bpftool: Return an error on prog dumps if both CFG and JSON are required

We do not support JSON output for control flow graphs of programs with
bpftool. So far, requiring both the CFG and JSON output would result in
producing a null JSON object. It makes more sense to raise an error
directly when parsing command line arguments and options, so that users
know they won't get any output they might expect.

If JSON is required for the graph, we leave it to Graphviz instead:

    # bpftool prog dump xlated <REF> visual | dot -Tjson
Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20230405132120.59886-5-quentin@isovalent.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/bash-completion/bpftool

@@ -255,16 +255,19 @@ _bpftool_map_update_get_name()
 
 _bpftool()
 {
-    local cur prev words objword
+    local cur prev words objword json=0
     _init_completion || return
 
     # Deal with options
     if [[ ${words[cword]} == -* ]]; then
         local c='--version --json --pretty --bpffs --mapcompat --debug \
-	       --use-loader --base-btf'
+            --use-loader --base-btf'
         COMPREPLY=( $( compgen -W "$c" -- "$cur" ) )
         return 0
     fi
+    if _bpftool_search_list -j --json -p --pretty; then
+        json=1
+    fi
 
     # Deal with simplest keywords
     case $prev in
@@ -367,7 +370,7 @@ _bpftool()
                             ;;
                         *)
                             _bpftool_once_attr 'file'
-                            if _bpftool_search_list 'xlated'; then
+                            if _bpftool_search_list 'xlated' && [[ "$json" == 0 ]]; then
                                 COMPREPLY+=( $( compgen -W 'opcodes visual linum' -- \
                                     "$cur" ) )
                             else

tools/bpf/bpftool/prog.c

@@ -849,9 +849,7 @@ prog_dump(struct bpf_prog_info *info, enum dump_mode mode,
 		dd.finfo_rec_size = info->func_info_rec_size;
 		dd.prog_linfo = prog_linfo;
 
-		if (json_output && visual)
-			jsonw_null(json_wtr);
-		else if (json_output)
+		if (json_output)
 			dump_xlated_json(&dd, buf, member_len, opcodes, linum);
 		else if (visual)
 			dump_xlated_cfg(&dd, buf, member_len);
@@ -940,6 +938,10 @@ static int do_dump(int argc, char **argv)
 		usage();
 		goto exit_close;
 	}
+	if (json_output && visual) {
+		p_err("'visual' is not compatible with JSON output");
+		goto exit_close;
+	}
 
 	if (json_output && nb_fds > 1)
 		jsonw_start_array(json_wtr);	/* root array */