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

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Mar 04, 2021 at 11:46:27AM +1100, Michael Ellerman wrote:
> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> writes:
> > On 2021/03/02 11:35AM, Jiri Olsa wrote:
> >> On Mon, Mar 01, 2021 at 02:58:53PM -0800, Yonghong Song wrote:
> >> > 
> >> > 
> >> > On 3/1/21 11:04 AM, Jiri Olsa 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__)
> >
> > 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.
> >
> >> > > +/*
> >> > > + * 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.

hm, so I need to do the instructions check below as well

> 	 *
> 	 * 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))

is this check/instructions specific to kernel code?

In the test prog I see following instructions:

Dump of assembler code for function get_base_addr:
   0x0000000010034cb0 <+0>:     lis     r2,4256
   0x0000000010034cb4 <+4>:     addi    r2,r2,31488
   ...

but first instruction does not match the check in kernel code above:

	1.insn value:	0x3c4010a0
	2.insn value:	0x38427b00

the used defines are:
	#define OP_RT_RA_MASK   0xffff0000UL
	#define LIS_R2          0x3c020000UL
	#define ADDIS_R2_R12    0x3c4c0000UL
	#define ADDI_R2_R2      0x38420000UL


maybe we could skip the check, and run the test twice: first on
kallsym address and if the uprobe is not hit we will run it again
on address + 8

thanks,
jirka

> 		return (unsigned long)(insn + 2);
> 	else
> 		return (unsigned long)func;
> 
> 
> cheers
>