns 0, the helper will iterate through next
element if available. If the callback_fn returns 1, the helper
will stop iterating and returns to the bpf program. Other return
values are not used for now.

Currently, this helper is only available with jit. It is possible
to make it work with interpreter with so effort but I leave it
as the future work.

[1]: https://lore.kernel.org/bpf/20210122205415.113822-1-xiyou.wangcong@gmail.com/

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204925.3884923-1-yhs@fb.com

1 files changed, 38 insertions, 0 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 79c893310492..b89af20cfa19 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -393,6 +393,15 @@ enum bpf_link_type {
  *                   is struct/union.
  */
 #define BPF_PSEUDO_BTF_ID	3
+/* insn[0].src_reg:  BPF_PSEUDO_FUNC
+ * insn[0].imm:      insn offset to the func
+ * insn[1].imm:      0
+ * insn[0].off:      0
+ * insn[1].off:      0
+ * ldimm64 rewrite:  address of the function
+ * verifier type:    PTR_TO_FUNC.
+ */
+#define BPF_PSEUDO_FUNC		4
 
 /* when bpf_call->src_reg == BPF_PSEUDO_CALL, bpf_call->imm == pc-relative
  * offset to another bpf function
@@ -3909,6 +3918,34 @@ union bpf_attr {
  *		* **BPF_MTU_CHK_RET_FRAG_NEEDED**
  *		* **BPF_MTU_CHK_RET_SEGS_TOOBIG**
  *
+ * long bpf_for_each_map_elem(struct bpf_map *map, void *callback_fn, void *callback_ctx, u64 flags)
+ *	Description
+ *		For each element in **map**, call **callback_fn** function with
+ *		**map**, **callback_ctx** and other map-specific parameters.
+ *		The **callback_fn** should be a static function and
+ *		the **callback_ctx** should be a pointer to the stack.
+ *		The **flags** is used to control certain aspects of the helper.
+ *		Currently, the **flags** must be 0.
+ *
+ *		The following are a list of supported map types and their
+ *		respective expected callback signatures:
+ *
+ *		BPF_MAP_TYPE_HASH, BPF_MAP_TYPE_PERCPU_HASH,
+ *		BPF_MAP_TYPE_LRU_HASH, BPF_MAP_TYPE_LRU_PERCPU_HASH,
+ *		BPF_MAP_TYPE_ARRAY, BPF_MAP_TYPE_PERCPU_ARRAY
+ *
+ *		long (\*callback_fn)(struct bpf_map \*map, const void \*key, void \*value, void \*ctx);
+ *
+ *		For per_cpu maps, the map_value is the value on the cpu where the
+ *		bpf_prog is running.
+ *
+ *		If **callback_fn** return 0, the helper will continue to the next
+ *		element. If return value is 1, the helper will skip the rest of
+ *		elements and return. Other return values are not used now.
+ *
+ *	Return
+ *		The number of traversed map elements for success, **-EINVAL** for
+ *		invalid **flags**.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -4075,6 +4112,7 @@ union bpf_attr {
 	FN(ima_inode_hash),		\
 	FN(sock_from_file),		\
 	FN(check_mtu),			\
+	FN(for_each_map_elem),		\
 	/* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper