netdev - Re: [PATCH bpf-next v7 4/7] bpftool: Implement "gen min_core

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Date:   Fri, 18 Feb 2022 19:52:31 +0000
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Mauricio Vásquez Bernal <mauricio@...volk.io>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Rafael David Tinoco <rafaeldtinoco@...il.com>,
        Lorenzo Fontana <lorenzo.fontana@...stic.co>,
        Leonardo Di Donato <leonardo.didonato@...stic.co>
Subject: Re: [PATCH bpf-next v7 4/7] bpftool: Implement "gen min_core_btf"
 logic

2022-02-18 11:48 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> On Fri, Feb 18, 2022 at 11:44 AM Mauricio Vásquez Bernal
> <mauricio@...volk.io> wrote:
>>
>> On Fri, Feb 18, 2022 at 11:20 AM Quentin Monnet <quentin@...valent.com> wrote:
>>>
>>> 2022-02-15 17:58 UTC-0500 ~ Mauricio Vásquez <mauricio@...volk.io>
>>>> This commit implements the logic for the gen min_core_btf command.
>>>> Specifically, it implements the following functions:
>>>>
>>>> - minimize_btf(): receives the path of a source and destination BTF
>>>> files and a list of BPF objects. This function records the relocations
>>>> for all objects and then generates the BTF file by calling
>>>> btfgen_get_btf() (implemented in the following commit).
>>>>
>>>> - btfgen_record_obj(): loads the BTF and BTF.ext sections of the BPF
>>>> objects and loops through all CO-RE relocations. It uses
>>>> bpf_core_calc_relo_insn() from libbpf and passes the target spec to
>>>> btfgen_record_reloc(), that calls one of the following functions
>>>> depending on the relocation kind.
>>>>
>>>> - btfgen_record_field_relo(): uses the target specification to mark all
>>>> the types that are involved in a field-based CO-RE relocation. In this
>>>> case types resolved and marked recursively using btfgen_mark_type().
>>>> Only the struct and union members (and their types) involved in the
>>>> relocation are marked to optimize the size of the generated BTF file.
>>>>
>>>> - btfgen_record_type_relo(): marks the types involved in a type-based
>>>> CO-RE relocation. In this case no members for the struct and union types
>>>> are marked as libbpf doesn't use them while performing this kind of
>>>> relocation. Pointed types are marked as they are used by libbpf in this
>>>> case.
>>>>
>>>> - btfgen_record_enumval_relo(): marks the whole enum type for enum-based
>>>> relocations.
>>>>
>>>> Signed-off-by: Mauricio Vásquez <mauricio@...volk.io>
>>>> Signed-off-by: Rafael David Tinoco <rafael.tinoco@...asec.com>
>>>> Signed-off-by: Lorenzo Fontana <lorenzo.fontana@...stic.co>
>>>> Signed-off-by: Leonardo Di Donato <leonardo.didonato@...stic.co>
>>>> ---
>>>>  tools/bpf/bpftool/Makefile |   8 +-
>>>>  tools/bpf/bpftool/gen.c    | 455 ++++++++++++++++++++++++++++++++++++-
>>>>  2 files changed, 457 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
>>>> index 94b2c2f4ad43..a137db96bd56 100644
>>>> --- a/tools/bpf/bpftool/Makefile
>>>> +++ b/tools/bpf/bpftool/Makefile
>>>> @@ -34,10 +34,10 @@ LIBBPF_BOOTSTRAP_INCLUDE := $(LIBBPF_BOOTSTRAP_DESTDIR)/include
>>>>  LIBBPF_BOOTSTRAP_HDRS_DIR := $(LIBBPF_BOOTSTRAP_INCLUDE)/bpf
>>>>  LIBBPF_BOOTSTRAP := $(LIBBPF_BOOTSTRAP_OUTPUT)libbpf.a
>>>>
>>>> -# We need to copy hashmap.h and nlattr.h which is not otherwise exported by
>>>> -# libbpf, but still required by bpftool.
>>>> -LIBBPF_INTERNAL_HDRS := $(addprefix $(LIBBPF_HDRS_DIR)/,hashmap.h nlattr.h)
>>>> -LIBBPF_BOOTSTRAP_INTERNAL_HDRS := $(addprefix $(LIBBPF_BOOTSTRAP_HDRS_DIR)/,hashmap.h)
>>>> +# We need to copy hashmap.h, nlattr.h, relo_core.h and libbpf_internal.h
>>>> +# which are not otherwise exported by libbpf, but still required by bpftool.
>>>> +LIBBPF_INTERNAL_HDRS := $(addprefix $(LIBBPF_HDRS_DIR)/,hashmap.h nlattr.h relo_core.h libbpf_internal.h)
>>>> +LIBBPF_BOOTSTRAP_INTERNAL_HDRS := $(addprefix $(LIBBPF_BOOTSTRAP_HDRS_DIR)/,hashmap.h relo_core.h libbpf_internal.h)
>>>>
>>>>  $(LIBBPF_OUTPUT) $(BOOTSTRAP_OUTPUT) $(LIBBPF_BOOTSTRAP_OUTPUT) $(LIBBPF_HDRS_DIR) $(LIBBPF_BOOTSTRAP_HDRS_DIR):
>>>>       $(QUIET_MKDIR)mkdir -p $@
>>>> diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
>>>> index 8e066c747691..806001020841 100644
>>>> --- a/tools/bpf/bpftool/gen.c
>>>> +++ b/tools/bpf/bpftool/gen.c
>>>> @@ -14,6 +14,7 @@
>>>>  #include <unistd.h>
>>>>  #include <bpf/bpf.h>
>>>>  #include <bpf/libbpf.h>
>>>> +#include <bpf/libbpf_internal.h>
>>>>  #include <sys/types.h>
>>>>  #include <sys/stat.h>
>>>>  #include <sys/mman.h>
>>>
>>> Mauricio, did you try this patch on a system with an old Glibc (< 2.26)
>>> by any chance? Haven't tried yet but I expect this might break bpftool's
>>> build when COMPAT_NEED_REALLOCARRAY is set, because in that case gen.c
>>> pulls <bpf/libbpf_internal.h>, and then <tools/libc_compat.h> (through
>>> main.h). And libc_compat.h defines reallocarray(), which
>>> libbpf_internal.h poisons with a GCC pragma.
>>>
>>
>> I just tried on Ubuntu 16.04 with Glibc 2.23 and got the error you mentioned.

Thanks a lot for testing!

>>
>>> At least this is what I observe when trying to add your patches to the
>>> kernel mirror, where reallocarray() is redefined unconditionally. I'm
>>> trying to figure out if we should fix this mirror-side, or kernel-side.
>>> (I suppose we still need this compatibility layer, Ubuntu 16.04 seems to
>>> use Glibc 2.23).
>>>
>>
>> I suppose this should be fixed kernel-side, I don't think it's a
>> particular problem with the mirror. What about only including
>> `<tools/libc_compat.h>` in the places where reallocarray() is used:
>> prog.c and xlated_dumper.c?
> 
> 
> libbpf abandoned feature probing for this and just uses its own
> libbpf_reallocarray() implementation. Simple and reliable. Detecting
> reallocarray() is PITA and isn't worth it.

We can do the same for bpftool, its mirror already does it [0]. We could
have this in bpftool's sources and use "bpftool_reallocarray()" instead
of "reallocarray()", and get rid of this probing. Mauricio are you
willing to take this?

[0]
https://github.com/libbpf/bpftool/blob/master/include/tools/libc_compat.h#L25

Quentin