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

[Jiri Olsa <olsajiri@gmail.com>]

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
>