netdev - Re: [PATCH bpf-next] selftests/bpf: Fix test_attach

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Date:   Wed, 3 Mar 2021 12:12:39 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Yonghong Song <yhs@...com>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Yauheni Kaliuta <ykaliuta@...hat.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix test_attach_probe for
 powerpc uprobes

On 2021/03/02 09:19AM, Yonghong Song wrote:
> 
> 
> On 3/2/21 3:14 AM, Jiri Olsa wrote:
> > On Mon, Mar 01, 2021 at 04:34:24PM -0800, Andrii Nakryiko wrote:
> > > On Mon, Mar 1, 2021 at 11:11 AM Jiri Olsa <jolsa@...nel.org> wrote:
> > > > 
> > > > When testing uprobes we the test gets GEP (Global Entry Point)
> > > > address from kallsyms, but then the function is called locally
> > > > so the uprobe is not triggered.
> > > > 
> > > > Fixing this by adjusting the address to LEP (Local Entry Point)
> > > > for powerpc arch.
> > > > 
> > > > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > > > ---
> > > >   .../selftests/bpf/prog_tests/attach_probe.c    | 18 +++++++++++++++++-
> > > >   1 file changed, 17 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
> > > > index a0ee87c8e1ea..c3cfb48d3ed0 100644
> > > > --- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
> > > > +++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
> > > > @@ -2,6 +2,22 @@
> > > >   #include <test_progs.h>
> > > >   #include "test_attach_probe.skel.h"
> > > > 
> > > > +#if defined(__powerpc64__)
> > > > +/*
> > > > + * We get the GEP (Global Entry Point) address from kallsyms,
> > > > + * but then the function is called locally, so we need to adjust
> > > > + * the address to get LEP (Local Entry Point).
> > > > + */
> > > > +#define LEP_OFFSET 8
> > > > +
> > > > +static ssize_t get_offset(ssize_t offset)
> > > 
> > > if we mark this function __weak global, would it work as is? Would it
> > > get an address of a global entry point? I know nothing about this GEP
> > > vs LEP stuff, interesting :)
> > 
> > you mean get_base_addr? it's already global
> > 
> > all the calls to get_base_addr within the object are made
> > to get_base_addr+0x8
> > 
> > 00000000100350c0 <test_attach_probe>:
> >      ...
> >      100350e0:   59 fd ff 4b     bl      10034e38 <get_base_addr+0x8>
> >      ...
> >      100358a8:   91 f5 ff 4b     bl      10034e38 <get_base_addr+0x8>
> > 
> > 
> > I'm following perf fix we had for similar issue:
> >    7b6ff0bdbf4f perf probe ppc64le: Fixup function entry if using kallsyms lookup
> > 
> > I'll get more info on that
> 
> Thanks. The patch
>    7b6ff0bdbf4f perf probe ppc64le: Fixup function entry if using kallsyms
> lookup
> talked about offset + 8 for kernel symbols.
> I guess uprobe symbol might be in the same situation if using the
> same compilation mechanism as kernel. But it would be good
> to get confirmation from ppc people.

Yes, this is part of the ELF V2 ABI, so it applies to both kernel and 
userspace.

- Naveen