On Thu, Apr 16, 2009 at 9:56 AM, Jeffrey Cao <jcao.li...@gmail.com> wrote:
> On 2009-04-16, Jeffrey Cao <jcao.li...@gmail.com> wrote:
>> On 2009-04-15, Peter Teoh <htmldevelo...@gmail.com> wrote:
>>
>>
>>> "No process context" actually means that the taskstruct's
>>> mm_struct->mm is NULL. This means that the pagetable CR3 are not
>> That's not correct. mm is NULL does not mean no porcess context.
>> Since all kernel processes/threads share the same upper 1G line address
>> space,
>> they do not have their own individual line address like user sapce proesses.
>> All user space processes have their individual page table maps, but kernel
>> processes share one page table maps. So mm is NULL for kernel processes.
> I did some kernel source code investigation to prove my thought.
> kernel_thread() calles do_fork() with the parameter CLONE_VM. that means
> the kernel process will share the memory space of its parent. And this way
> all the kernel threads share one memory space.
Which is memory space of kernel itself, right?
And off course this means a kernel thread can be cloned from an existing kernel
thread only.
CMIIW
Thanks,
>
>>
>>> changed from its previous value. Therefore, whatever u read/write
>>> to, u are reading/writing to the previous owner of the address space,
>>> which is why when u do things like copy_to_user() from kernel threads,
>>> u are copying to any arbitrary process that happened to be running
>>> BEFORE the kernel thread is switched.
>>>
>>> For eg,
>>>
>>> /*
>>> * Access another process' address space.
>>> * Source/target buffer must be kernel space,
>>> * Do not walk the page table directly, use get_user_pages
>>> */
>>> int access_process_vm(struct task_struct *tsk, unsigned long addr,
>>> void *buf, int len, int write)
>>> {
>>> struct mm_struct *mm;
>>> struct vm_area_struct *vma;
>>> void *old_buf = buf;
>>>
>>> mm = get_task_mm(tsk);
>>> if (!mm)
>>> return 0;
>>>
>>> The above (!mm) check actually means that the API access_process_vm()
>>> MUST NOT be executed from a kernel thread env, which does not have any
>>> process context.
>>>
>> mm is NULL means the process is a kernel process, and it does not allow
>> such access.
>>
>>
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>>
>
>
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>
--
Pradeep
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