Date:   Fri, 18 Feb 2022 11:48:10 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Mauricio Vásquez Bernal <mauricio@...volk.io>
Cc:     Quentin Monnet <quentin@...valent.com>,
        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

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.
>
> > 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.

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