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

On Wed, Mar 03, 2021 at 12:10:43PM +0530, Naveen N. Rao wrote:

On 2021/03/02 11:35AM, Jiri Olsa wrote:

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On Mon, Mar 01, 2021 at 02:58:53PM -0800, Yonghong Song wrote:

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On 3/1/21 11:04 AM, Jiri Olsa wrote:

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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__)

This needs to be specific to ELF v2 ABI, so you'll need to check 
_CALL_ELF. See commit d5c2e2c17ae1d6 ("perf probe ppc64le: Prefer symbol 
table lookup over DWARF") for an example.

i see, there's the outer #if _CALL_ELF I missed

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+/*
+ * 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).

Any documentation in the kernel about this behavior? This will
help to validate the change without trying with powerpc64 qemu...

I don't think we have documented this in the kernel anywhere, but this 
is specific to the ELF v2 ABI and is described there:
- 2.3.2.1.  Function Prologue: 
  http://cdn.openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655240___RefHeading___Toc377640597.html
- 3.4.1.  Symbol Values:
   http://cdn.openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655241_95185.html

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we got similar fix in perf:

7b6ff0bdbf4f perf probe ppc64le: Fixup function entry if using kallsyms lookup

CC-ing few other folks from ppc land for more info

Thanks.

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+ */
+#define LEP_OFFSET 8
+
+static ssize_t get_offset(ssize_t offset)
+{
+	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"))

As documented in the links above, the right way to identify local entry 
point (LEP) is by looking at the symbol table. Falling back to using a 
hardcoded offset of 8 is a reasonable workaround if we don't have access 
to the symbol table.

thanks for the all the info, I'll send v2 with _CALL_ELF check

jirka