Re: [PATCH v3 bpf-next] libbpf: add LIBBPF_DEPRECATED_SINCE macro for scheduling API deprecations

[Daniel Borkmann <daniel@iogearbox.net>]

On 9/9/21 8:08 PM, Andrii Nakryiko wrote:
> On Thu, Sep 9, 2021 at 9:37 AM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
>> On Thu, Sep 9, 2021 at 5:58 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>> On 9/8/21 11:32 PM, Andrii Nakryiko wrote:
>>>> From: Quentin Monnet <quentin@isovalent.com>
>>>>
>>>> Introduce a macro LIBBPF_DEPRECATED_SINCE(major, minor, message) to prepare
>>>> the deprecation of two API functions. This macro marks functions as deprecated
>>>> when libbpf's version reaches the values passed as an argument.
>>>>
>>>> As part of this change libbpf_version.h header is added with recorded major
>>>> (LIBBPF_MAJOR_VERSION) and minor (LIBBPF_MINOR_VERSION) libbpf version macros.
>>>> They are now part of libbpf public API and can be relied upon by user code.
>>>> libbpf_version.h is installed system-wide along other libbpf public headers.
>>>>
>>>> Due to this new build-time auto-generated header, in-kernel applications
>>>> relying on libbpf (resolve_btfids, bpftool, bpf_preload) are updated to
>>>> include libbpf's output directory as part of a list of include search paths.
>>>> Better fix would be to use libbpf's make_install target to install public API
>>>> headers, but that clean up is left out as a future improvement. The build
>>>> changes were tested by building kernel (with KBUILD_OUTPUT and O= specified
>>>> explicitly), bpftool, libbpf, selftests/bpf, and resolve_btfids builds. No
>>>> problems were detected.
>>>>
>>>> Note that because of the constraints of the C preprocessor we have to write
>>>> a few lines of macro magic for each version used to prepare deprecation (0.6
>>>> for now).
>>>>
>>>> Also, use LIBBPF_DEPRECATED_SINCE() to schedule deprecation of
>>>> btf__get_from_id() and btf__load(), which are replaced by
>>>> btf__load_from_kernel_by_id() and btf__load_into_kernel(), respectively,
>>>> starting from future libbpf v0.6. This is part of libbpf 1.0 effort ([0]).
>>>>
>>>>     [0] Closes: https://github.com/libbpf/libbpf/issues/278
>>>>
>>>> Co-developed-by: Quentin Monnet <quentin@isovalent.com>
>>>> Signed-off-by: Quentin Monnet <quentin@isovalent.com>
>>>> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>>>> ---
>>>> v2->v3:
>>>>     - adding `sleep 10` revealed two more missing dependencies in resolve_btfids
>>>>       and selftest/bpf's bench, which were fixed (BPF CI);
>>>> v1->v2:
>>>>     - fix bpf_preload build by adding dependency for iterators/iterators.o on
>>>>       libbpf.a generation (caught by BPF CI);
>>>>
>>>>    kernel/bpf/preload/Makefile          |  7 +++++--
>>>>    tools/bpf/bpftool/Makefile           |  4 ++++
>>>>    tools/bpf/resolve_btfids/Makefile    |  6 ++++--
>>>>    tools/lib/bpf/Makefile               | 24 +++++++++++++++++-------
>>>>    tools/lib/bpf/btf.h                  |  2 ++
>>>>    tools/lib/bpf/libbpf_common.h        | 19 +++++++++++++++++++
>>>>    tools/testing/selftests/bpf/Makefile |  4 ++--
>>>>    7 files changed, 53 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/kernel/bpf/preload/Makefile b/kernel/bpf/preload/Makefile
>>>> index 1951332dd15f..ac29d4e9a384 100644
>>>> --- a/kernel/bpf/preload/Makefile
>>>> +++ b/kernel/bpf/preload/Makefile
>>>> @@ -10,12 +10,15 @@ LIBBPF_OUT = $(abspath $(obj))
>>>>    $(LIBBPF_A):
>>>>        $(Q)$(MAKE) -C $(LIBBPF_SRCS) O=$(LIBBPF_OUT)/ OUTPUT=$(LIBBPF_OUT)/ $(LIBBPF_OUT)/libbpf.a
>>>>
>>>> -userccflags += -I $(srctree)/tools/include/ -I $(srctree)/tools/include/uapi \
>>>> +userccflags += -I$(LIBBPF_OUT) -I $(srctree)/tools/include/ \
>>>> +     -I $(srctree)/tools/include/uapi \
>>>>        -I $(srctree)/tools/lib/ -Wno-unused-result
