Re: [PATCH bpf-next] libbpf: Support Debian in resolve_full_path()

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Mon, 2022-04-04 at 14:58 -0700, Andrii Nakryiko wrote:
> On Mon, Apr 4, 2022 at 3:29 AM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > attach_probe selftest fails on Debian-based distros with `failed to
> > resolve full path for 'libc.so.6'`. The reason is that these
> > distros
> > embraced multiarch to the point where even for the "main"
> > architecture
> > they store libc in /lib/<triple>.
> > 
> > This is configured in /etc/ld.so.conf and in theory it's possible
> > to
> > replicate the loader's parsing and processing logic in libbpf,
> > however
> > a much simpler solution is to just enumerate the known library
> > paths.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> >  tools/lib/bpf/libbpf.c | 54 ++++++++++++++++++++++++++++++++++++--
> > ----
> >  1 file changed, 47 insertions(+), 7 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 6d2be53e4ba9..4f616b11564f 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -10707,21 +10707,61 @@ static long elf_find_func_offset(const
> > char *binary_path, const char *name)
> >         return ret;
> >  }
> > 
> > +static void add_debian_library_paths(const char **search_paths,
> > int *n)
> > +{
> > +       /*
> > +        * Based on https://packages.debian.org/sid/libc6.
> > +        *
> > +        * Assume that the traced program is built for the same
> > architecture
> > +        * as libbpf, which should cover the vast majority of
> > cases.
> > +        */
> > +#if defined(__x86_64__)
> 
> can you please also drop defined() where possible, it looks cleaner
> to me:
> 
> #if __x86_64__
> 
> vs
> 
> #if defined(__x86_64__)

The consensus in the existing kernel and tools code (including libbpf
itself) seems to be to use #if defined() or #ifdef for such macros:

$ git grep __x86_64__ | wc -l
306

$ git grep __x86_64__ | grep -v \
    -e '#\s*ifdef __x86_64__' \
    -e 'defined\s*(__x86_64__)' \
    -e '#\s*ifndef __x86_64__' \
    -e '#\s*else' \
    -e '#\s*endif'
arch/x86/Makefile:        CHECKFLAGS += -D__x86_64__
arch/x86/Makefile.um:CHECKFLAGS  += -m64 -D__x86_64__
tools/lib/bpf/libbpf.c:#if __x86_64__
tools/testing/selftests/ipc/Makefile:	CFLAGS := -DCONFIG_X86_64 -
D__x86_64__
tools/testing/selftests/rcutorture/bin/mkinitrd.sh:if echo -e "#if
__x86_64__||__i386__||__i486__||__i586__||__i686__" \
tools/testing/selftests/x86/mov_ss_trap.c:#if __x86_64__

I think `#if __x86_64__` should work in most cases, but I'd rather
stick with the existing style if you don't mind.