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
>
> 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.
> >
> > > + */
> > > +#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.
- Naveen
