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:

quoted

On Mon, Mar 01, 2021 at 04:34:24PM -0800, Andrii Nakryiko wrote:

quoted

On Mon, Mar 1, 2021 at 11:11 AM Jiri Olsa [off-list ref] wrote:

quoted

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

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