linux-kernel - Re: [PATCH v8 8/8] livepatch: Detect offset for the ftrace location during build

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Message-ID: <1455293609.16012.9.camel@gmail.com>
Date:	Sat, 13 Feb 2016 03:13:29 +1100
From:	Balbir Singh <bsingharora@...il.com>
To:	Torsten Duwe <duwe@....de>, Michael Ellerman <mpe@...erman.id.au>
Cc:	Jiri Kosina <jkosina@...e.cz>, Miroslav Benes <mbenes@...e.cz>,
	Petr Mladek <pmladek@...e.com>, Jessica Yu <jeyu@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	live-patching@...r.kernel.org
Subject: Re: [PATCH v8 8/8] livepatch: Detect offset for the ftrace location
 during build

On Thu, 2016-01-28 at 16:32 +0100, Torsten Duwe wrote:
> From: Petr Mladek <pmladek@...e.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