On Thu, 2016-01-28 at 16:32 +0100, Torsten Duwe wrote:

From: Petr Mladek <pmladek@suse.com>

Livepatch works on x86_64 and s390 only when the ftrace call
is at the very beginning of the function. But PPC is different.
We need to handle TOC and save LR there before calling the
global ftrace handler.

Now, the problem is that the extra operations have different
length on PPC depending on the used gcc version. It is
4 instructions (16 bytes) before gcc-6 and only 3 instructions
(12 bytes) with gcc-6.

This patch tries to detect the offset a generic way during
build. It assumes that the offset of the ftrace location
is the same for all functions. It modifies the existing
recordmcount tool that is able to find read mcount locations
directly from the object files. It adds an option -p
to print the first found offset.

The recordmcount tool is then used in the kernel/livepatch
subdirectory to generate a header file. It defines
a constant that is used to compute the ftrace location
from the function address.

Finally, we have to enable the C implementation of the
recordmcount tool to be used on PPC and S390. It seems
to work fine there. It should be more reliable because
it reads the standardized elf structures. The old perl
implementation uses rather complex regular expressions
to parse objdump output and is therefore much more tricky.

I'm still missing something, I'm getting offset as 8

When I run, I get

scripts/recordmcount -p kernel/livepatch/core.o 
#define KLP_FTRACE_LOCATION 8

scripts/recordmcount -p kernel/livepatch/ftrace-test.o 
#define KLP_FTRACE_LOCATION 8

My sample fails as well, since the expected offset is 16.
I guess the script is being run against a not so good
test.

A quick hack (no signoff below, its just an experiment),
seems to do the trick for the provided sample-livepatch.
It is hacky because it uses the sample object and due to
lack of a better description of srctree, it uses 
srctree/../..

I suspect the usage of recordmcount needs to be revisited

diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile
index 65a44b68..10b5f38 100644
--- a/kernel/livepatch/Makefile
+++ b/kernel/livepatch/Makefile

@@ -2,7 +2,7 @@ obj-$(CONFIG_LIVEPATCH) += livepatch.o

 
 livepatch-objs := core.o
 
-always		:= $(hostprogs-y) ftrace-test.o
+always		:= $(hostprogs-y) $(srctree)/../../samples/livepatch/livepatch-sample.o
 
 # dependencies on generated files need to be listed explicitly
 $(obj)/core.o: $(obj)/livepatch-ftrace.h

@@ -10,7 +10,7 @@ $(obj)/core.o: $(obj)/livepatch-ftrace.h

 quiet_cmd_livepatch-rmcount = RMCOUNT $@
       cmd_livepatch-rmcount = $(objtree)/scripts/recordmcount -p $< > $@
 
-$(obj)/livepatch-ftrace.h: $(obj)/ftrace-test.o $(objtree)/scripts/recordmcount
+$(obj)/livepatch-ftrace.h: $(obj)/../../samples/livepatch/livepatch-sample.o $(objtree)/scripts/recordmcount
 	$(call if_changed,livepatch-rmcount)
 
 targets += livepatch-ftrace.h