Re: [PATCH] libbpf: add support for printing BTF character arrays as strings

On Sat, May 31, 2025 at 11:20 AM Alexei Starovoitov alexei.starovoitov@gmail.com wrote:

On Sat, May 31, 2025 at 12:20 AM Blake Jones blakejones@google.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@google.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.

Just for clarity on this. We'll need a "libbpf-strings" feature like the existing "libbpf" one: https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git...

Currently these features are only used if perf is built with LIBBPF_DYNAMIC=1 as part of the build arguments (ie its not the default): https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git...

If no suitable libbpf is detected then the build will error out. I guess if feature-libbpf is present but not feature-libbpf-strings then we'll need a perf #define so that the string feature won't cause perf's build to fail. We could make it so that perf's build fails if feature-libbpf and feature-libbpf-strings are missing, but that's likely too much for people using LIBBPF_DYNAMIC=1 today.

Thanks, Ian

...

};

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.

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