[RFC PATCH 4/9] bpf: Add bpf_struct_ops_for_each_prog()

[Tejun Heo <tj@kernel.org>]

Add a helper that walks the member progs of the struct_ops map
containing a given @kdata vmtable. struct_ops ->reg() callbacks (and
similar) sometimes need to inspect the loaded BPF programs, e.g. to
discover maps they reference via prog->aux->used_maps.

The implementation mirrors bpf_struct_ops_id(): container_of @kdata
to recover the bpf_struct_ops_map, then iterate st_map->links[i]->prog
for i in [0, funcs_cnt). Same access pattern, no new locking - by the
time ->reg() fires st_map is fully populated and stable.

A sched_ext follow-up uses this to require cid-form schedulers to use
exactly one BPF_F_ARENA_MAP_ALWAYS arena across their member progs,
without requiring the BPF program to call a registration kfunc.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 include/linux/bpf.h         |  3 +++
 kernel/bpf/bpf_struct_ops.c | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index af54705611d7..f4e4360b81f6 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2128,6 +2128,9 @@ int bpf_prog_assoc_struct_ops(struct bpf_prog *prog, struct bpf_map *map);
 void bpf_prog_disassoc_struct_ops(struct bpf_prog *prog);
 void *bpf_prog_get_assoc_struct_ops(const struct bpf_prog_aux *aux);
 u32 bpf_struct_ops_id(const void *kdata);
+int bpf_struct_ops_for_each_prog(const void *kdata,
+				 int (*cb)(struct bpf_prog *prog, void *data),
+				 void *data);
 
 #ifdef CONFIG_NET
 /* Define it here to avoid the use of forward declaration */
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 05b366b821c3..16aec18ed31b 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -1203,6 +1203,42 @@ u32 bpf_struct_ops_id(const void *kdata)
 }
 EXPORT_SYMBOL_GPL(bpf_struct_ops_id);
 
+/**
+ * bpf_struct_ops_for_each_prog - Invoke @cb for each member prog
+ * @kdata: kernel-side struct_ops vmtable (the @kdata arg to ->reg/->update/->unreg)
+ * @cb: callback invoked once per member prog; non-zero return stops iteration
+ * @data: opaque argument passed to @cb
+ *
+ * Walks the struct_ops member progs registered on the map containing @kdata.
+ * Intended for use from struct_ops ->reg() callbacks (and similar) that need to
+ * inspect the loaded BPF programs (for example to discover maps they reference
+ * via @prog->aux->used_maps).
+ *
+ * Return 0 if iteration completed, otherwise the first non-zero @cb return.
+ */
+int bpf_struct_ops_for_each_prog(const void *kdata,
+				 int (*cb)(struct bpf_prog *prog, void *data),
+				 void *data)
+{
+	struct bpf_struct_ops_value *kvalue;
+	struct bpf_struct_ops_map *st_map;
+	u32 i;
+	int ret;
+
+	kvalue = container_of(kdata, struct bpf_struct_ops_value, data);
+	st_map = container_of(kvalue, struct bpf_struct_ops_map, kvalue);
+
+	for (i = 0; i < st_map->funcs_cnt; i++) {
+		if (!st_map->links[i])
+			continue;
+		ret = cb(st_map->links[i]->prog, data);
+		if (ret)
+			return ret;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(bpf_struct_ops_for_each_prog);
+
 static bool bpf_struct_ops_valid_to_reg(struct bpf_map *map)
 {
 	struct bpf_struct_ops_map *st_map = (struct bpf_struct_ops_map *)map;
-- 
2.53.0