[PATCH 1/3] perf: add support for printing BTF character arrays as strings

[Blake Jones <blakejones@google.com>]

The BTF dumper code currently displays arrays of characters as just that -
arrays, with each character formatted individually. Sometimes this is what
makes sense, but it's nice to be able to treat that array as a string.

This change adds a special case to the btf_dump functionality to allow
arrays of single-byte integer values to be printed as character strings.
Characters for which isprint() returns false are printed as hex-escaped
values. This is enabled when the new ".print_strings" is set to 1 in the
btf_dump_type_data_opts structure.

As an example, here's what it looks like to dump the string "hello" using
a few different field values for btf_dump_type_data_opts (.compact = 1):

- .print_strings = 0, .skip_names = 0:  (char[6])['h','e','l','l','o',]
- .print_strings = 0, .skip_names = 1:  ['h','e','l','l','o',]
- .print_strings = 1, .skip_names = 0:  (char[6])"hello"
- .print_strings = 1, .skip_names = 1:  "hello"

Here's the string "h\xff", dumped with .compact = 1 and .skip_names = 1:

- .print_strings = 0:  ['h',-1,]
- .print_strings = 1:  "h\xff"

Signed-off-by: Blake Jones <blakejones@google.com>
---
 tools/lib/bpf/btf.h      |  3 ++-
 tools/lib/bpf/btf_dump.c | 51 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 4392451d634b..be8e8e26d245 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -326,9 +326,10 @@ struct btf_dump_type_data_opts {
 	bool compact;		/* no newlines/indentation */
 	bool skip_names;	/* skip member/type names */
 	bool emit_zeroes;	/* show 0-valued fields */
+	bool print_strings;	/* print char arrays as strings */
 	size_t :0;
 };
-#define btf_dump_type_data_opts__last_field emit_zeroes
+#define btf_dump_type_data_opts__last_field print_strings
 
 LIBBPF_API int
 btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 460c3e57fadb..a07dd5accdd8 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -75,6 +75,7 @@ struct btf_dump_data {
 	bool is_array_member;
 	bool is_array_terminated;
 	bool is_array_char;
+	bool print_strings;
 };
 
 struct btf_dump {
@@ -2028,6 +2029,50 @@ static int btf_dump_var_data(struct btf_dump *d,
 	return btf_dump_dump_type_data(d, NULL, t, type_id, data, 0, 0);
 }
 
+static int btf_dump_string_data(struct btf_dump *d,
+				const struct btf_type *t,
+				__u32 id,
+				const void *data)
+{
+	const struct btf_array *array = btf_array(t);
+	__u32 i;
+
+	if (!btf_is_int(skip_mods_and_typedefs(d->btf, array->type, NULL)) ||
+	    btf__resolve_size(d->btf, array->type) != 1 ||
+	    !d->typed_dump->print_strings) {
+		pr_warn("unexpected %s() call for array type %u\n",
+			__func__, array->type);
+		return -EINVAL;
+	}
+
+	btf_dump_data_pfx(d);
+	btf_dump_printf(d, "\"");
+
+	for (i = 0; i < array->nelems; i++, data++) {
+		char c;
+
+		if (data >= d->typed_dump->data_end)
+			return -E2BIG;
+
+		c = *(char *)data;
+		if (c == '\0') {
+			/* When printing character arrays as strings, NUL bytes
+			 * are always treated as string terminators; they are
+			 * never printed.
+			 */
+			break;
+		}
+		if (isprint(c))
+			btf_dump_printf(d, "%c", c);
+		else
+			btf_dump_printf(d, "\\x%02x", *(__u8 *)data);
+	}
+
+	btf_dump_printf(d, "\"");
+
+	return 0;
+}
+
 static int btf_dump_array_data(struct btf_dump *d,
 			       const struct btf_type *t,
 			       __u32 id,
@@ -2055,8 +2100,11 @@ static int btf_dump_array_data(struct btf_dump *d,
 		 * char arrays, so if size is 1 and element is
 		 * printable as a char, we'll do that.
 		 */
-		if (elem_size == 1)
+		if (elem_size == 1) {
+			if (d->typed_dump->print_strings)
+				return btf_dump_string_data(d, t, id, data);
 			d->typed_dump->is_array_char = true;
+		}
 	}
 
 	/* note that we increment depth before calling btf_dump_print() below;
@@ -2544,6 +2592,7 @@ int btf_dump__dump_type_data(struct btf_dump *d, __u32 id,
 	d->typed_dump->compact = OPTS_GET(opts, compact, false);
 	d->typed_dump->skip_names = OPTS_GET(opts, skip_names, false);
 	d->typed_dump->emit_zeroes = OPTS_GET(opts, emit_zeroes, false);
+	d->typed_dump->print_strings = OPTS_GET(opts, print_strings, false);
 
 	ret = btf_dump_dump_type_data(d, NULL, t, id, data, 0, 0);
 
-- 
2.49.0.1143.g0be31eac6b-goog