Kernel: Assert if rounding-up-to-page-size would wrap around to 0

If we try to align a number above 0xfffff000 to the next multiple of
the page size (4 KiB), it would wrap around to 0. This is most likely
never what we want, so let's assert if that happens.

Kernel/Arch/PC/BIOS.cpp

@@ -36,7 +36,7 @@ MappedROM map_bios()
     MappedROM mapping;
     mapping.size = 128 * KiB;
     mapping.paddr = PhysicalAddress(0xe0000);
-    mapping.region = MM.allocate_kernel_region(mapping.paddr, PAGE_ROUND_UP(mapping.size), {}, Region::Access::Read);
+    mapping.region = MM.allocate_kernel_region(mapping.paddr, page_round_up(mapping.size), {}, Region::Access::Read);
     return mapping;
 }
 
@@ -49,7 +49,7 @@ MappedROM map_ebda()
     size_t ebda_size = *ebda_length_ptr;
 
     MappedROM mapping;
-    mapping.region = MM.allocate_kernel_region(ebda_paddr.page_base(), PAGE_ROUND_UP(ebda_size), {}, Region::Access::Read);
+    mapping.region = MM.allocate_kernel_region(ebda_paddr.page_base(), page_round_up(ebda_size), {}, Region::Access::Read);
     mapping.offset = ebda_paddr.offset_in_page();
     mapping.size = ebda_size;
     mapping.paddr = ebda_paddr;

Kernel/Devices/BXVGADevice.cpp

@@ -179,7 +179,7 @@ KResultOr<Region*> BXVGADevice::mmap(Process& process, FileDescription&, const R
         return ENODEV;
     if (offset != 0)
         return ENXIO;
-    if (range.size() != PAGE_ROUND_UP(framebuffer_size_in_bytes()))
+    if (range.size() != page_round_up(framebuffer_size_in_bytes()))
         return EOVERFLOW;
 
     auto vmobject = AnonymousVMObject::create_for_physical_range(m_framebuffer_address, framebuffer_size_in_bytes());

Kernel/Devices/MBVGADevice.cpp

@@ -58,7 +58,7 @@ KResultOr<Region*> MBVGADevice::mmap(Process& process, FileDescription&, const R
         return ENODEV;
     if (offset != 0)
         return ENXIO;
-    if (range.size() != PAGE_ROUND_UP(framebuffer_size_in_bytes()))
+    if (range.size() != page_round_up(framebuffer_size_in_bytes()))
         return EOVERFLOW;
 
     auto vmobject = AnonymousVMObject::create_for_physical_range(m_framebuffer_address, framebuffer_size_in_bytes());

Kernel/Heap/kmalloc.cpp

@@ -108,7 +108,7 @@ struct KmallocGlobalHeap {
             // was big enough to likely satisfy the request
             if (subheap.free_bytes() < allocation_request) {
                 // Looks like we probably need more
-                size_t memory_size = PAGE_ROUND_UP(decltype(m_global_heap.m_heap)::calculate_memory_for_bytes(allocation_request));
+                size_t memory_size = page_round_up(decltype(m_global_heap.m_heap)::calculate_memory_for_bytes(allocation_request));
                 // Add some more to the new heap. We're already using it for other
                 // allocations not including the original allocation_request
                 // that triggered heap expansion. If we don't allocate

Kernel/Interrupts/APIC.cpp

@@ -309,7 +309,7 @@ void APIC::do_boot_aps()
     // Also account for the data appended to:
     // * aps_to_enable u32 values for ap_cpu_init_stacks
     // * aps_to_enable u32 values for ap_cpu_init_processor_info_array
-    auto apic_startup_region = MM.allocate_kernel_region_identity(PhysicalAddress(0x8000), PAGE_ROUND_UP(apic_ap_start_size + (2 * aps_to_enable * sizeof(u32))), {}, Region::Access::Read | Region::Access::Write | Region::Access::Execute);
+    auto apic_startup_region = MM.allocate_kernel_region_identity(PhysicalAddress(0x8000), page_round_up(apic_ap_start_size + (2 * aps_to_enable * sizeof(u32))), {}, Region::Access::Read | Region::Access::Write | Region::Access::Execute);
     memcpy(apic_startup_region->vaddr().as_ptr(), reinterpret_cast<const void*>(apic_ap_start), apic_ap_start_size);
 
