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

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);
}
}

Answer 3

All process related information can be found at /proc filesystem at the userspace level. Inside the kernel, these information are generated via fs/proc/*.c
http://lxr.linux.no/linux+v3.2.4/fs/proc/
Looking at the file task_mmu.c, which printing all the vm_start information u can observe that all handling of vm_start field always require the mmap_sem to be locked:
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

For kernel threads mm will be null. So whenever you read the mm do it in the following manner.
down_read(&p->mm->mmap_sem)
if(mm) {
/* read the contents of mm*/
}
up_read(&p->mm->mmap_sem)
Also you may use get_task_mm(). With get_task_mm() you need not acquire the lock. Here is how you use it :
struct mm_struct *mm;
mm = get_task_mm(p);
if (mm) {
/* read the mm contents */
}