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

[Jiri Olsa <jolsa@xxxxxxxxxx>]

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@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >  .../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

jirka

> 
> > +{
> > +       return offset + LEP_OFFSET;
> > +}
> > +#else
> > +#define get_offset(offset) (offset)
> > +#endif
> > +
> >  ssize_t get_base_addr() {
> >         size_t start, offset;
> >         char buf[256];
> > @@ -36,7 +52,7 @@ void test_attach_probe(void)
> >         if (CHECK(base_addr < 0, "get_base_addr",
> >                   "failed to find base addr: %zd", base_addr))
> >                 return;
> > -       uprobe_offset = (size_t)&get_base_addr - base_addr;
> > +       uprobe_offset = get_offset((size_t)&get_base_addr - base_addr);
> >
> >         skel = test_attach_probe__open_and_load();
> >         if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
> > --
> > 2.29.2
> >
>