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

"Naveen N. Rao" [off-list ref] writes:

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

quoted

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

quoted

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

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.

quoted

quoted

quoted

+/*
+ * 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

There's a comment in ppc_function_entry(), but I don't think we have any
actual "documentation".

static inline unsigned long ppc_function_entry(void *func)
{
#ifdef PPC64_ELF_ABI_v2
	u32 *insn = func;

	/*
	 * A PPC64 ABIv2 function may have a local and a global entry
	 * point. We need to use the local entry point when patching
	 * functions, so identify and step over the global entry point
	 * sequence.
	 *
	 * The global entry point sequence is always of the form:
	 *
	 * addis r2,r12,XXXX
	 * addi  r2,r2,XXXX
	 *
	 * A linker optimisation may convert the addis to lis:
	 *
	 * lis   r2,XXXX
	 * addi  r2,r2,XXXX
	 */
	if ((((*insn & OP_RT_RA_MASK) == ADDIS_R2_R12) ||
	     ((*insn & OP_RT_RA_MASK) == LIS_R2)) &&
	    ((*(insn+1) & OP_RT_RA_MASK) == ADDI_R2_R2))
		return (unsigned long)(insn + 2);
	else
		return (unsigned long)func;


cheers