where are the bug ?

[Rohan Puri]

On Sat, Oct 20, 2012 at 7:35 AM, Fan Yang <lljyangfan at gmail.com> wrote:

>
>
> 2012/10/19 Rohan Puri <rohan.puri15 at gmail.com>
>
>>
>>
>> On Fri, Oct 19, 2012 at 7:38 PM, Anuz Pratap Singh Tomar <
>> chambilkethakur at gmail.com> wrote:
>>
>>>
>>>
>>> On Fri, Oct 19, 2012 at 2:46 PM, Fan Yang <lljyangfan at gmail.com> wrote:
>>>
>>>>
>>>>
>>>> 2012/10/19 Arun KS <getarunks at gmail.com>
>>>> >
>>>> > Hi Fan,
>>>> >
>>>> > On Fri, Oct 19, 2012 at 6:50 PM, Fan Yang <lljyangfan at gmail.com>
>>>> wrote:
>>>> >>
>>>> >> HI ALL:
>>>> >>     I just run a module on my machine, but it  can't work. When the
>>>> module run the kernel will painc. I don't know where is wrong. This is my
>>>> code:
>>>> >>
>>>> >>  1 #include<linux/module.h>
>>>> >>   2 #include<linux/kernel.h>
>>>> >>   3 #include<linux/init.h>
>>>> >>   4 #include<linux/sched.h>
>>>> >>   5
>>>> >>   6 int input = 1;
>>>> >>   7 module_param (input, int, S_IRUGO);
>>>> >>   8
>>>> >>   9 static int __init printvma_init (void)
>>>> >>  10 {
>>>> >>  11     struct vm_area_struct *p, *start;
>>>> >>  12     int i;
>>>> >>  13     struct task_struct *thread;
>>>>
>>>> >>  14
>>>> >>  15     thread = current;
>>>> >>  16
>>>> >>  17     while (1)
>>>> >>  18     {
>>>> >>  19         if (thread->pid == input)
>>>> >>  20             break;
>>>> >>  21         thread = list_entry (thread->tasks.next, struct
>>>> task_struct, tasks);
>>>> >>  22     }
>>>> >>  23     p = thread->mm->mmap;
>>>> >>  24
>>>> >>  25     do{
>>>> >>  26         printk ("%lx\t%lx\t%s\n", p->vm_start,\
>>>> >>  27                 p->vm_end, p->vm_file->f_path.dentry->d_iname);
>>>> >>  28         p = p->vm_next;
>>>> >>  29     }while (p != NULL);
>>>> >>  30
>>>> >>  31     printk ("vm_file address is:%d\tf_path address is:%d\
>>>> >>  32             \tname is:%s",& p->vm_file->f_path,\
>>>> >>  33             p->vm_file->f_path.dentry->d_iname);
>>>> >>  34
>>>> >>  35     printk ("info from the kernel space:%s\n", thread->comm);
>>>> >>  36     return 0;
>>>> >>  37 }
>>>> >>  38
>>>> >>  39 static void __exit printvma_exit (void)
>>>> >>  40 {
>>>> >>  41     printk ("the module will leave the kernel space..\n");
>>>> >>  42 }
>>>> >>  43
>>>> >>  44 module_init (printvma_init);
>>>> >>  45 module_exit (printvma_exit);
>>>> >>  46 MODULE_LICENSE ("GPL");
>>>> >>
>>>> >>
>>>> >> what's wrong?
>>>> >
>>>> >
>>>> > It would be good if you paste your crash log here.
>>>> >
>>>> > Thanks,
>>>> > Arun
>>>> >>
>>>> >>
>>>> >> thanks
>>>> >>
>>>> >> _______________________________________________
>>>> >> Kernelnewbies mailing list
>>>> >> Kernelnewbies at kernelnewbies.org
>>>> >> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>> >>
>>>> >
>>>> The module run in a virtual machine, I can't control the machine when
>>>> it crashed, so I just got a picture when the kernel panic.
>>>>
>>>> you can run the module under uml, it wont be hard to copy  the crash
>>> log from terminal in uml.
>>>
>>>>
>>>>
>>>>
>>>>
>>>> Thanks
>>>> Fan
>>>>
>>>> _______________________________________________
>>>> Kernelnewbies mailing list
>>>> Kernelnewbies at kernelnewbies.org
>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>>
>>>>
>>>
>>> _______________________________________________
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>>>
>>>
>> Hi Fan,
>>
>> See the issue is thread->mm is NULL in your case. The simplest way to
>> test this in your case is by the following : -
>>
>> Put these statements after the while loop
>>
>> if(!thread->mm) { printk("thread->mm is NULL\n"); return 0; }
>>
>> After this compile and load the module, you will see this statement
>> printed in dmesg command output.
>>
>>
>> General programming practice : -
>>
>> Always make checks for NULL pointer in your code, before dereferencing
>> your code.
>>
>> - Rohan
>>
> Hi Rohan,
>
> I don't think the thread->mm is NULL, because when I print the several
> vm_area_struct of the thread->mm it work well, but if put the code in the
> loop to print all the vma, it crashed.
>
> Thinks
> Fan
>

Hi Fan,

Yes Fan, you are right, its NOT thread->mm NULL, but p->vm_file is NULL, to
verify put the following as the fist statement in do {}while; loop

if(!p->vm_file) { printk("p->vm_file NULL\n"); return 0;}


This message gets printed to kernel log buffer.

Also, you still need to NULL check pointer before dereferencing them. Let
me know, whats the result on your system.

- Rohan
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