>>>>
>>>>    userprogs := bpf_preload_umd
>>>>
>>>> -clean-files := $(userprogs) bpf_helper_defs.h FEATURE-DUMP.libbpf staticobjs/ feature/
>>>> +clean-files := $(userprogs) libbpf_version.h bpf_helper_defs.h FEATURE-DUMP.libbpf staticobjs/ feature/
>>>> +
>>>> +$(obj)/iterators/iterators.o: $(LIBBPF_A)
>>>>
>>>>    bpf_preload_umd-objs := iterators/iterators.o
>>>>    bpf_preload_umd-userldlibs := $(LIBBPF_A) -lelf -lz
>>>
>>> One small issue I ran into by accident while testing:
>>>
>>> [root@linux bpf-next]# make -j8 kernel/bpf/
>>>     SYNC    include/config/auto.conf.cmd
>>>     DESCEND objtool
>>>     CALL    scripts/atomic/check-atomics.sh
>>>     CALL    scripts/checksyscalls.sh
>>>     CC      kernel/bpf/syscall.o
>>>     AR      kernel/bpf/preload/built-in.a
>>>     CC [M]  kernel/bpf/preload/bpf_preload_kern.o
>>>     CC [U]  kernel/bpf/preload/iterators/iterators.o
>>> In file included from ./tools/lib/bpf/libbpf.h:20,
>>>                    from kernel/bpf/preload/iterators/iterators.c:10:
>>> ./tools/lib/bpf/libbpf_common.h:13:10: fatal error: libbpf_version.h: No such file or directory
>>>      13 | #include "libbpf_version.h"
>>>         |          ^~~~~~~~~~~~~~~~~~
>>> compilation terminated.
>>> make[3]: *** [scripts/Makefile.userprogs:43: kernel/bpf/preload/iterators/iterators.o] Error 1
>>> make[3]: *** Waiting for unfinished jobs....
>>> make[2]: *** [scripts/Makefile.build:540: kernel/bpf/preload] Error 2
>>> make[2]: *** Waiting for unfinished jobs....
>>> make[1]: *** [scripts/Makefile.build:540: kernel/bpf] Error 2
>>> make: *** [Makefile:1872: kernel] Error 2
>>>
>>> For me it was the case where tools/lib/bpf/ was already built _before_ this patch
>>> was applied, then I applied it, and just ran make -j8 kernel/bpf/ where the above
>>> can then be reproduced. I'd assume that as-is, this would affect many folks on update.
>>
>> We had a similar issue even before these changes with resolve_btfids
>> build, because Kbuild doesn't record dependency on libbpf build
>> properly. I'll see how hard it is to record that in a non-intrusive
>> way for both resolve_btfids and preload/iterators, because doing `make
>> resolve_btfids_clean` or, worse, `make clean` isn't great.
> 
> Hm... I can't repro this. Both preload/iterators and resolve_btfids
> have $(wildcard $(LIBBPF_SRC)/*.[ch] $(LIBBPF_SRC)/Makefile)
> dependency which will trigger libbpf rebuild if any of libbpf source
> files changes. I added `sleep 1000` and everything under kernel/bpf is
> blocked on this until libbpf build completes. For full kernel build it
> also shouldn't happen because we build resolve_btfids first before
> proceeding to building other files and resolve_btfids should trigger
> libbpf build.
> 
> I assume `make clean kernel/bpf` fixed the issue for you in that case?

No, fixing on that node was done via re-building libbpf via tools/lib/bpf/.

> Also, how did you build tools/lib/bpf before running `make -j8 kernel/bpf`?

Basically just i) cd tools/lib/bpf, ii) make clean, iii) make, then applying
the patch, and then running make -j8 kernel/bpf. I just tried on a different
build node with newer user space, and there it indeed triggers a rebuild of
libbpf. On my laptop where I was running into this it didn't. I would suspect
a potential diff in make's behavior (not seen in GNU Make 4.3, seen in GNU
Make 4.2.1, fwiw). Anyway, for this niche case, manual rebuild of tools/lib/bpf
does it ... given it's neither seen in your case nor on newer uspace, I took
it in, lets see how it goes. Thanks!

> Honestly, hard to tell how this happens without being able to repro
> this. So far everything seems to be working for me. I've tried with
> and without the KBUILD_OUTPUT envvar set. I also tried O=<path>.
> 
>>>
>>> Thanks,
>>> Daniel