bpf: add new BPF_BTF_GET_NEXT_ID syscall command

Add a new command for the bpf() system call: BPF_BTF_GET_NEXT_ID is used
to cycle through all BTF objects loaded on the system.

The motivation is to be able to inspect (list) all BTF objects presents
on the system.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -24,6 +24,9 @@ struct seq_file;
 struct btf;
 struct btf_type;
 
+extern struct idr btf_idr;
+extern spinlock_t btf_idr_lock;
+
 /* map is generic key/value storage optionally accesible by eBPF programs */
 struct bpf_map_ops {
 	/* funcs callable from userspace (via syscall) */

include/uapi/linux/bpf.h

@@ -106,6 +106,7 @@ enum bpf_cmd {
 	BPF_TASK_FD_QUERY,
 	BPF_MAP_LOOKUP_AND_DELETE_ELEM,
 	BPF_MAP_FREEZE,
+	BPF_BTF_GET_NEXT_ID,
 };
 
 enum bpf_map_type {

kernel/bpf/btf.c

@@ -195,8 +195,8 @@
 	     i < btf_type_vlen(struct_type);					\
 	     i++, member++)
 
-static DEFINE_IDR(btf_idr);
-static DEFINE_SPINLOCK(btf_idr_lock);
+DEFINE_IDR(btf_idr);
+DEFINE_SPINLOCK(btf_idr_lock);
 
 struct btf {
 	void *data;

kernel/bpf/syscall.c

@@ -2884,6 +2884,10 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
 		err = bpf_obj_get_next_id(&attr, uattr,
 					  &map_idr, &map_idr_lock);
 		break;
+	case BPF_BTF_GET_NEXT_ID:
+		err = bpf_obj_get_next_id(&attr, uattr,
+					  &btf_idr, &btf_idr_lock);
+		break;
 	case BPF_PROG_GET_FD_BY_ID:
 		err = bpf_prog_get_fd_by_id(&attr);
 		break;