A char[] is currently printed as an integer array.
This patch will print it as a string when
1) The array element type is an one byte int
2) The array element type has a BTF_INT_CHAR encoding or
   the array element type's name is "char"
3) All characters is between (0x1f, 0x7f) and it is terminated
   by a null character.

Signed-off-by: Martin KaFai Lau <redacted>
---
 tools/bpf/bpftool/btf_dumper.c | 41 ++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/tools/bpf/bpftool/btf_dumper.c b/tools/bpf/bpftool/btf_dumper.c
index 57bd6c0fafc9..1d2d8d2cedea 100644
--- a/tools/bpf/bpftool/btf_dumper.c
+++ b/tools/bpf/bpftool/btf_dumper.c

@@ -77,6 +77,42 @@ static void btf_dumper_enum(const struct btf_dumper *d,
        jsonw_int(d->jw, value);
 }

+static bool is_str_array(const struct btf *btf, const struct btf_array *arr,
+                        const char *s)
+{
+       const struct btf_type *elem_type;
+       const char *end_s;
+
+       if (!arr->nelems)
+               return false;
+
+       elem_type = btf__type_by_id(btf, arr->type);
+       /* Not skipping typedef.  typedef to char does not count as
+        * a string now.
+        */
+       while (elem_type && btf_is_mod(elem_type))
+               elem_type = btf__type_by_id(btf, elem_type->type);
+
+       if (!elem_type || !btf_is_int(elem_type) || elem_type->size != 1)
+               return false;
+
+       if (btf_int_encoding(elem_type) != BTF_INT_CHAR &&
+           strcmp("char", btf__name_by_offset(btf, elem_type->name_off)))
+               return false;
+
+       end_s = s + arr->nelems;
+       while (s < end_s) {
+               if (!*s)
+                       return true;
+               if (*s <= 0x1f || *s >= 0x7f)
+                       return false;
+               s++;
+       }
+
+       /* '\0' is not found */
+       return false;
+}
+
 static int btf_dumper_array(const struct btf_dumper *d, __u32 type_id,
                            const void *data)
 {

@@ -86,6 +122,11 @@ static int btf_dumper_array(const struct btf_dumper *d, __u32 type_id,
        int ret = 0;
        __u32 i;

+       if (is_str_array(d->btf, arr, data)) {
+               jsonw_string(d->jw, data);
+               return 0;
+       }
+