サンプル

#include <linux/module.h>
#include <linux/kernel.h>

static int          val1 = 1;
static int          val2;
static int          val3;
const static int    val4 = 1;
int __weak          val5;

int __weak func1(void)
{
       return 1;
}

static int func2(void)
{
       return 1;
}

int     func3(void)
{
       return 1;
}

static int __init babakaka_init( void )
{
    printk("val1:%x\n", &val1);
    printk("val2:%x\n", &val2);
    printk("val3:%x\n", &val3);
    printk("val4:%x\n", &val4);
    printk("val5:%x\n", &val5);
    printk("func1:%x\n", &func1);
    printk("func2:%x\n", &func2);
    printk("func3:%x\n", &func3);
    return 0;
}

static void __exit babakaka_exit( void )
{
}

EXPORT_SYMBOL(val3);
EXPORT_SYMBOL(func2);

module_init(babakaka_init);
module_exit(babakaka_exit);

検証

[root@localhost lkm]# insmod babakaka.ko
[root@localhost lkm]# dmesg
 :
[ 1747.914420] val1:e0b6d000
[ 1747.914433] val2:e0b6d1a0
[ 1747.914440] val3:e0b6d1a4
[ 1747.914446] val4:e0b6c080
[ 1747.914452] val5:e0b6d19c
[ 1747.914458] func1:e0b6b000
[ 1747.914465] func2:e0b6b010
[ 1747.914471] func3:e0b6b020

[root@localhost lkm]# cat /proc/kallsyms | grep babakaka
e0b6b010 T func2        [babakaka]         <-EXPORT_SYMBOされたコード
e0b6d000 d val1 [babakaka]                 <-初期化データ
e0b6d1a0 b val2 [babakaka]                 <-未初期データ
e0b6d1a4 B val3 [babakaka]                 <-EXPORT_SYMBOLされた未初期データ
e0b6c080 r val4 [babakaka]                 <-読み込み専用データ
e0b6b030 t babakaka_exit        [babakaka]
e0b6c024 r __ksymtab_func2      [babakaka]
e0b6c084 r __kstrtab_func2      [babakaka]
e0b6c02c r __ksymtab_val3       [babakaka]
e0b6c08a r __kstrtab_val3       [babakaka]
e0b6d020 d __this_module        [babakaka]
e0b6b030 t cleanup_module       [babakaka]
e0b6b020 t func3        [babakaka]         <-コード
e0b6d19c v val5 [babakaka]                 <-weakデータ
e0b6b000 w func1        [babakaka]         <-weakコード

実装

#define __weak                          __attribute__((weak))

#define EXPORT_SYMBOL(sym)                                      \
       __EXPORT_SYMBOL(sym, "")

#define __EXPORT_SYMBOL(sym, sec)                               \
       extern typeof(sym) sym;                                 \
       __CRC_SYMBOL(sym, sec)                                  \
       static const char __kstrtab_##sym[]                     \
       __attribute__((section("__ksymtab_strings"), aligned(1))) \
       = MODULE_SYMBOL_PREFIX #sym;                            \
       static const struct kernel_symbol __ksymtab_##sym       \
       __used                                                  \
       __attribute__((section("___ksymtab" sec "+" #sym), unused))     \
       = { (unsigned long)&sym, __kstrtab_##sym }

LKMでEXPORT_SYMBOLされているオブジェクトのタイプ属性は大文字となります。カーネル本体でのEXPORTされているオブジェクトには影響しません。

static const struct seq_operations kallsyms_op = {
       .start = s_start,
       .next = s_next,
       .stop = s_stop,
       .show = s_show
};

static int s_show(struct seq_file *m, void *p)
{
       struct kallsym_iter *iter = m->private;

       if (!iter->name[0])
               return 0;

       if (iter->module_name[0]) {
               char type;

               type = iter->exported ? toupper(iter->type) :
                                       tolower(iter->type);
               seq_printf(m, "%pK %c %s\t[%s]\n", (void *)iter->value,
                          type, iter->name, iter->module_name);
       } else
               seq_printf(m, "%pK %c %s\n", (void *)iter->value,
                          iter->type, iter->name);
       return 0;
}