On 2009-04-15, Peter Teoh <htmldeveloper@xxxxxxxxx> 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.
> 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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