Linux Kernel programming: trying to get vm_area_struct-> vm_start kernel crash

Question

this is for a job at school where I need to determine the size of processes in the system using a system call. My code is as follows:

...
struct task_struct *p;
struct vm_area_struct *v;
struct mm_struct *m;
read_lock(&tasklist_lock);
for_each_process(p) {
    printk("%ld\n", p->pid);
    m = p->mm;
    v = m->mmap;
    long start = v->vm_start;
    printk("vm_start is %ld\n", start);
}
read_unlock(&tasklist_lock);
...

When I run a user level program that calls this system call, the output I get is:

1
vm_start - 134512640
2

EIP: 0073: [< 0806e352 > ] CPU: 0 ESP: 007b: 0f7ecf04 EFLAGS: 00010246   
EAX: 00000000 EBX: 0fc587c0 ECX: 081fbb58 EDX: 00000000
ESI: bf88efe0 EDI: 0f482284 EBP: 0f7ecf10 DS: 007b ES: 007b
081f9bc0: [< 08069ae8 > ] show_regs + 0xb4/0xb9
081f9bec: [< 080587ac]] segv + 0x225/0x23d
081f9c8c: [< 08058582 > ] segv_handler + 0x4f/0x54
081f9cac: [< 08067453 > ] sig_handler_common_skas + 0xb7/0xd4
081f9cd4: [< 08064748 > ] sig_handler + 0x34/0x44
081f9cec: [< 080648b5 > ] handle_signal + 0x4c/0x7a
081f9d0c: [< 08066227 > ] hard_handler + 0xf/0x14
081f9d1c: [< 00776420 > ] 0x776420


Kernel panic - : addr 0x0, ip 0x806e352


EIP: 0073: [< 400ea0f2 > ] CPU: 0 ESP: 007b: bf88ef9c EFLAGS: 00000246   
EAX: ffffffda EBX: 00000000 ECX: bf88efc8 EDX: 080483c8
ESI: 00000000 EDI: bf88efe0 EBP: bf88f038 DS: 007b ES: 007b
081f9b28: [< 08069ae8]] show_regs + 0xb4/0xb9
081f9b54: [< 08058a1a > ] panic_exit + 0x25/0x3f
081f9b68: [< 08084f54 > ] notifier_call_chain + 0x21/0x46
081f9b88: [< 08084fef > ] __atomic_notifier_call_chain + 0x17/0x19
081f9ba4: [< 08085006 > ] atomic_notifier_call_chain + 0x15/0x17
081f9bc0: [< 0807039a > ] + 0x52/0xd8
081f9be0: [< 080587ba > ] segv + 0x233/0x23d
081f9c8c: [< 08058582 > ] segv_handler + 0x4f/0x54
081f9cac: [< 08067453 > ] sig_handler_common_skas + 0xb7/0xd4
081f9cd4: [< 08064748 > ] sig_handler + 0x34/0x44
081f9cec: [< 080648b5 > ] handle_signal + 0x4c/0x7a
081f9d0c: [< 08066227 > ] hard_handler + 0xf/0x14
081f9d1c: [< 00776420 > ] 0x776420

(pid = 1) vm_start - , , ​​. - , , , , ? !

( ....)

edit: Fedora 2.6 uml.

Answer 1

mm fill - check p->mm NULL.

Answer 2

:

m = p->mm;
if (m != 0) {
    v = m->mmap;
    if (v != 0) {
        long start = v->vm_start;
        printk("vm_start is %ld\n", start);
    }
}

Answer 3

/proc . fs/proc/*. C

http://lxr.linux.no/linux+v3.2.4/fs/proc/

task_mmu.c, vm_start, , vm_start mmap_sem:

           down_read(&mm->mmap_sem);
            for (vma = mm->mmap; vma; vma = vma->vm_next) {
                    clear_refs_walk.private = vma;
...
                    walk_page_range(vma->vm_start, vma->vm_end,
                                    &clear_refs_walk);

Answer 4

mm null. , mm, .

    down_read(&p->mm->mmap_sem)
          if(mm) {
                 /* read the contents of mm*/
          }
    up_read(&p->mm->mmap_sem)

get_task_mm(). get_task_mm() . :

   struct mm_struct *mm;
   mm = get_task_mm(p);
   if (mm) {
           /* read the mm contents */
   }