On Thu, Mar 04, 2021 at 11:46:27AM +1100, Michael Ellerman wrote:
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> 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@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__)
> >
> > 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
>
