Re: [PATCH bpf-next] selftests/bpf: Fix test_attach_probe for powerpc uprobes

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Mar 02, 2021 at 10:31:41AM -0800, Andrii Nakryiko wrote:
> On Tue, Mar 2, 2021 at 3:14 AM Jiri Olsa <jolsa@redhat.com> 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@kernel.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@kernel.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
> 
> My thinking was that if you mark the function as __weak, then the
> compiler is not allowed to assume that the actual implementation of
> that function will come from the same object (because it might be
> replaced by the linker later), so it has to be pessimistic and use
> global entry, no? Totally theoritizing here, of course.

ah ok.. good idea, but it's still jumping to +8 in my test

    # nm test_progs | grep get_base_addr
    0000000010034e30 W get_base_addr

    100350e0:   59 fd ff 4b     bl      10034e38 <get_base_addr+0x8>

looks like it's linker, because compiler leaves just jump to next instruction

jirka