Subject: Re: [PATCH 1/2] libbpf: Fix endianness macro usage for some compilers

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Fri, 19 Jul 2019 15:30:57 -0300

Em Fri, Jul 19, 2019 at 09:25:07AM -0700, Y Song escreveu:
> On Fri, Jul 19, 2019 at 7:35 AM Arnaldo Carvalho de Melo <[email-masked]> wrote:
> > Using endian.h and its endianness macros makes this code build in a
> > wider range of compilers, as some don't have those macros
> > (__BYTE_ORDER__, __ORDER_LITTLE_ENDIAN__, __ORDER_BIG_ENDIAN__),
> > so use instead endian.h's macros (__BYTE_ORDER, __LITTLE_ENDIAN,
> > __BIG_ENDIAN) which makes this code even shorter :-)
> 
> gcc 4.6.0 starts to support __BYTE_ORDER__, __ORDER_LITTLE_ENDIAN__, etc.
> I guess those platforms with failing compilation have gcc < 4.6.0.
> Agree that for libbpf, which will be used outside kernel bpf selftest should
> try to compile with lower versions of gcc.

Yeah, I wouldn't mind at all if the answer to this patch was hey, the
minimal version for building libbpf is X.Y, no problem, its just that in
this case there is an alternative that works everywhere and the
resulting source code is even more compact :-)

- Arnaldo
 
> > Acked-by: Andrii Nakryiko <[email-masked]>
> > Cc: Adrian Hunter <[email-masked]>
> > Cc: Alexei Starovoitov <[email-masked]>
> > Cc: Daniel Borkmann <[email-masked]>
> > Cc: Jiri Olsa <[email-masked]>
> > Cc: Namhyung Kim <[email-masked]>
> > Fixes: 12ef5634a855 ("libbpf: simplify endianness check")
> > Fixes: e6c64855fd7a ("libbpf: add btf__parse_elf API to load .BTF and .BTF.ext")
> > Link: https://lkml.kernel.org/n/[email-masked]
> > Signed-off-by: Arnaldo Carvalho de Melo <[email-masked]>
> > ---
> >  tools/lib/bpf/btf.c    | 5 +++--
> >  tools/lib/bpf/libbpf.c | 5 +++--
> >  2 files changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index 467224feb43b..d821107f55f9 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -1,6 +1,7 @@
> >  // SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
> >  /* Copyright (c) 2018 Facebook */
> >
> > +#include <endian.h>
> >  #include <stdio.h>
> >  #include <stdlib.h>
> >  #include <string.h>
> > @@ -419,9 +420,9 @@ struct btf *btf__new(__u8 *data, __u32 size)
> >
> >  static bool btf_check_endianness(const GElf_Ehdr *ehdr)
> >  {
> > -#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> > +#if __BYTE_ORDER == __LITTLE_ENDIAN
> >         return ehdr->e_ident[EI_DATA] == ELFDATA2LSB;
> > -#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> > +#elif __BYTE_ORDER == __BIG_ENDIAN
> >         return ehdr->e_ident[EI_DATA] == ELFDATA2MSB;
> >  #else
> >  # error "Unrecognized __BYTE_ORDER__"
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 794dd5064ae8..b1dec5b1de54 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -20,6 +20,7 @@
> >  #include <inttypes.h>
> >  #include <string.h>
> >  #include <unistd.h>
> > +#include <endian.h>
> >  #include <fcntl.h>
> >  #include <errno.h>
> >  #include <asm/unistd.h>
> > @@ -612,10 +613,10 @@ static int bpf_object__elf_init(struct bpf_object *obj)
> >
> >  static int bpf_object__check_endianness(struct bpf_object *obj)
> >  {
> > -#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> > +#if __BYTE_ORDER == __LITTLE_ENDIAN
> >         if (obj->efile.ehdr.e_ident[EI_DATA] == ELFDATA2LSB)
> >                 return 0;
> > -#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> > +#elif __BYTE_ORDER == __BIG_ENDIAN
> >         if (obj->efile.ehdr.e_ident[EI_DATA] == ELFDATA2MSB)
> >                 return 0;
> >  #else
> > --
> > 2.21.0
> >

-- 

- Arnaldo