bpf: Support associating BPF program with struct_ops

Add a new BPF command BPF_PROG_ASSOC_STRUCT_OPS to allow associating
a BPF program with a struct_ops map. This command takes a file
descriptor of a struct_ops map and a BPF program and set
prog->aux->st_ops_assoc to the kdata of the struct_ops map.

The command does not accept a struct_ops program nor a non-struct_ops
map. Programs of a struct_ops map is automatically associated with the
map during map update. If a program is shared between two struct_ops
maps, prog->aux->st_ops_assoc will be poisoned to indicate that the
associated struct_ops is ambiguous. The pointer, once poisoned, cannot
be reset since we have lost track of associated struct_ops. For other
program types, the associated struct_ops map, once set, cannot be
changed later. This restriction may be lifted in the future if there is
a use case.

A kernel helper bpf_prog_get_assoc_struct_ops() can be used to retrieve
the associated struct_ops pointer. The returned pointer, if not NULL, is
guaranteed to be valid and point to a fully updated struct_ops struct.
For struct_ops program reused in multiple struct_ops map, the return
will be NULL.

prog->aux->st_ops_assoc is protected by bumping the refcount for
non-struct_ops programs and RCU for struct_ops programs. Since it would
be inefficient to track programs associated with a struct_ops map, every
non-struct_ops program will bump the refcount of the map to make sure
st_ops_assoc stays valid. For a struct_ops program, it is protected by
RCU as map_free will wait for an RCU grace period before disassociating
the program with the map. The helper must be called in BPF program
context or RCU read-side critical section.

struct_ops implementers should note that the struct_ops returned may not
be initialized nor attached yet. The struct_ops implementer will be
responsible for tracking and checking the state of the associated
struct_ops map if the use case expects an initialized or attached
struct_ops.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20251203233748.668365-3-ameryhung@gmail.com

1 files changed, 46 insertions, 0 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6589acc89ef8..3080cc48bfc3 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -6122,6 +6122,49 @@ static int prog_stream_read(union bpf_attr *attr)
 	return ret;
 }
 
+#define BPF_PROG_ASSOC_STRUCT_OPS_LAST_FIELD prog_assoc_struct_ops.prog_fd
+
+static int prog_assoc_struct_ops(union bpf_attr *attr)
+{
+	struct bpf_prog *prog;
+	struct bpf_map *map;
+	int ret;
+
+	if (CHECK_ATTR(BPF_PROG_ASSOC_STRUCT_OPS))
+		return -EINVAL;
+
+	if (attr->prog_assoc_struct_ops.flags)
+		return -EINVAL;
+
+	prog = bpf_prog_get(attr->prog_assoc_struct_ops.prog_fd);
+	if (IS_ERR(prog))
+		return PTR_ERR(prog);
+
+	if (prog->type == BPF_PROG_TYPE_STRUCT_OPS) {
+		ret = -EINVAL;
+		goto put_prog;
+	}
+
+	map = bpf_map_get(attr->prog_assoc_struct_ops.map_fd);
+	if (IS_ERR(map)) {
+		ret = PTR_ERR(map);
+		goto put_prog;
+	}
+
+	if (map->map_type != BPF_MAP_TYPE_STRUCT_OPS) {
+		ret = -EINVAL;
+		goto put_map;
+	}
+
+	ret = bpf_prog_assoc_struct_ops(prog, map);
+
+put_map:
+	bpf_map_put(map);
+put_prog:
+	bpf_prog_put(prog);
+	return ret;
+}
+
 static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size)
 {
 	union bpf_attr attr;
@@ -6261,6 +6304,9 @@ static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size)
 	case BPF_PROG_STREAM_READ_BY_FD:
 		err = prog_stream_read(&attr);
 		break;
+	case BPF_PROG_ASSOC_STRUCT_OPS:
+		err = prog_assoc_struct_ops(&attr);
+		break;
 	default:
 		err = -EINVAL;
 		break;