tools: bpftool: add an option to prevent auto-mount of bpffs, tracefs

In order to make life easier for users, bpftool automatically attempts
to mount the BPF virtual file system, if it is not mounted already,
before trying to pin objects in it. Similarly, it attempts to mount
tracefs if necessary before trying to dump the trace pipe to the
console.

While mounting file systems on-the-fly can improve user experience, some
administrators might prefer to avoid that. Let's add an option to block
these mount attempts. Note that it does not prevent automatic mounting
of tracefs by debugfs for the "bpftool prog tracelog" command.

Signed-off-by: Quentin Monnet <[e-mail]>
Reviewed-by: Jakub Kicinski <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/bpf/bpftool/Documentation/bpftool-map.rst

···
 	-f, --bpffs
 		  Show file names of pinned maps.
 
+	-n, --nomount
+		  Do not automatically attempt to mount any virtual file system
+		  (such as tracefs or BPF virtual file system) when necessary.
+
 EXAMPLES
 ========
 **# bpftool map show**

tools/bpf/bpftool/Documentation/bpftool-prog.rst

···
 	-m, --mapcompat
 		  Allow loading maps with unknown map definitions.
 
+	-n, --nomount
+		  Do not automatically attempt to mount any virtual file system
+		  (such as tracefs or BPF virtual file system) when necessary.
+
 EXAMPLES
 ========
 **# bpftool prog show**

tools/bpf/bpftool/Documentation/bpftool.rst

···
 	-m, --mapcompat
 		  Allow loading maps with unknown map definitions.
 
+	-n, --nomount
+		  Do not automatically attempt to mount any virtual file system
+		  (such as tracefs or BPF virtual file system) when necessary.
+
 
 SEE ALSO
 ========

tools/bpf/bpftool/common.c

···
 		/* nothing to do if already mounted */
 		goto out_free;
 
+	if (block_mount) {
+		p_err("no BPF file system found, not mounting it due to --nomount option");
+		err = -1;
+		goto out_free;
+	}
+
 	err = mnt_fs(dir, "bpf", err_str, ERR_MAX_LEN);
 	if (err) {
 		err_str[ERR_MAX_LEN - 1] = '\0';

tools/bpf/bpftool/main.c

···
 bool pretty_output;
 bool json_output;
 bool show_pinned;
+bool block_mount;
 int bpf_flags;
 struct pinned_obj_table prog_table;
 struct pinned_obj_table map_table;
···
 		{ "version",	no_argument,	NULL,	'V' },
 		{ "bpffs",	no_argument,	NULL,	'f' },
 		{ "mapcompat",	no_argument,	NULL,	'm' },
+		{ "nomount",	no_argument,	NULL,	'n' },
 		{ 0 }
 	};
 	int opt, ret;
···
 	pretty_output = false;
 	json_output = false;
 	show_pinned = false;
+	block_mount = false;
 	bin_name = argv[0];
 
 	hash_init(prog_table.table);
 	hash_init(map_table.table);
 
 	opterr = 0;
-	while ((opt = getopt_long(argc, argv, "Vhpjfm",
+	while ((opt = getopt_long(argc, argv, "Vhpjfmn",
 				  options, NULL)) >= 0) {
 		switch (opt) {
 		case 'V':
···
 			break;
 		case 'm':
 			bpf_flags = MAPS_RELAX_COMPAT;
+			break;
+		case 'n':
+			block_mount = true;
 			break;
 		default:
 			p_err("unrecognized option '%s'", argv[optind - 1]);

tools/bpf/bpftool/main.h

···
 #define HELP_SPEC_PROGRAM						\
 	"PROG := { id PROG_ID | pinned FILE | tag PROG_TAG }"
 #define HELP_SPEC_OPTIONS						\
-	"OPTIONS := { {-j|--json} [{-p|--pretty}] | {-f|--bpffs} | {-m|--mapcompat}"
+	"OPTIONS := { {-j|--json} [{-p|--pretty}] | {-f|--bpffs} |\n"	\
+	"\t            {-m|--mapcompat} | {-n|--nomount} }"
 #define HELP_SPEC_MAP							\
 	"MAP := { id MAP_ID | pinned FILE }"
 
···
 extern json_writer_t *json_wtr;
 extern bool json_output;
 extern bool show_pinned;
+extern bool block_mount;
 extern int bpf_flags;
 extern struct pinned_obj_table prog_table;
 extern struct pinned_obj_table map_table;

tools/bpf/bpftool/tracelog.c

···
 	if (found && validate_tracefs_mnt(mnt, TRACEFS_MAGIC))
 		goto exit_found;
 
+	if (block_mount)
+		return false;
+
 	p_info("could not find tracefs, attempting to mount it now");
 	/* Most of the time, tracefs is automatically mounted by debugfs at
 	 * /sys/kernel/debug/tracing when we try to access it. If we could not