     // Allocate enough stacks for all APs

Kernel/KBuffer.h

@@ -50,7 +50,7 @@ class KBufferImpl : public RefCounted<KBufferImpl> {
 public:
     static RefPtr<KBufferImpl> try_create_with_size(size_t size, u8 access, const char* name = "KBuffer", AllocationStrategy strategy = AllocationStrategy::Reserve)
     {
-        auto region = MM.allocate_kernel_region(PAGE_ROUND_UP(size), name, access, strategy);
+        auto region = MM.allocate_kernel_region(page_round_up(size), name, access, strategy);
         if (!region)
             return nullptr;
         return adopt(*new KBufferImpl(region.release_nonnull(), size, strategy));
@@ -58,7 +58,7 @@ public:
 
     static RefPtr<KBufferImpl> try_create_with_bytes(ReadonlyBytes bytes, u8 access, const char* name = "KBuffer", AllocationStrategy strategy = AllocationStrategy::Reserve)
     {
-        auto region = MM.allocate_kernel_region(PAGE_ROUND_UP(bytes.size()), name, access, strategy);
+        auto region = MM.allocate_kernel_region(page_round_up(bytes.size()), name, access, strategy);
         if (!region)
             return nullptr;
         memcpy(region->vaddr().as_ptr(), bytes.data(), bytes.size());
@@ -81,7 +81,7 @@ public:
 
     bool expand(size_t new_capacity)
     {
-        auto new_region = MM.allocate_kernel_region(PAGE_ROUND_UP(new_capacity), m_region->name(), m_region->access(), m_allocation_strategy);
+        auto new_region = MM.allocate_kernel_region(page_round_up(new_capacity), m_region->name(), m_region->access(), m_allocation_strategy);
         if (!new_region)
             return false;
         if (m_region && m_size > 0)

Kernel/KBufferBuilder.cpp

@@ -42,7 +42,7 @@ inline bool KBufferBuilder::check_expand(size_t size)
     size_t new_buffer_size = m_size + size;
     if (Checked<size_t>::addition_would_overflow(new_buffer_size, 1 * MiB))
         return false;
-    new_buffer_size = PAGE_ROUND_UP(new_buffer_size + 1 * MiB);
+    new_buffer_size = page_round_up(new_buffer_size + 1 * MiB);
     return m_buffer->expand(new_buffer_size);
 }
 

Kernel/Net/E1000NetworkAdapter.cpp

@@ -196,8 +196,8 @@ void E1000NetworkAdapter::detect()
 E1000NetworkAdapter::E1000NetworkAdapter(PCI::Address address, u8 irq)
     : PCI::Device(address, irq)
     , m_io_base(PCI::get_BAR1(pci_address()) & ~1)
-    , m_rx_descriptors_region(MM.allocate_contiguous_kernel_region(PAGE_ROUND_UP(sizeof(e1000_rx_desc) * number_of_rx_descriptors + 16), "E1000 RX", Region::Access::Read | Region::Access::Write))
-    , m_tx_descriptors_region(MM.allocate_contiguous_kernel_region(PAGE_ROUND_UP(sizeof(e1000_tx_desc) * number_of_tx_descriptors + 16), "E1000 TX", Region::Access::Read | Region::Access::Write))
+    , m_rx_descriptors_region(MM.allocate_contiguous_kernel_region(page_round_up(sizeof(e1000_rx_desc) * number_of_rx_descriptors + 16), "E1000 RX", Region::Access::Read | Region::Access::Write))
+    , m_tx_descriptors_region(MM.allocate_contiguous_kernel_region(page_round_up(sizeof(e1000_tx_desc) * number_of_tx_descriptors + 16), "E1000 TX", Region::Access::Read | Region::Access::Write))
 {
     set_interface_name("e1k");
 
