On Sat, May 31, 2025 at 11:20 AM Alexei Starovoitov
<alexei.starovoi...@gmail.com> wrote:
>
> On Sat, May 31, 2025 at 12:20 AM Blake Jones <blakejo...@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 <blakejo...@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/tree/tools/build/feature/test-libbpf.c?h=perf-tools-next
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/tree/tools/perf/Makefile.config?h=perf-tools-next#n580
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.
>
> pw-bot: cr
