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

Question

this is for an assignment at school, where I need to determine the size of the processes on 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 that I get is:

1
vm_start is 134512640
2

EIP: 0073:[<0806e352>] CPU: 0 Not tainted ESP: 007b:0f7ecf04 EFLAGS: 00010246 Not tainted
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 - not syncing: Kernel mode fault at addr 0x0, ip 0x806e352


EIP: 0073:[<400ea0f2>] CPU: 0 Not tainted ESP: 007b:bf88ef9c EFLAGS: 00000246 Not tainted
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>] panic+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

The first process (pid = 1) gave me the vm_start without any problems, but when I try to access the second process, the kernel crashes. Can anyone tell me what's wrong, and maybe how to fix it as well? Thanks a lot!

(sorry for the bad formatting....)

edit: This is done in a Fedora 2.6 core in an uml environment.

Answer 1

Some kernel threads might not have mm filled - check p->mm for NULL.

Answer 2

Changed the code to check for null pointers:

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