@@ -206,7 +206,7 @@ E1000NetworkAdapter::E1000NetworkAdapter(PCI::Address address, u8 irq)
     enable_bus_mastering(pci_address());
 
     size_t mmio_base_size = PCI::get_BAR_space_size(pci_address(), 0);
-    m_mmio_region = MM.allocate_kernel_region(PhysicalAddress(page_base_of(PCI::get_BAR0(pci_address()))), PAGE_ROUND_UP(mmio_base_size), "E1000 MMIO", Region::Access::Read | Region::Access::Write, Region::Cacheable::No);
+    m_mmio_region = MM.allocate_kernel_region(PhysicalAddress(page_base_of(PCI::get_BAR0(pci_address()))), page_round_up(mmio_base_size), "E1000 MMIO", Region::Access::Read | Region::Access::Write, Region::Cacheable::No);
     m_mmio_base = m_mmio_region->vaddr();
     m_use_mmio = true;
     m_interrupt_line = PCI::get_interrupt_line(pci_address());

Kernel/Net/RTL8139NetworkAdapter.cpp

@@ -141,8 +141,8 @@ void RTL8139NetworkAdapter::detect()
 RTL8139NetworkAdapter::RTL8139NetworkAdapter(PCI::Address address, u8 irq)
     : PCI::Device(address, irq)
     , m_io_base(PCI::get_BAR0(pci_address()) & ~1)
-    , m_rx_buffer(MM.allocate_contiguous_kernel_region(PAGE_ROUND_UP(RX_BUFFER_SIZE + PACKET_SIZE_MAX), "RTL8139 RX", Region::Access::Read | Region::Access::Write))
-    , m_packet_buffer(MM.allocate_contiguous_kernel_region(PAGE_ROUND_UP(PACKET_SIZE_MAX), "RTL8139 Packet buffer", Region::Access::Read | Region::Access::Write))
+    , m_rx_buffer(MM.allocate_contiguous_kernel_region(page_round_up(RX_BUFFER_SIZE + PACKET_SIZE_MAX), "RTL8139 RX", Region::Access::Read | Region::Access::Write))
+    , m_packet_buffer(MM.allocate_contiguous_kernel_region(page_round_up(PACKET_SIZE_MAX), "RTL8139 Packet buffer", Region::Access::Read | Region::Access::Write))
 {
     m_tx_buffers.ensure_capacity(RTL8139_TX_BUFFER_COUNT);
     set_interface_name("rtl8139");
@@ -161,7 +161,7 @@ RTL8139NetworkAdapter::RTL8139NetworkAdapter(PCI::Address address, u8 irq)
     klog() << "RTL8139: RX buffer: " << m_rx_buffer->physical_page(0)->paddr();
 
     for (int i = 0; i < RTL8139_TX_BUFFER_COUNT; i++) {
-        m_tx_buffers.append(MM.allocate_contiguous_kernel_region(PAGE_ROUND_UP(TX_BUFFER_SIZE), "RTL8139 TX", Region::Access::Write | Region::Access::Read));
+        m_tx_buffers.append(MM.allocate_contiguous_kernel_region(page_round_up(TX_BUFFER_SIZE), "RTL8139 TX", Region::Access::Write | Region::Access::Read));
         klog() << "RTL8139: TX buffer " << i << ": " << m_tx_buffers[i]->physical_page(0)->paddr();
     }
 

Kernel/PCI/MMIOAccess.cpp

@@ -51,7 +51,7 @@ private:
 
 DeviceConfigurationSpaceMapping::DeviceConfigurationSpaceMapping(Address device_address, const MMIOSegment& mmio_segment)
     : m_device_address(device_address)
-    , m_mapped_region(MM.allocate_kernel_region(PAGE_ROUND_UP(PCI_MMIO_CONFIG_SPACE_SIZE), "PCI MMIO Device Access", Region::Access::Read | Region::Access::Write).release_nonnull())
+    , m_mapped_region(MM.allocate_kernel_region(page_round_up(PCI_MMIO_CONFIG_SPACE_SIZE), "PCI MMIO Device Access", Region::Access::Read | Region::Access::Write).release_nonnull())
 {
     PhysicalAddress segment_lower_addr = mmio_segment.get_paddr();
     PhysicalAddress device_physical_mmio_space = segment_lower_addr.offset(
@@ -106,7 +106,7 @@ MMIOAccess::MMIOAccess(PhysicalAddress p_mcfg)
     klog() << "PCI: MCFG, length - " << length << ", revision " << revision;
     checkup_region->unmap();
 
