bpf: Create a new btf_name_by_offset() for non type name use case

The current btf_name_by_offset() is returning "(anon)" type name for
the offset == 0 case and "(invalid-name-offset)" for the out-of-bound
offset case.

It fits well for the internal BTF verbose log purpose which
is focusing on type.  For example,
offset == 0 => "(anon)" => anonymous type/name.
Returning non-NULL for the bad offset case is needed
during the BTF verification process because the BTF verifier may
complain about another field first before discovering the name_off
is invalid.

However, it may not be ideal for the newer use case which does not
necessary mean type name.  For example, when logging line_info
in the BPF verifier in the next patch, it is better to log an
empty src line instead of logging "(anon)".

The existing bpf_name_by_offset() is renamed to __bpf_name_by_offset()
and static to btf.c.

A new bpf_name_by_offset() is added for generic context usage.  It
returns "\0" for name_off == 0 (note that btf->strings[0] is "\0")
and NULL for invalid offset.  It allows the caller to decide
what is the best output in its context.

The new btf_name_by_offset() is overlapped with btf_name_offset_valid().
Hence, btf_name_offset_valid() is removed from btf.h to keep the btf.h API
minimal.  The existing btf_name_offset_valid() usage in btf.c could also be
replaced later.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/btf.h

@@ -46,7 +46,6 @@ void btf_type_seq_show(const struct btf *btf, u32 type_id, void *obj,
 		       struct seq_file *m);
 int btf_get_fd_by_id(u32 id);
 u32 btf_id(const struct btf *btf);
-bool btf_name_offset_valid(const struct btf *btf, u32 offset);
 bool btf_type_is_reg_int(const struct btf_type *t, u32 expected_size);
 
 #ifdef CONFIG_BPF_SYSCALL

kernel/bpf/btf.c

@@ -474,7 +474,7 @@ static bool btf_name_valid_identifier(const struct btf *btf, u32 offset)
 	return !*src;
 }
 
-const char *btf_name_by_offset(const struct btf *btf, u32 offset)
+static const char *__btf_name_by_offset(const struct btf *btf, u32 offset)
 {
 	if (!offset)
 		return "(anon)";
@@ -484,6 +484,14 @@ const char *btf_name_by_offset(const struct btf *btf, u32 offset)
 		return "(invalid-name-offset)";
 }
 
+const char *btf_name_by_offset(const struct btf *btf, u32 offset)
+{
+	if (offset < btf->hdr.str_len)
+		return &btf->strings[offset];
+
+	return NULL;
+}
+
 const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id)
 {
 	if (type_id > btf->nr_types)
@@ -576,7 +584,7 @@ __printf(4, 5) static void __btf_verifier_log_type(struct btf_verifier_env *env,
 	__btf_verifier_log(log, "[%u] %s %s%s",
 			   env->log_type_id,
 			   btf_kind_str[kind],
-			   btf_name_by_offset(btf, t->name_off),
+			   __btf_name_by_offset(btf, t->name_off),
 			   log_details ? " " : "");
 
 	if (log_details)
@@ -620,7 +628,7 @@ static void btf_verifier_log_member(struct btf_verifier_env *env,
 		btf_verifier_log_type(env, struct_type, NULL);
 
 	__btf_verifier_log(log, "\t%s type_id=%u bits_offset=%u",
-			   btf_name_by_offset(btf, member->name_off),
+			   __btf_name_by_offset(btf, member->name_off),
 			   member->type, member->offset);
 
 	if (fmt && *fmt) {
@@ -1872,7 +1880,7 @@ static s32 btf_enum_check_meta(struct btf_verifier_env *env,
 
 
 		btf_verifier_log(env, "\t%s val=%d\n",
-				 btf_name_by_offset(btf, enums[i].name_off),
+				 __btf_name_by_offset(btf, enums[i].name_off),
 				 enums[i].val);
 	}
 
@@ -1896,7 +1904,8 @@ static void btf_enum_seq_show(const struct btf *btf, const struct btf_type *t,
 	for (i = 0; i < nr_enums; i++) {
 		if (v == enums[i].val) {
 			seq_printf(m, "%s",
-				   btf_name_by_offset(btf, enums[i].name_off));
+				   __btf_name_by_offset(btf,
+							enums[i].name_off));
 			return;
 		}
 	}
@@ -1954,11 +1963,11 @@ static void btf_func_proto_log(struct btf_verifier_env *env,
 	}
 
 	btf_verifier_log(env, "%u %s", args[0].type,
-			 btf_name_by_offset(env->btf,
+			 __btf_name_by_offset(env->btf,
 					      args[0].name_off));
 	for (i = 1; i < nr_args - 1; i++)
 		btf_verifier_log(env, ", %u %s", args[i].type,
-				 btf_name_by_offset(env->btf,
+				 __btf_name_by_offset(env->btf,
 						      args[i].name_off));
 
 	if (nr_args > 1) {
@@ -1966,7 +1975,7 @@ static void btf_func_proto_log(struct btf_verifier_env *env,
 
 		if (last_arg->type)
 			btf_verifier_log(env, ", %u %s", last_arg->type,
-					 btf_name_by_offset(env->btf,
+					 __btf_name_by_offset(env->btf,
 							      last_arg->name_off));
 		else
 			btf_verifier_log(env, ", vararg");

kernel/bpf/verifier.c

@@ -4910,8 +4910,8 @@ static int check_btf_line(struct bpf_verifier_env *env,
 			goto err_free;
 		}
 
-		if (!btf_name_offset_valid(btf, linfo[i].line_off) ||
+		if (!btf_name_by_offset(btf, linfo[i].line_off) ||
-		    !btf_name_offset_valid(btf, linfo[i].file_name_off)) {
+		    !btf_name_by_offset(btf, linfo[i].file_name_off)) {
 			verbose(env, "Invalid line_info[%u].line_off or .file_name_off\n", i);
 			err = -EINVAL;
 			goto err_free;