Re: Handle kernel page faults using task gate

[Chuck Ebbert <76306.1226@compuserve.com>]

On Wed, 29 Jun 2005 at 18:43:47 +0300, eliad lubovsky wrote:

> my page fault handler:
> static void pagefault_fn(void)
> {
>   unsigned int address, aligned_page_fault_address;
>   struct vm_struct *area;
> 
>   /* retrieve the page fault address */ 
>   __asm__("movl %%cr2,%0":"=r" (address));
> 
>   aligned_page_fault_address = ((address+PAGE_SIZE)&(~(4096-1)));
> 
>   area = find_vm_area((void*)(aligned_page_fault_address));
> 
>   /* allocate a new physical page, expand the stack size */
>   expend_stack_size(area);
> 
>  // asm ("pushf; orl  $0x00004000, (%esp); popf; iret"); /* sets NT   */
>  // asm ("pushf; andl $0xffffbfff, (%esp); popf; iret"); /* clears NT */
>  asm ("iret");
> }


 That will work exactly once. :(

 On the next fault, control wil be transferred to the instruction after
the iret.  It needs to be like this:

======================

static void pagefault_fn(void) {
        unsigned int address, aligned_page_fault_address;
        struct vm_struct *area;

        /* put one-time initialization code here */

        goto handle_fault;

return_from_fault:
        asm("iret");

handle_fault:
        aligned_page_fault_address = ((address+PAGE_SIZE)&(~(4096-1)));
        area = find_vm_area((void*)(aligned_page_fault_address));

        /* allocate a new physical page, expand the stack size */
        expend_stack_size(area);

        goto return_from_fault;
}

======================

 (You also need a private stack to hold the local vars for each TSS.)

--
Chuck