void do_page_fault()

in mm/fault.c [35:234]

• 134 lines of code
• 40 McCabe index (conditional complexity)

void do_page_fault(struct pt_regs *regs)
{
	struct vm_area_struct * vma;
	struct mm_struct *mm = current->mm;
	unsigned int exccause = regs->exccause;
	unsigned int address = regs->excvaddr;
	int code;

	int is_write, is_exec;
	vm_fault_t fault;
	unsigned int flags = FAULT_FLAG_DEFAULT;

	code = SEGV_MAPERR;

	/* We fault-in kernel-space virtual memory on-demand. The
	 * 'reference' page table is init_mm.pgd.
	 */
	if (address >= TASK_SIZE && !user_mode(regs))
		goto vmalloc_fault;

	/* If we're in an interrupt or have no user
	 * context, we must not take the fault..
	 */
	if (faulthandler_disabled() || !mm) {
		bad_page_fault(regs, address, SIGSEGV);
		return;
	}

	is_write = (exccause == EXCCAUSE_STORE_CACHE_ATTRIBUTE) ? 1 : 0;
	is_exec =  (exccause == EXCCAUSE_ITLB_PRIVILEGE ||
		    exccause == EXCCAUSE_ITLB_MISS ||
		    exccause == EXCCAUSE_FETCH_CACHE_ATTRIBUTE) ? 1 : 0;

	pr_debug("[%s:%d:%08x:%d:%08lx:%s%s]\n",
		 current->comm, current->pid,
		 address, exccause, regs->pc,
		 is_write ? "w" : "", is_exec ? "x" : "");

	if (user_mode(regs))
		flags |= FAULT_FLAG_USER;

	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);

retry:
	mmap_read_lock(mm);
	vma = find_vma(mm, address);

	if (!vma)
		goto bad_area;
	if (vma->vm_start <= address)
		goto good_area;
	if (!(vma->vm_flags & VM_GROWSDOWN))
		goto bad_area;
	if (expand_stack(vma, address))
		goto bad_area;

	/* Ok, we have a good vm_area for this memory access, so
	 * we can handle it..
	 */

good_area:
	code = SEGV_ACCERR;

	if (is_write) {
		if (!(vma->vm_flags & VM_WRITE))
			goto bad_area;
		flags |= FAULT_FLAG_WRITE;
	} else if (is_exec) {
		if (!(vma->vm_flags & VM_EXEC))
			goto bad_area;
	} else	/* Allow read even from write-only pages. */
		if (!(vma->vm_flags & (VM_READ | VM_WRITE)))
			goto bad_area;

	/* If for any reason at all we couldn't handle the fault,
	 * make sure we exit gracefully rather than endlessly redo
	 * the fault.
	 */
	fault = handle_mm_fault(vma, address, flags, regs);

	if (fault_signal_pending(fault, regs)) {
		if (!user_mode(regs))
			goto bad_page_fault;
		return;
	}

	if (unlikely(fault & VM_FAULT_ERROR)) {
		if (fault & VM_FAULT_OOM)
			goto out_of_memory;
		else if (fault & VM_FAULT_SIGSEGV)
			goto bad_area;
		else if (fault & VM_FAULT_SIGBUS)
			goto do_sigbus;
		BUG();
	}

	if (fault & VM_FAULT_RETRY) {
		flags |= FAULT_FLAG_TRIED;

		/* No need to mmap_read_unlock(mm) as we would
		 * have already released it in __lock_page_or_retry
		 * in mm/filemap.c.
		 */

		goto retry;
	}

	mmap_read_unlock(mm);
	return;

	/* Something tried to access memory that isn't in our memory map..
	 * Fix it, but check if it's kernel or user first..
	 */
bad_area:
	mmap_read_unlock(mm);
	if (user_mode(regs)) {
		current->thread.bad_vaddr = address;
		current->thread.error_code = is_write;
		force_sig_fault(SIGSEGV, code, (void *) address);
		return;
	}
	bad_page_fault(regs, address, SIGSEGV);
	return;


	/* We ran out of memory, or some other thing happened to us that made
	 * us unable to handle the page fault gracefully.
	 */
out_of_memory:
	mmap_read_unlock(mm);
	if (!user_mode(regs))
		bad_page_fault(regs, address, SIGKILL);
	else
		pagefault_out_of_memory();
	return;

do_sigbus:
	mmap_read_unlock(mm);

	/* Send a sigbus, regardless of whether we were in kernel
	 * or user mode.
	 */
	current->thread.bad_vaddr = address;
	force_sig_fault(SIGBUS, BUS_ADRERR, (void *) address);

	/* Kernel mode? Handle exceptions or die */
	if (!user_mode(regs))
		bad_page_fault(regs, address, SIGBUS);
	return;

vmalloc_fault:
	{
		/* Synchronize this task's top level page-table
		 * with the 'reference' page table.
		 */
		struct mm_struct *act_mm = current->active_mm;
		int index = pgd_index(address);
		pgd_t *pgd, *pgd_k;
		p4d_t *p4d, *p4d_k;
		pud_t *pud, *pud_k;
		pmd_t *pmd, *pmd_k;
		pte_t *pte_k;

		if (act_mm == NULL)
			goto bad_page_fault;

		pgd = act_mm->pgd + index;
		pgd_k = init_mm.pgd + index;

		if (!pgd_present(*pgd_k))
			goto bad_page_fault;

		pgd_val(*pgd) = pgd_val(*pgd_k);

		p4d = p4d_offset(pgd, address);
		p4d_k = p4d_offset(pgd_k, address);
		if (!p4d_present(*p4d) || !p4d_present(*p4d_k))
			goto bad_page_fault;

		pud = pud_offset(p4d, address);
		pud_k = pud_offset(p4d_k, address);
		if (!pud_present(*pud) || !pud_present(*pud_k))
			goto bad_page_fault;

		pmd = pmd_offset(pud, address);
		pmd_k = pmd_offset(pud_k, address);
		if (!pmd_present(*pmd) || !pmd_present(*pmd_k))
			goto bad_page_fault;

		pmd_val(*pmd) = pmd_val(*pmd_k);
		pte_k = pte_offset_kernel(pmd_k, address);

		if (!pte_present(*pte_k))
			goto bad_page_fault;
		return;
	}
bad_page_fault:
	bad_page_fault(regs, address, SIGKILL);
	return;
}