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"
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);
When this mode is turned on, "emit_zeroes" and "compact" have no effect, and embedded NUL characters always terminate printing of an array.
Signed-off-by: Blake Jones blakejones@google.com --- .../selftests/bpf/prog_tests/btf_dump.c | 118 ++++++++++++++++++ 1 file changed, 118 insertions(+)
diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c index c0a776feec23..82903585c870 100644 --- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c +++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c @@ -879,6 +879,122 @@ static void test_btf_dump_var_data(struct btf *btf, struct btf_dump *d, "static int bpf_cgrp_storage_busy = (int)2", 2); }
+struct btf_dump_string_ctx { + struct btf *btf; + struct btf_dump *d; + char *str; + struct btf_dump_type_data_opts *opts; + int array_id; +}; + +static int btf_dump_one_string(struct btf_dump_string_ctx *ctx, + char *ptr, size_t ptr_sz, + const char *expected_val) +{ + size_t type_sz; + int ret; + + ctx->str[0] = '\0'; + type_sz = btf__resolve_size(ctx->btf, ctx->array_id); + ret = btf_dump__dump_type_data(ctx->d, ctx->array_id, ptr, ptr_sz, ctx->opts); + if (type_sz <= ptr_sz) { + if (!ASSERT_EQ(ret, type_sz, "failed/unexpected type_sz")) + return -EINVAL; + } + if (!ASSERT_STREQ(ctx->str, expected_val, "ensure expected/actual match")) + return -EFAULT; + return 0; +} + +static void btf_dump_strings(struct btf_dump_string_ctx *ctx) +{ + struct btf_dump_type_data_opts *opts = ctx->opts; + + opts->emit_strings = true; + + opts->compact = true; + opts->emit_zeroes = false; + + opts->skip_names = false; + btf_dump_one_string(ctx, "foo", 4, "(char[4])"foo""); + + opts->skip_names = true; + btf_dump_one_string(ctx, "foo", 4, ""foo""); + + /* This should have no effect. */ + opts->emit_zeroes = false; + btf_dump_one_string(ctx, "foo", 4, ""foo""); + + /* This should have no effect. */ + opts->compact = false; + btf_dump_one_string(ctx, "foo", 4, ""foo""); + + /* Non-printable characters come out as hex. */ + btf_dump_one_string(ctx, "fo\xff", 4, ""fo\xff""); + btf_dump_one_string(ctx, "fo\x7", 4, ""fo\x07""); + + /* + * Strings that are too long for the specified type ("char[4]") + * should fall back to the current behavior. + */ + opts->compact = true; + btf_dump_one_string(ctx, "abcde", 6, "['a','b','c','d',]"); + + /* + * Strings that are too short for the specified type ("char[4]") + * should work normally. + */ + btf_dump_one_string(ctx, "ab", 3, ""ab""); + + /* Non-NUL-terminated arrays don't get printed as strings. */ + char food[4] = { 'f', 'o', 'o', 'd' }; + char bye[3] = { 'b', 'y', 'e' }; + + btf_dump_one_string(ctx, food, 4, "['f','o','o','d',]"); + btf_dump_one_string(ctx, bye, 3, "['b','y','e',]"); + + /* The embedded NUL should terminate the string. */ + char embed[4] = { 'f', 'o', '\0', 'd' }; + + btf_dump_one_string(ctx, embed, 4, ""fo""); +} + +static void test_btf_dump_string_data(void) +{ + struct test_ctx t = {}; + char str[STRSIZE]; + struct btf_dump *d; + DECLARE_LIBBPF_OPTS(btf_dump_type_data_opts, opts); + struct btf_dump_string_ctx ctx; + int char_id, int_id, array_id; + + if (test_ctx__init(&t)) + return; + + d = btf_dump__new(t.btf, btf_dump_snprintf, str, NULL); + if (!ASSERT_OK_PTR(d, "could not create BTF dump")) + return; + + /* Generate BTF for a four-element char array. */ + char_id = btf__add_int(t.btf, "char", 1, BTF_INT_CHAR); + ASSERT_EQ(char_id, 1, "char_id"); + int_id = btf__add_int(t.btf, "int", 4, BTF_INT_SIGNED); + ASSERT_EQ(int_id, 2, "int_id"); + array_id = btf__add_array(t.btf, int_id, char_id, 4); + ASSERT_EQ(array_id, 3, "array_id"); + + ctx.btf = t.btf; + ctx.d = d; + ctx.str = str; + ctx.opts = &opts; + ctx.array_id = array_id; + + btf_dump_strings(&ctx); + + btf_dump__free(d); + test_ctx__free(&t); +} + static void test_btf_datasec(struct btf *btf, struct btf_dump *d, char *str, const char *name, const char *expected_val, void *data, size_t data_sz) @@ -970,6 +1086,8 @@ void test_btf_dump() { test_btf_dump_struct_data(btf, d, str); if (test__start_subtest("btf_dump: var_data")) test_btf_dump_var_data(btf, d, str); + if (test__start_subtest("btf_dump: string_data")) + test_btf_dump_string_data(); btf_dump__free(d); btf__free(btf);
On Tue, Jun 3, 2025 at 1:37 PM Blake Jones blakejones@google.com wrote:
When this mode is turned on, "emit_zeroes" and "compact" have no effect, and embedded NUL characters always terminate printing of an array.
Signed-off-by: Blake Jones blakejones@google.com
.../selftests/bpf/prog_tests/btf_dump.c | 118 ++++++++++++++++++ 1 file changed, 118 insertions(+)
Nice set of tests, thank you! Applied to bpf-next.
[...]
Hello:
This series was applied to bpf/bpf-next.git (master) by Andrii Nakryiko andrii@kernel.org:
On Tue, 3 Jun 2025 13:37:00 -0700 you 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.
[...]
Here is the summary with links: - [v3,1/2] libbpf: add support for printing BTF character arrays as strings https://git.kernel.org/bpf/bpf-next/c/87c9c79a02b4 - [v3,2/2] Tests for the ".emit_strings" functionality in the BTF dumper. https://git.kernel.org/bpf/bpf-next/c/a570f386f3d1
You are awesome, thank you!
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);elsebtf_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
Hi Jiri,
On Wed, Jun 11, 2025 at 3:34 AM Jiri Olsa olsajiri@gmail.com wrote:
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
I actually started this work by looking at bpftool's dumper, and only after a bit of time did I realize that there was a more general dumper in libbpf. I suspect that part of why no one has updated bpftool to use libbpf for this is that bpftool's dumper also has a "display in JSON" mode which libbpf doesn't support. So yes, it could be done, but it would take some work.
Blake
linux-kselftest-mirror@lists.linaro.org
-
Andrii Nakryiko -
Blake Jones -
Jiri Olsa -
patchwork-bot+netdevbpf＠kernel.org