-    auto mcfg_region = MM.allocate_kernel_region(p_mcfg.page_base(), PAGE_ROUND_UP(length) + PAGE_SIZE, "PCI Parsing MCFG", Region::Access::Read | Region::Access::Write);
+    auto mcfg_region = MM.allocate_kernel_region(p_mcfg.page_base(), page_round_up(length) + PAGE_SIZE, "PCI Parsing MCFG", Region::Access::Read | Region::Access::Write);
 
     auto& mcfg = *(ACPI::Structures::MCFG*)mcfg_region->vaddr().offset(p_mcfg.offset_in_page()).as_ptr();
     dbgln_if(PCI_DEBUG, "PCI: Checking MCFG @ {}, {}", VirtualAddress(&mcfg), PhysicalAddress(p_mcfg.get()));

Kernel/Storage/RamdiskController.cpp

@@ -68,7 +68,7 @@ RamdiskController::RamdiskController()
     size_t count = 0;
     for (auto used_memory_range : MemoryManager::the().used_memory_ranges()) {
         if (used_memory_range.type == UsedMemoryRangeType::BootModule) {
-            size_t length = PAGE_ROUND_UP(used_memory_range.end.get()) - used_memory_range.start.get();
+            size_t length = page_round_up(used_memory_range.end.get()) - used_memory_range.start.get();
             auto region = MemoryManager::the().allocate_kernel_region(used_memory_range.start, length, "Ramdisk", Region::Access::Read | Region::Access::Write);
             m_devices.append(RamdiskDevice::create(*this, move(region), 6, count));
             count++;

Kernel/Syscalls/execve.cpp

@@ -167,7 +167,7 @@ static KResultOr<RequiredLoadRange> get_required_load_range(FileDescription& pro
 
     size_t executable_size = inode.size();
 
-    auto region = MM.allocate_kernel_region_with_vmobject(*vmobject, PAGE_ROUND_UP(executable_size), "ELF memory range calculation", Region::Access::Read);
+    auto region = MM.allocate_kernel_region_with_vmobject(*vmobject, page_round_up(executable_size), "ELF memory range calculation", Region::Access::Read);
     if (!region) {
         dbgln("Could not allocate memory for ELF");
         return ENOMEM;
@@ -203,7 +203,7 @@ static KResultOr<FlatPtr> get_interpreter_load_offset(const Elf32_Ehdr& main_pro
     constexpr FlatPtr minimum_interpreter_load_offset_randomization_size = 10 * MiB;
 
     auto random_load_offset_in_range([](auto start, auto size) {
-        return PAGE_ROUND_DOWN(start + get_good_random<FlatPtr>() % size);
+        return page_round_down(start + get_good_random<FlatPtr>() % size);
     });
 
     if (main_program_header.e_type == ET_DYN) {
@@ -263,7 +263,7 @@ static KResultOr<LoadResult> load_elf_object(NonnullOwnPtr<Space> new_space, Fil
 
     size_t executable_size = inode.size();
 
-    auto executable_region = MM.allocate_kernel_region_with_vmobject(*vmobject, PAGE_ROUND_UP(executable_size), "ELF loading", Region::Access::Read);
+    auto executable_region = MM.allocate_kernel_region_with_vmobject(*vmobject, page_round_up(executable_size), "ELF loading", Region::Access::Read);
     if (!executable_region) {
         dbgln("Could not allocate memory for ELF loading");
         return ENOMEM;

Kernel/Syscalls/mmap.cpp

@@ -160,7 +160,10 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
     if (alignment & ~PAGE_MASK)
         return (void*)-EINVAL;
 
-    if (!is_user_range(VirtualAddress(addr), PAGE_ROUND_UP(size)))
+    if (page_round_up_would_wrap(size))
+        return (void*)-EINVAL;
+
+    if (!is_user_range(VirtualAddress(addr), page_round_up(size)))
         return (void*)-EFAULT;
 
     String name;
@@ -204,7 +207,7 @@ void* Process::sys$mmap(Userspace<const Syscall::SC_mmap_params*> user_params)
     Optional<Range> range;
 
     if (map_randomized) {
-        range = space().page_directory().range_allocator().allocate_randomized(PAGE_ROUND_UP(size), alignment);
+        range = space().page_directory().range_allocator().allocate_randomized(page_round_up(size), alignment);
     } else {
         range = space().allocate_range(VirtualAddress(addr), size, alignment);
         if (!range.has_value()) {
@@ -272,7 +275,10 @@ int Process::sys$mprotect(void* addr, size_t size, int prot)
         REQUIRE_PROMISE(prot_exec);
     }
 
