Re: [PATCH v3 1/2] libbpf: add support for printing BTF character arrays as strings

On Tue, Jun 03, 2025 at 01:37:00PM -0700, Blake Jones wrote:

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 0-terminated 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 ".emit_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):

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

hi, could this be used in bpftool map dump? ;-) I checked, but it looks like bpftool map dump is using something else to dump data.. I admit I haven't spent much on time that

thanks, jirka

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

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

Signed-off-by: Blake Jones blakejones@google.com

tools/lib/bpf/btf.h | 3 ++- tools/lib/bpf/btf_dump.c | 55 +++++++++++++++++++++++++++++++++++++++- 2 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h index 4392451d634b..ccfd905f03df 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 emit_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 emit_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..7c2f1f13f958 100644 --- a/tools/lib/bpf/btf_dump.c +++ b/tools/lib/bpf/btf_dump.c @@ -68,6 +68,7 @@ struct btf_dump_data { bool compact; bool skip_names; bool emit_zeroes;

• bool emit_strings; __u8 indent_lvl; /* base indent level */ char indent_str[BTF_DATA_INDENT_STR_LEN]; /* below are used during iteration */

@@ -2028,6 +2029,52 @@ 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);
• const char *chars = data;
• __u32 i;
• /* Make sure it is a NUL-terminated string. */
• for (i = 0; i < array->nelems; i++) {

• if ((void *)(chars + i) >= d->typed_dump->data_end)
  

• 	return -E2BIG;
  

• if (chars[i] == '\0')
  

• 	break;
  

• }
• if (i == array->nelems) {

• /* The caller will print this as a regular array. */
  

• return -EINVAL;
  

• }
• btf_dump_data_pfx(d);
• btf_dump_printf(d, """);
• for (i = 0; i < array->nelems; i++) {

• char c = chars[i];
  

• 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)c);
  

• }
• 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 +2102,13 @@ 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->emit_strings &&
  

• 	    btf_dump_string_data(d, t, id, data) == 0) {
  

• 		return 0;
  

• 	}
  d->typed_dump->is_array_char = true;
  

• }
  

  }

/* note that we increment depth before calling btf_dump_print() below; @@ -2544,6 +2596,7 @@ 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->emit_strings = OPTS_GET(opts, emit_strings, false);

ret = btf_dump_dump_type_data(d, NULL, t, id, data, 0, 0); -- 2.49.0.1204.g71687c7c1d-goog