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

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

On 1 November 2018 at 17:07, Will Deacon <will.deacon@....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@....com>
>

For the simplified version:

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