-    Range range_to_mprotect = { VirtualAddress((FlatPtr)addr & PAGE_MASK), PAGE_ROUND_UP(size) };
+    if (page_round_up_would_wrap(size))
+        return -EINVAL;
+
+    Range range_to_mprotect = { VirtualAddress((FlatPtr)addr & PAGE_MASK), page_round_up(size) };
 
     if (!range_to_mprotect.size())
         return -EINVAL;
@@ -343,7 +349,10 @@ int Process::sys$madvise(void* address, size_t size, int advice)
 {
     REQUIRE_PROMISE(stdio);
 
-    Range range_to_madvise { VirtualAddress((FlatPtr)address & PAGE_MASK), PAGE_ROUND_UP(size) };
+    if (page_round_up_would_wrap(size))
+        return -EINVAL;
+
+    Range range_to_madvise { VirtualAddress((FlatPtr)address & PAGE_MASK), page_round_up(size) };
 
     if (!range_to_madvise.size())
         return -EINVAL;
@@ -415,7 +424,10 @@ int Process::sys$munmap(void* addr, size_t size)
     if (!size)
         return -EINVAL;
 
-    Range range_to_unmap { VirtualAddress(addr), PAGE_ROUND_UP(size) };
+    if (page_round_up_would_wrap(size))
+        return -EINVAL;
+
+    Range range_to_unmap { VirtualAddress(addr), page_round_up(size) };
 
     if (!is_user_range(range_to_unmap))
         return -EFAULT;

Kernel/VM/InodeVMObject.cpp

@@ -77,7 +77,7 @@ void InodeVMObject::inode_size_changed(Badge<Inode>, size_t old_size, size_t new
 
     InterruptDisabler disabler;
 
-    auto new_page_count = PAGE_ROUND_UP(new_size) / PAGE_SIZE;
+    auto new_page_count = page_round_up(new_size) / PAGE_SIZE;
     m_physical_pages.resize(new_page_count);
 
     m_dirty_pages.grow(new_page_count, false);

Kernel/VM/MemoryManager.cpp

@@ -167,7 +167,7 @@ void MemoryManager::parse_memory_map()
     // Register used memory regions that we know of.
     m_used_memory_ranges.ensure_capacity(4);
     m_used_memory_ranges.append(UsedMemoryRange { UsedMemoryRangeType::LowMemory, PhysicalAddress(0x00000000), PhysicalAddress(1 * MiB) });
-    m_used_memory_ranges.append(UsedMemoryRange { UsedMemoryRangeType::Kernel, PhysicalAddress(virtual_to_low_physical(FlatPtr(&start_of_kernel_image))), PhysicalAddress(PAGE_ROUND_UP(virtual_to_low_physical(FlatPtr(&end_of_kernel_image)))) });
+    m_used_memory_ranges.append(UsedMemoryRange { UsedMemoryRangeType::Kernel, PhysicalAddress(virtual_to_low_physical(FlatPtr(&start_of_kernel_image))), PhysicalAddress(page_round_up(virtual_to_low_physical(FlatPtr(&end_of_kernel_image)))) });
 
     if (multiboot_info_ptr->flags & 0x4) {
         auto* bootmods_start = multiboot_copy_boot_modules_array;

Kernel/VM/MemoryManager.h

@@ -39,8 +39,23 @@
 
 namespace Kernel {
 
-#define PAGE_ROUND_UP(x) ((((FlatPtr)(x)) + PAGE_SIZE - 1) & (~(PAGE_SIZE - 1)))
-#define PAGE_ROUND_DOWN(x) (((FlatPtr)(x)) & ~(PAGE_SIZE - 1))
+constexpr bool page_round_up_would_wrap(FlatPtr x)
+{
+    return x > 0xfffff000u;
+}
+
+constexpr FlatPtr page_round_up(FlatPtr x)
+{
+    FlatPtr rounded = (((FlatPtr)(x)) + PAGE_SIZE - 1) & (~(PAGE_SIZE - 1));
+    // Rounding up >0xffff0000 wraps back to 0. That's never what we want.
+    ASSERT(x == 0 || rounded != 0);
+    return rounded;
+}
+
+constexpr FlatPtr page_round_down(FlatPtr x)
+{
+    return ((FlatPtr)(x)) & ~(PAGE_SIZE - 1);
+}
 
