How my custom module on linux 3.2.28 can make a call to print_cpu_info?

Question

I'm going to implement my custom module in which information for CPU is printed using print_cpu_info(). In order to call print_cpu_info(), I have included the needed header file, but it doesn't work. Here is my module.

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <asm/alternative.h>
#include <asm/bugs.h>
#include <asm/processor.h>
#include <asm/mtrr.h>
#include <asm/cacheflush.h>
extern struct cpuinfo_x86 boot_cpu_data;
int cpuinfox86_init(void)
{

    print_cpu_info(&boot_cpu_data);
    return 0;

}

void cpuinfox86_exit(void)
{
    printk("good bye cpu\n");
}

module_init(cpuinfox86_init);
module_exit(cpuinfox86_exit);
MODULE_LICENSE("GPL");

After compiling this module, I get

make -C /lib/modules/3.2.28-2009720166/build  SUBDIRS=/home/tracking/1031_oslab modules
make[1]: Entering directory `/usr/src/linux-3.2.28'
  CC [M]  /home/tracking/1031_oslab/module.o
  Building modules, stage 2.
  MODPOST 1 modules
WARNING: "print_cpu_info" [/home/tracking/1031_oslab/module.ko] undefined!
  CC      /home/tracking/1031_oslab/module.mod.o
  LD [M]  /home/tracking/1031_oslab/module.ko
make[1]: Leaving directory `/usr/src/linux-3.2.28'

any idea?

Does 'cat /proc/kallsyms | grep print_cpu_info' show that this is available? — Peter L., Oct 30 '13 at 21:19

score 6 · Accepted Answer · answered Oct 31 '13 at 02:42

6

"print_cpu_info" is not exported symbol, so it can not be used by modules. However, you can use "kallsyms_lookup_name", which is exported, to get the address of "print_cpu_info" and execute the function call using a function pointer.

answered Oct 31 '13 at 02:42

Gyan Gupta

986
8
15

you said that calling `kallsyms_lookup_name("print_cpu_info")` give me the address of the function `print_cpu_info()`, Right? – inherithandle Oct 31 '13 at 03:24

score 0 · Answer 2 · answered Oct 10 '17 at 18:13

static void* find_sym( const char *sym ) {  // find address kernel symbol sym
   static unsigned long faddr = 0;          // static !!!
   // ----------- nested functions are a GCC extension ---------
   int symb_fn( void* data, const char* sym, struct module* mod, unsigned long addr ) {
      if( 0 == strcmp( (char*)data, sym ) ) {
         faddr = addr;
         return 1;
      }
      else return 0;
   };
   // --------------------------------------------------------
   kallsyms_on_each_symbol( symb_fn, (void*)sym );
   return (void*)faddr;
}

static void (*cpuInfo)(struct cpuinfo_x86 *);

How to use it:

unsigned int cpu = 0;
    struct cpuinfo_x86 *c;
    cpuInfo = find_sym("print_cpu_info");
    for_each_online_cpu(cpu)
    {
        c = &cpu_data(cpu);
        cpuInfo(c);
    }

How my custom module on linux 3.2.28 can make a call to print_cpu_info?

2 Answers2

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