tools: Add commands to list BPF links and BTF objects to BPF script

Add two commands to the BPF script, "b" and "l", to list the BTF objects
and the BPF link (respectively) loaded on the system. These commands
make use of the recently introduced helpers for iterating over those
objects.

Older kernels do not have eBPF links, so make sure we handle the lookup
exception properly.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>

tools/bpf_inspect.py

@@ -7,12 +7,18 @@
 from drgn import container_of
 from drgn.helpers import enum_type_to_class
 from drgn.helpers.linux import (
+    bpf_btf_for_each,
     bpf_link_for_each,
     bpf_map_for_each,
     bpf_prog_for_each,
     hlist_for_each_entry,
 )
 
+try:
+    BpfLinkType = enum_type_to_class(prog.type("enum bpf_link_type"), "BpfLinkType")
+except LookupError:
+    BpfLinkType = None
+
 BpfMapType = enum_type_to_class(prog.type("enum bpf_map_type"), "BpfMapType")
 BpfProgType = enum_type_to_class(prog.type("enum bpf_prog_type"), "BpfProgType")
 BpfAttachType = enum_type_to_class(prog.type("enum bpf_attach_type"), "BpfAttachType")
@@ -145,9 +151,35 @@ def list_bpf_maps(args):
         print(f"{id_:>6}: {type_:32} {name}")
 
 
+def list_bpf_btf(args):
+    for btf in bpf_btf_for_each(prog):
+        id_ = btf.id.value_()
+        name = btf.name.string_().decode() or "<anon>"
+        kernel = "kernel" if btf.kernel_btf.value_() == 1 else ""
+
+        print(f"{id_:>6}: {kernel:6} {name}")
+
+
+def list_bpf_links(args):
+    if BpfLinkType is None:
+        # BpfLinkType was not initialized properly, likely because the kernel
+        # is too old to support link. So there is no link to list, simply
+        # return.
+        return
+
+    for link in bpf_link_for_each(prog):
+        id_ = link.id.value_()
+        type_ = BpfLinkType(link.type).name
+        prog_id = ""
+        if link.prog:
+            prog_id = link.prog.aux.id.value_()
+
+        print(f"{id_:>6}: {type_:32} {prog_id:>6}")
+
+
 def main():
     parser = argparse.ArgumentParser(
-        description="drgn script to list BPF programs or maps and their properties unavailable via kernel API"
+        description="drgn script to list BPF objects and their properties unavailable via kernel API"
     )
 
     subparsers = parser.add_subparsers(title="subcommands", dest="subcommand")
@@ -159,6 +191,12 @@ def main():
     map_parser = subparsers.add_parser("map", aliases=["m"], help="list BPF maps")
     map_parser.set_defaults(func=list_bpf_maps)
 
+    link_parser = subparsers.add_parser("link", aliases=["l"], help="list BPF links")
+    link_parser.set_defaults(func=list_bpf_links)
+
+    btf_parser = subparsers.add_parser("btf", aliases=["b"], help="list BTF objects")
+    btf_parser.set_defaults(func=list_bpf_btf)
+
     args = parser.parse_args()
     args.func(args)