linux-kernel - Re: [PATCH v9 0/6] add support for relative references in special sections

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Date:   Sun, 1 Jul 2018 19:39:43 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Garnier <thgarnie@...gle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Russell King <linux@...linux.org.uk>,
        Paul Mackerras <paulus@...ba.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Petr Mladek <pmladek@...e.com>,
        James Morris <jmorris@...ei.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Pitre <nico@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v9 0/6] add support for relative references in special sections

On 27 June 2018 at 17:15, Will Deacon <will.deacon@....com> wrote:
> Hi Ard,
>
> On Tue, Jun 26, 2018 at 08:27:55PM +0200, Ard Biesheuvel wrote:
>> This adds support for emitting special sections such as initcall arrays,
>> PCI fixups and tracepoints as relative references rather than absolute
>> references. This reduces the size by 50% on 64-bit architectures, but
>> more importantly, it removes the need for carrying relocation metadata
>> for these sections in relocatable kernels (e.g., for KASLR) that needs
>> to be fixed up at boot time. On arm64, this reduces the vmlinux footprint
>> of such a reference by 8x (8 byte absolute reference + 24 byte RELA entry
>> vs 4 byte relative reference)
>>
>> Patch #3 was sent out before as a single patch. This series supersedes
>> the previous submission. This version makes relative ksymtab entries
>> dependent on the new Kconfig symbol HAVE_ARCH_PREL32_RELOCATIONS rather
>> than trying to infer from kbuild test robot replies for which architectures
>> it should be blacklisted.
>>
>> Patch #1 introduces the new Kconfig symbol HAVE_ARCH_PREL32_RELOCATIONS,
>> and sets it for the main architectures that are expected to benefit the
>> most from this feature, i.e., 64-bit architectures or ones that use
>> runtime relocations.
>>
>> Patch #2 add support for #define'ing __DISABLE_EXPORTS to get rid of
>> ksymtab/kcrctab sections in decompressor and EFI stub objects when
>> rebuilding existing C files to run in a different context.
>
> I had a small question on patch 3, but it's really for my understanding.
> So, for patches 1-3:
>
> Reviewed-by: Will Deacon <will.deacon@....com>
>

Thanks all.

Thomas, Ingo,

Except for the below tweak against patch #3 for powerpc, which may
apparently get confused by an input section called .discard without
any suffixes, this series is good to go, but requires your ack to
proceed, so I would like to ask you to share your comments and/or
objections. Also, any suggestions or recommendations regarding the
route these patches should take are highly appreciated.

Ard.


diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 2d9c63f41031..61c844d4ab48 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -287,7 +287,7 @@ unsigned long read_word_at_a_time(const void *addr)
  * visible to the compiler.
  */
 #define __ADDRESSABLE(sym) \
-       static void * __attribute__((section(".discard"), used))        \
+       static void * __attribute__((section(".discard.addressable"), used)) \
                __PASTE(__addressable_##sym, __LINE__) = (void *)&sym;

 /**