Re: [PATCH v5 bpf-next 9/9] kbuild,bpf: add module-specific pahole flags for distilled base BTF

[Alan Maguire <alan.maguire@oracle.com>]

On 31/05/2024 20:06, Andrii Nakryiko wrote:
> On Tue, May 28, 2024 at 5:25 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> Support creation of module BTF along with distilled base BTF;
>> the latter is stored in a .BTF.base ELF section and supplements
>> split BTF references to base BTF with information about base types,
>> allowing for later relocation of split BTF with a (possibly
>> changed) base.  resolve_btfids detects the presence of a .BTF.base
>> section and will use it instead of the base BTF it is passed in
>> BTF id resolution.
>>
>> Modules will be built with a distilled .BTF.base section for external
>> module build, i.e.
>>
>> make -C. -M=path2/module
>>
>> ...while in-tree module build as part of a normal kernel build will
>> not generate distilled base BTF; this is because in-tree modules
>> change with the kernel and do not require BTF relocation for the
>> running vmlinux.
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
>> ---
>>  scripts/Makefile.btf      | 5 +++++
>>  scripts/Makefile.modfinal | 2 +-
>>  2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
>> index bca8a8f26ea4..191b4903e864 100644
>> --- a/scripts/Makefile.btf
>> +++ b/scripts/Makefile.btf
>> @@ -21,8 +21,13 @@ else
>>  # Switch to using --btf_features for v1.26 and later.
>>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func
>>
>> +ifneq ($(KBUILD_EXTMOD),)
>> +module-pahole-flags-$(call test-ge, $(pahole-ver), 126) += --btf_features=distilled_base
> 
> Remind me, please. What's the state of pahole patches? Are they
> waiting on these libbpf changes to land first, right?
>

Exactly. The idea would be this; we land the libbpf patches first, and
then update the dependent commit in the dwarves repo to point at one
with the relocation APIs. In the interim - where we have the code in the
library but not in pahole - everything continues to work as before. It's
just that the above --btf_feature creating distilled BTF is ignored,
which means we don't generate any distilled BTF in any modules until
that support is in the pahole used.

Thanks!

Alan

>> +endif
>> +
>>  endif
>>
>>  pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)         += --lang_exclude=rust
>>
>>  export PAHOLE_FLAGS := $(pahole-flags-y)
>> +export MODULE_PAHOLE_FLAGS := $(module-pahole-flags-y)
>> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
>> index 79fcf2731686..6d2b8da98ee5 100644
>> --- a/scripts/Makefile.modfinal
>> +++ b/scripts/Makefile.modfinal
>> @@ -39,7 +39,7 @@ quiet_cmd_btf_ko = BTF [M] $@
>>         if [ ! -f vmlinux ]; then                                       \
>>                 printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
>>         else                                                            \
>> -               LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) --btf_base vmlinux $@; \
>> +               LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) $(MODULE_PAHOLE_FLAGS) --btf_base vmlinux $@; \
>>                 $(RESOLVE_BTFIDS) -b vmlinux $@;                        \
>>         fi;
>>
>> --
>> 2.31.1
>>