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

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Date:	Tue, 16 Feb 2016 11:17:02 +0530
From:	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:	Petr Mladek <pmladek@...e.com>
Cc:	Balbir Singh <bsingharora@...il.com>, Jessica Yu <jeyu@...hat.com>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Torsten Duwe <duwe@....de>, Jiri Kosina <jkosina@...e.cz>,
	live-patching@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>,
	linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v8 8/8] livepatch: Detect offset for the ftrace location
 during build

* Petr Mladek <pmladek@...e.com> [2016-02-12 17:45:17]:

[...]
> I guess that you used a broken gcc and cheated the check
> to pass the compilation. Did you, please?
> 
> The test used to detect the offset is using a minimalistic
> function is is afftected by the gcc bug.
> 
> The patch below might be used to cheat the offset check as well.
> 
> Torsten, please mention this somewhere if you, just by chance,
> send a new version of the patchset.
> 
> From f6a438a3f2f60cc1acc859b41d0cc9259c9a331e Mon Sep 17 00:00:00 2001
> From: root <root@....arch.suse.de>
> Date: Tue, 2 Feb 2016 15:35:06 +0100
> Subject: [PATCH 2/2] livepatch: Make sure the TOC is handled when detecting
>  ftrace location
> 
> There seems to be a bug in gcc on PPC. It does not handle TOC
> if the function does not access global variables or functions
> by default. But it should when profiling is enabled.
> 
> This patch works around this problem by adding a call
> to a global function.
> 
> This patch is for testing only!
> ---
>  kernel/livepatch/ftrace-test.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/livepatch/ftrace-test.c b/kernel/livepatch/ftrace-test.c
> index 22f0c54bf7b3..a3b7aabb67e5 100644
> --- a/kernel/livepatch/ftrace-test.c
> +++ b/kernel/livepatch/ftrace-test.c
> @@ -1,6 +1,9 @@
>  /* Sample code to figure out mcount location offset */
> +#include <linux/printk.h>
> +
>  
>  int test(int a)
>  {
> +	printk("%d\n", a);
>  	return ++a;
>  }

Thanks. This workaround, helped to load sample livepatch module.

Thanks,
Kamalesh.