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Message-ID: <CAADnVQJv_FVciT9LC+W=sVtWAt9oXeAACzmTHzyqY-2svi4ugA@mail.gmail.com>
Date: Sat, 31 May 2025 11:20:44 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Blake Jones <blakejones@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
Ihor Solodrai <ihor.solodrai@...ux.dev>, Namhyung Kim <namhyung@...nel.org>,
Ian Rogers <irogers@...gle.com>, bpf <bpf@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH] libbpf: add support for printing BTF character arrays as strings
On Sat, May 31, 2025 at 12:20 AM Blake Jones <blakejones@...gle.com> 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
> 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@...gle.com>
> ---
> tools/lib/bpf/btf.h | 3 +-
> tools/lib/bpf/btf_dump.c | 51 ++++++++-
> .../selftests/bpf/prog_tests/btf_dump.c | 102 ++++++++++++++++++
> 3 files changed, 154 insertions(+), 2 deletions(-)
Please split selftests vs main libbpf parts.
> 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;
Looks useful, but make sure to add a feature detection
to perf, since it has to work with old and new libbpf.
> };
>
> 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.
> + */
Please use normal kernel style comments.
We're gradually getting away from networking style.
> + 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);
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
> index c0a776feec23..70e51943f148 100644
> --- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c
> +++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
> @@ -879,6 +879,106 @@ static void test_btf_dump_var_data(struct btf *btf, struct btf_dump *d,
> "static int bpf_cgrp_storage_busy = (int)2", 2);
> }
>
> +/*
> + * String-like types are generally not named, so they need to be
> + * found this way rather than via btf__find_by_name().
> + */
This is the correct style of comments.
> +static int find_char_array_type(struct btf *btf, int nelems)
> +{
> + const int nr_types = btf__type_cnt(btf);
> + const int char_type = btf__find_by_name(btf, "char");
> +
> + for (int i = 1; i < nr_types; i++) {
> + const struct btf_type *t;
> + const struct btf_array *at;
> +
> + t = btf__type_by_id(btf, i);
> + if (btf_kind(t) != BTF_KIND_ARRAY)
> + continue;
> +
> + at = btf_array(t);
> + if (at->nelems == nelems && at->type == char_type)
> + return i;
> + }
> +
> + return -ENOENT;
> +}
> +
> +static int btf_dump_string_data(struct btf *btf, struct btf_dump *d,
> + char *str, struct btf_dump_type_data_opts *opts,
> + char *ptr, size_t ptr_sz,
> + const char *expected_val)
> +{
> + char name[64];
> + size_t type_sz;
> + int type_id;
> + int ret = 0;
> +
> + snprintf(name, sizeof(name), "char[%zu]", ptr_sz);
> + type_id = find_char_array_type(btf, ptr_sz);
> + if (!ASSERT_GE(type_id, 0, "find type id"))
> + return -ENOENT;
> + type_sz = btf__resolve_size(btf, type_id);
> + str[0] = '\0';
> + ret = btf_dump__dump_type_data(d, type_id, ptr, ptr_sz, opts);
> + if (type_sz <= ptr_sz) {
> + if (!ASSERT_EQ(ret, type_sz, "failed/unexpected type_sz"))
> + return -EINVAL;
> + } else {
> + if (!ASSERT_EQ(ret, -E2BIG, "failed to return -E2BIG"))
> + return -EINVAL;
> + }
> + if (!ASSERT_STREQ(str, expected_val, "ensure expected/actual match"))
> + return -EFAULT;
> + return 0;
> +}
> +
> +static void test_btf_dump_string_data(struct btf *btf, struct btf_dump *d,
> + char *str)
> +{
> + DECLARE_LIBBPF_OPTS(btf_dump_type_data_opts, opts);
> +
> + opts.compact = true;
> + opts.emit_zeroes = false;
> + opts.print_strings = true;
> +
> + opts.skip_names = false;
> + btf_dump_string_data(btf, d, str, &opts, "foo", 4,
> + "(char[4])\"foo\"");
we allow up to 100 char per line.
Don't split it that short.
pw-bot: cr
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