Error with call_usermodehelper API

Dec. 30, 2013

      I am having a problem with the use of call_usermodehelper API. I am using
this API inside Kernel jprobes module to trap start_thread function. This
API works well if I provide UMH_WAIT_EXEC as wait parameter, but it fails to
load the process when UMH_WAIT_PROC is passed as parameter. 

Specifically, the following code works properly.

    #include <linux/kernel.h>
    #include <linux/module.h>
    #include <linux/sched.h>
    #include <linux/kprobes.h>
    #include <linux/kallsyms.h>
    #include <linux/ptrace.h>
    #include <linux/kmod.h>
    #include <linux/syscalls.h>
    #include <linux/delay.h>

    static struct jprobe start_thread_jprobe;

    static asmlinkage int kp_start_thread(struct pt_regs* regs, 
                            unsigned long new_ip,unsigned long new_sp){

        int retval;

        char * envp[] = { NULL };
        char* argv[]={"/bin/ls",NULL};

        //Only modifying this for a process named test1
        if(strcmp(current->comm,"test1")==0){

        retval=call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);

        if(retval<0){
                printk("Failed in starting! %d\n",retval);
        }
        else {
            printk("Succeded in starting %d\n",retval);
        }
    }

    jprobe_return();
    /*NOTREACHED*/
    return (0);
    }

    int init_module(void)
    {
    int ret;
    start_thread_jprobe.entry = (kprobe_opcode_t *)kp_start_thread;
    start_thread_jprobe.kp.addr = (kprobe_opcode_t *)
    kallsyms_lookup_name("start_thread");

    if (!start_thread_jprobe.kp.addr) {
    printk("unable to lookup symbol\n");
    return (-1);
    }

    if ((ret = register_jprobe(&start_thread_jprobe)) <0) {
    printk("register_jprobe failed, returned %d\n", ret);
    return (-1);
    }
    return (0);
    }

But if I update the call to call_usemodehelper as follows.

     retval=call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);

The above call fails to load *ls* using this API. It gives an error number
-14 (EFAULT).

A simple test is as follows:

test1.c

    #include <stdio.h>
       int main(){
              printf("This is test1\n");
              return 0;
       }

Makefile for module:

    obj-m := kp-start-thread.o
    KDIR := /lib/modules/$(shell uname -r)/build
    PWD := $(shell pwd)
    default:
            $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
    clean:
            rm -f *.mod.c *.ko *.o

Test:

    gcc test1.c -o test1 
       make 
       sudo insmod ./kp-start-thread.ko
       ./test1  
       dmesg ---> Will demonstrate success message with EXEC and failure
with WAIT.

Error with call_usermodehelper API

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