 inline u32 low_physical_to_virtual(u32 physical)
 {

Kernel/VM/Region.cpp

@@ -154,8 +154,8 @@ bool Region::is_volatile(VirtualAddress vaddr, size_t size) const
         return false;
 
     auto offset_in_vmobject = vaddr.get() - (this->vaddr().get() - m_offset_in_vmobject);
-    size_t first_page_index = PAGE_ROUND_DOWN(offset_in_vmobject) / PAGE_SIZE;
-    size_t last_page_index = PAGE_ROUND_UP(offset_in_vmobject + size) / PAGE_SIZE;
+    size_t first_page_index = page_round_down(offset_in_vmobject) / PAGE_SIZE;
+    size_t last_page_index = page_round_up(offset_in_vmobject + size) / PAGE_SIZE;
     return is_volatile_range({ first_page_index, last_page_index - first_page_index });
 }
 
@@ -171,16 +171,16 @@ auto Region::set_volatile(VirtualAddress vaddr, size_t size, bool is_volatile, b
         // partial pages volatile to prevent potentially non-volatile
         // data to be discarded. So rund up the first page and round
         // down the last page.
-        size_t first_page_index = PAGE_ROUND_UP(offset_in_vmobject) / PAGE_SIZE;
-        size_t last_page_index = PAGE_ROUND_DOWN(offset_in_vmobject + size) / PAGE_SIZE;
+        size_t first_page_index = page_round_up(offset_in_vmobject) / PAGE_SIZE;
+        size_t last_page_index = page_round_down(offset_in_vmobject + size) / PAGE_SIZE;
         if (first_page_index != last_page_index)
             add_volatile_range({ first_page_index, last_page_index - first_page_index });
     } else {
         // If marking pages as non-volatile, round down the first page
         // and round up the last page to make sure the beginning and
         // end of the range doesn't inadvertedly get discarded.
-        size_t first_page_index = PAGE_ROUND_DOWN(offset_in_vmobject) / PAGE_SIZE;
-        size_t last_page_index = PAGE_ROUND_UP(offset_in_vmobject + size) / PAGE_SIZE;
+        size_t first_page_index = page_round_down(offset_in_vmobject) / PAGE_SIZE;
+        size_t last_page_index = page_round_up(offset_in_vmobject + size) / PAGE_SIZE;
         switch (remove_volatile_range({ first_page_index, last_page_index - first_page_index }, was_purged)) {
         case PurgeablePageRanges::RemoveVolatileError::Success:
         case PurgeablePageRanges::RemoveVolatileError::SuccessNoChange:

Kernel/VM/Space.cpp

@@ -57,7 +57,7 @@ Space::~Space()
 Optional<Range> Space::allocate_range(VirtualAddress vaddr, size_t size, size_t alignment)
 {
     vaddr.mask(PAGE_MASK);
-    size = PAGE_ROUND_UP(size);
+    size = page_round_up(size);
     if (vaddr.is_null())
         return page_directory().range_allocator().allocate_anywhere(size, alignment);
     return page_directory().range_allocator().allocate_specific(vaddr, size);
@@ -137,7 +137,7 @@ Region* Space::find_region_from_range(const Range& range)
     if (m_region_lookup_cache.range.has_value() && m_region_lookup_cache.range.value() == range && m_region_lookup_cache.region)
         return m_region_lookup_cache.region.unsafe_ptr();
 
-    size_t size = PAGE_ROUND_UP(range.size());
+    size_t size = page_round_up(range.size());
     for (auto& region : m_regions) {
         if (region.vaddr() == range.base() && region.size() == size) {
             m_region_lookup_cache.range = range;

Kernel/VM/TypedMapping.h

@@ -47,7 +47,7 @@ template<typename T>
 static TypedMapping<T> map_typed(PhysicalAddress paddr, size_t length, u8 access = Region::Access::Read)
 {
     TypedMapping<T> table;
-    table.region = MM.allocate_kernel_region(paddr.page_base(), PAGE_ROUND_UP(length), {}, access);
+    table.region = MM.allocate_kernel_region(paddr.page_base(), page_round_up(length), {}, access);
     table.offset = paddr.offset_in_page();
     return table;
 }