linux-kernel - Re: [PATCH v9 3/6] module: use relative references for _

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Message-ID: <CAKv+Gu8v4VGzKSQ1F6r6B_U2xhffM1EJQWCJYuf+4-xsf_hjuw@mail.gmail.com>
Date:   Wed, 27 Jun 2018 17:21:33 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Will Deacon <will.deacon@....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>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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>, Ingo Molnar <mingo@...hat.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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v9 3/6] module: use relative references for __ksymtab entries

On 27 June 2018 at 17:13, Will Deacon <will.deacon@....com> wrote:
> Hi Ard,
>
> On Tue, Jun 26, 2018 at 08:27:58PM +0200, Ard Biesheuvel wrote:
>> An ordinary arm64 defconfig build has ~64 KB worth of __ksymtab
>> entries, each consisting of two 64-bit fields containing absolute
>> references, to the symbol itself and to a char array containing
>> its name, respectively.
>
> [...]
>
>> diff --git a/include/linux/export.h b/include/linux/export.h
>> index ea7df303d68d..ae072bc5aacf 100644
>> --- a/include/linux/export.h
>> +++ b/include/linux/export.h
>> @@ -18,12 +18,6 @@
>>  #define VMLINUX_SYMBOL_STR(x) __VMLINUX_SYMBOL_STR(x)
>>
>>  #ifndef __ASSEMBLY__
>> -struct kernel_symbol
>> -{
>> -     unsigned long value;
>> -     const char *name;
>> -};
>> -
>>  #ifdef MODULE
>>  extern struct module __this_module;
>>  #define THIS_MODULE (&__this_module)
>> @@ -54,17 +48,47 @@ extern struct module __this_module;
>>  #define __CRC_SYMBOL(sym, sec)
>>  #endif
>>
>> +#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
>> +#include <linux/compiler.h>
>> +/*
>> + * Emit the ksymtab entry as a pair of relative references: this reduces
>> + * the size by half on 64-bit architectures, and eliminates the need for
>> + * absolute relocations that require runtime processing on relocatable
>> + * kernels.
>> + */
>> +#define __KSYMTAB_ENTRY(sym, sec)                                    \
>> +     __ADDRESSABLE(sym)                                              \
>> +     asm("   .section \"___ksymtab" sec "+" #sym "\", \"a\"  \n"     \
>> +         "   .balign 8                                       \n"     \
>
> Can we use KSYM_ALIGN here instead of 8, or do we need the 8-byte alignment
> even on 32-bit architectures?
>

We don't *need* 8 byte alignment on any architecture, but since the
structure itself is 8 bytes in size and we have a sizable array of
them, it makes sense to align them to 8 bytes.