bpftool: auto-complete BTF IDs for btf dump

Auto-complete BTF IDs for `btf dump id` sub-command. List of possible BTF
IDs is scavenged from loaded BPF programs that have associated BTFs, as
there is currently no API in libbpf to fetch list of all BTFs in the
system.

Suggested-by: Quentin Monnet <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Reviewed-by: Quentin Monnet <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

tools/bpf/bpftool/bash-completion/bpftool

@@ -71,6 +71,12 @@ _bpftool_get_prog_tags()
         command sed -n 's/.*"tag": "\(.*\)",$/\1/p' )" -- "$cur" ) )
 }
 
+_bpftool_get_btf_ids()
+{
+    COMPREPLY+=( $( compgen -W "$( bpftool -jp prog 2>&1 | \
+        command sed -n 's/.*"btf_id": \(.*\),\?$/\1/p' )" -- "$cur" ) )
+}
+
 _bpftool_get_obj_map_names()
 {
     local obj
@@ -635,6 +641,9 @@ _bpftool()
                                 map)
                                     _bpftool_get_map_ids
                                     ;;
+                                dump)
+                                    _bpftool_get_btf_ids
+                                    ;;
                             esac
                             return 0
                             ;;