From: Martin KaFai Lau <kafai@fb.com>
Date: Thu, 13 Dec 2018 10:41:46 -0800
Subject: bpf: Create a new btf_name_by_offset() for non type name use case
Patch-mainline: v5.0-rc1
Git-commit: 23127b33ec80e656921362d7dc82a0064bac20a2
References: bsc#1109837

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>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 kernel/bpf/btf.c |   19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -451,7 +451,7 @@ static bool btf_name_valid_identifier(co
 	return !*src;
 }
 
-static 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)";
@@ -461,6 +461,14 @@ static const char *btf_name_by_offset(co
 		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;
+}
+
 static const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id)
 {
 	if (type_id > btf->nr_types)
@@ -531,7 +539,7 @@ __printf(4, 5) static void __btf_verifie
 	__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)
@@ -575,7 +583,7 @@ static void btf_verifier_log_member(stru
 		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) {
@@ -1822,7 +1830,7 @@ static s32 btf_enum_check_meta(struct bt
 
 
 		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);
 	}
 
@@ -1846,7 +1854,8 @@ static void btf_enum_seq_show(const stru
 	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;
 		}
 	}