linux-kernel - Re: module: preserve Elf information for livepatch modules

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Message-ID: <20160208203441.GB27290@packer-debian-8-amd64.digitalocean.com>
Date:	Mon, 8 Feb 2016 15:34:42 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>,
	Jonathan Corbet <corbet@....net>,
	Miroslav Benes <mbenes@...e.cz>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-doc@...r.kernel.org
Subject: Re: module: preserve Elf information for livepatch modules

+++ Josh Poimboeuf [08/02/16 14:10 -0600]:
>On Wed, Feb 03, 2016 at 08:11:07PM -0500, Jessica Yu wrote:
>> For livepatch modules, copy Elf section, symbol, and string information
>> from the load_info struct in the module loader. Persist copies of the
>> original symbol table and string table.
>>
>> Livepatch manages its own relocation sections in order to reuse module
>> loader code to write relocations. Livepatch modules must preserve Elf
>> information such as section indices in order to apply livepatch relocation
>> sections using the module loader's apply_relocate_add() function.
>>
>> In order to apply livepatch relocation sections, livepatch modules must
>> keep a complete copy of their original symbol table in memory. Normally, a
>> stripped down copy of a module's symbol table (containing only "core"
>> symbols) is made available through module->core_symtab. But for livepatch
>> modules, the symbol table copied into memory on module load must be exactly
>> the same as the symbol table produced when the patch module was compiled.
>> This is because the relocations in each livepatch relocation section refer
>> to their respective symbols with their symbol indices, and the original
>> symbol indices (and thus the symtab ordering) must be preserved in order
>> for apply_relocate_add() to find the right symbol.
>
>This patch didn't apply clean to linux-next/master.  I didn't
>investigate why, but maybe it depends on the other patch set which
>removes the notifiers?  (If so, that should be mentioned in the cover
>letter.)

A very recent commit (8244062ef) introduced some changes
kernel/module.c that intersect with this patch, so I think this is why
the patch didn't apply cleanly to today's tree...Anyhow I will rebase
again before sending out v5. Thanks for the comments :-)

Jessica