Re: [PATCH v2] arm64/module: use mod->klp_info section header information for livepatch modules

From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Jessica Yu <jeyu@kernel.org>, Torsten Duwe <duwe@lst.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Julien Thierry <julien.thierry@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	Miroslav Benes <mbenes@suse.cz>, Petr Mladek <pmladek@suse.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	live-patching@vger.kernel.org
Subject: Re: [PATCH v2] arm64/module: use mod->klp_info section header information for livepatch modules
Date: Mon, 5 Nov 2018 13:30:37 +0100	[thread overview]
Message-ID: <CAKv+Gu-nq+zWur+swiCxsGtMFpm1KtA4KTdkejJQdjuS9Rg10Q@mail.gmail.com> (raw)
In-Reply-To: <20181101160759.GB17659@arm.com>

On 1 November 2018 at 17:07, Will Deacon <will.deacon@arm.com> wrote:
> Hello, Jessica,
>
> On Tue, Oct 30, 2018 at 02:19:10PM +0100, Jessica Yu wrote:
>> +++ Will Deacon [29/10/18 15:28 +0000]:
>> >On Fri, Oct 26, 2018 at 07:25:01PM +0200, Jessica Yu wrote:
>> >>diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
>> >>index fef773c94e9d..ac9b97f9ae5e 100644
>> >>--- a/arch/arm64/include/asm/module.h
>> >>+++ b/arch/arm64/include/asm/module.h
>> >>@@ -25,6 +25,7 @@ struct mod_plt_sec {
>> >>    struct elf64_shdr       *plt;
>> >>    int                     plt_num_entries;
>> >>    int                     plt_max_entries;
>> >>+   int                     plt_shndx;
>> >>};
>> >
>> >Does this mean we can drop the plt pointer from this struct altogether, and
>> >simply offset into the section headers when applying the relocations?
>>
>> Hmm, if everyone is OK with dropping the plt pointer from struct
>> mod_plt_sec, then I think we can simplify this patch even further.
>>
>> With the plt shndx saved, we can additionally pass a pointer to
>> sechdrs to module_emit_plt_entry(), and with that just offset into the
>> section headers as you suggest. Since livepatch *already* passes in
>> the correct copy of the section headers (mod->klp_info->sechdrs) to
>> apply_relocate_add(), we wouldn't even need to modify the arm64
>> module_finalize() to change mod->arch.core.plt to point into
>> mod->klp_info->sechdrs anymore and we can drop all the changes to the
>> module loader too.
>>
>> Something like the following maybe?
>
> This looks pretty good, thanks! My only (minor) objection is that the
> renaming of plt_sec -> plt_info throughout makes the patch a lot more
> churny than it needs to be, for questionable gain.
>
> Anyway, it looks functionally correct and I've tested loading/unloading
> the "hello world" test module with both PLTs enabled and disabled.
>
> Acked-by: Will Deacon <will.deacon@arm.com>
>

For the simplified version:

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>