linux-kernel - Re: [RFC PATCH 3/5] livepatch: reuse module loader code to write relocations

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Date:	Wed, 11 Nov 2015 16:22:57 +0100
From:	Petr Mladek <pmladek@...e.com>
To:	Jessica Yu <jeyu@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Miroslav Benes <mbenes@...e.cz>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/5] livepatch: reuse module loader code to write
 relocations

On Mon 2015-11-09 23:45:53, Jessica Yu wrote:
> Reuse module loader code to write relocations, thereby eliminating the
> need for architecture specific code in livepatch. Namely, we reuse
> apply_relocate_add() in the module loader to write relocs instead of
> duplicating functionality in livepatch's klp_write_module_reloc(). To
> apply relocation sections, remaining SHN_LIVEPATCH symbols referenced by
> relocs are resolved and then apply_relocate_add() is called to apply
> those relocations.
> 
> Signed-off-by: Jessica Yu <jeyu@...hat.com>
> ---
>  include/linux/livepatch.h | 11 ++++--
>  include/linux/module.h    |  6 ++++
>  kernel/livepatch/core.c   | 89 +++++++++++++++++++++++++++++------------------
>  3 files changed, 70 insertions(+), 36 deletions(-)
> 
> index 087a8c7..26c419f 100644
> --- a/kernel/livepatch/core.c
> +++ b/kernel/livepatch/core.c
> @@ -28,6 +28,8 @@
>  #include <linux/list.h>
>  #include <linux/kallsyms.h>
>  #include <linux/livepatch.h>
> +#include <linux/elf.h>
> +#include <asm/cacheflush.h>
>  
>  /**
>   * struct klp_ops - structure for tracking registered ftrace ops structs
> @@ -281,46 +283,54 @@ static int klp_find_external_symbol(struct module *pmod, const char *name,
>  }
>  
>  static int klp_write_object_relocations(struct module *pmod,
> -					struct klp_object *obj)
> +					struct klp_object *obj,
> +					struct klp_patch *patch)
>  {
[...]

> +
> +	/* For each __klp_rela section for this object */
> +	list_for_each_entry(reloc_sec, &obj->reloc_secs, list) {

Who, when, and how will define reloc_secs, please?

I guess that it is just an optimization. It helps to avoid going
through all elf sections of all livepatch modules. Therefore, I think
that we might fill this when the livepatch module is loaded. But
I do not see the code for this.

Best Regards,
Petr
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