Re: [PATCH 1/1] libbpf: Convert ELF notes into read-only maps

[Blaise Boscaccy <bboscaccy@linux.microsoft.com>]

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Wed, Feb 5, 2025 at 11:09 AM Blaise Boscaccy
> <bboscaccy@linux.microsoft.com> wrote:
>>
>> Add a flexible mechanism, using existing ELF constructs, to attach
>> additional metadata to BPF programs for possible use by BPF
>> gatekeepers and skeletons.
>>
>> During object file parsing, note sections are no longer skipped and
>> now treated as read-only data. During libbpf-based loading or skeleton
>> generation, those sections are then transformed into read-only maps
>> which are subsequently passed into the kernel.
>
> We already have this mechanism, it's .rodata (and
> .rodata.<customname>) section(s). Adding .note sections as BPF maps
> make no sense to me. Just piggy-back on .rodata for storing any
> necessary metadata.
>
> pw-bot: cr
>

The ELF specification clearly states:
"Sometimes a vendor or system builder needs to mark an object file with
special information that other programs will check for conformance,
compatibility, etc. Sections of type SHT_NOTE and program header
elements of type PT_NOTE can be used for this purpose."

Which is exactly what we are trying to do. They make no mention of
piggy-backing off of .rodata.

Further, one can generally strip away .note sections and be left with an
object that's still functioning. The same cannot be said about .rodata. 



>>
>> Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
>> ---
>>  tools/bpf/bpftool/gen.c | 4 ++--
>>  tools/lib/bpf/libbpf.c  | 6 ++++++
>>  2 files changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
>> index 5a4d3240689ed..311d6a3f1c4bb 100644
>> --- a/tools/bpf/bpftool/gen.c
>> +++ b/tools/bpf/bpftool/gen.c
>> @@ -92,7 +92,7 @@ static void get_header_guard(char *guard, const char *obj_name, const char *suff
>>
>>  static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
>>  {
>> -       static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
>> +       static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
>>         const char *name = bpf_map__name(map);
>>         int i, n;
>>
>> @@ -117,7 +117,7 @@ static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
>>
>>  static bool get_datasec_ident(const char *sec_name, char *buf, size_t buf_sz)
>>  {
>> -       static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
>> +       static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig", ".note" };
>>         int i, n;
>>
>>         /* recognize hard coded LLVM section name */
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 194809da51725..be6af0fece040 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -523,6 +523,7 @@ struct bpf_struct_ops {
>>  #define STRUCT_OPS_SEC ".struct_ops"
>>  #define STRUCT_OPS_LINK_SEC ".struct_ops.link"
>>  #define ARENA_SEC ".addr_space.1"
>> +#define NOTE_SEC ".note"
>>
>>  enum libbpf_map_type {
>>         LIBBPF_MAP_UNSPEC,
>> @@ -3977,6 +3978,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
>>                         sec_desc->sec_type = SEC_BSS;
>>                         sec_desc->shdr = sh;
>>                         sec_desc->data = data;
>> +               } else if (sh->sh_type == SHT_NOTE && (strcmp(name, NOTE_SEC) == 0 ||
>> +                                                      str_has_pfx(name, NOTE_SEC "."))) {
>> +                       sec_desc->sec_type = SEC_RODATA;
>> +                       sec_desc->shdr = sh;
>> +                       sec_desc->data = data;
>>                 } else {
>>                         pr_info("elf: skipping section(%d) %s (size %zu)\n", idx, name,
>>                                 (size_t)sh->sh_size);
>> --
>> 2.48.1
>>