Re: [PATCH v9 3/6] module: use relative references for __ksymtab entries

[Ard Biesheuvel]

On 27 June 2018 at 17:13, Will Deacon <will.deacon@xxxxxxx> 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.