Adjust table accessing code to make it safer (always out of bounds checked)

1 files changed, 33 insertions, 14 deletions

diff --git a/arch/x86_64/src/memory/paging.cpp b/arch/x86_64/src/memory/paging.cpp
index a07b2c0..a8f2c40 100644
--- a/arch/x86_64/src/memory/paging.cpp
+++ b/arch/x86_64/src/memory/paging.cpp
@@ -40,25 +40,21 @@ namespace teachos::arch::memory
     flags = std::bitset<64U>(frame.start_address()) | flags;
   }
 
-  auto page_table::zero_entries() -> void
+  page_table::page_table()
+      : entries()
+      , p4(reinterpret_cast<page_table *>(0xfffffffffffff000))
   {
-    for (size_t i = 0; i < sizeof(entries) / sizeof(entries[0]); ++i)
-    {
-      entries[i].set_unused();
-    }
+    // Nothing to do
   }
 
-  auto page_table::next_table_address(std::size_t index) const -> std::optional<std::size_t>
+  auto page_table::zero_entries() -> void
   {
-    auto entry = entries[index];
-
-    if (entry.contains_flags(entry.PRESENT) && !entry.contains_flags(entry.HUGE_PAGE))
+    constexpr size_t entry_amount = sizeof(entries) / sizeof(entries[0]);
+    for (size_t i = 0; i < entry_amount; ++i)
     {
-      std::size_t table_address = reinterpret_cast<std::size_t>(this);
-      return (table_address << 9) | (index << 12);
+      auto entry = this->operator[](i);
+      entry.set_unused();
     }
-    // TODO: Implement behaviour for huge pages currently not done
-    return std::nullopt;
   }
 
   auto page_table::next_table(std::size_t index) const -> std::optional<page_table const *>
@@ -73,9 +69,32 @@ namespace teachos::arch::memory
     return std::nullopt;
   }
 
-  entry & page_table::operator[](std::size_t index)
+  auto page_table::operator[](std::size_t index) -> entry &
+  {
+    // C array is not bounds checked, therefore we have to check ourselves, to ensure no out of bounds reads, which
+    // could be incredibly hard to debug later.
+    arch::exception_handling::assert(index < PAGE_TABLE_ENTRY_COUNT, "[Page Table] index out of bounds");
+    return entries[index];
+  }
+
+  auto page_table::operator[](std::size_t index) const -> entry const &
   {
+    // C array is not bounds checked, therefore we have to check ourselves, to ensure no out of bounds reads, which
+    // could be incredibly hard to debug later.
     arch::exception_handling::assert(index < PAGE_TABLE_ENTRY_COUNT, "[Page Table] index out of bounds");
     return entries[index];
   }
+
+  auto page_table::next_table_address(std::size_t index) const -> std::optional<std::size_t>
+  {
+    auto entry = this->operator[](index);
+
+    if (entry.contains_flags(entry::PRESENT) && !entry.contains_flags(entry::HUGE_PAGE))
+    {
+      std::size_t const table_address = reinterpret_cast<std::size_t>(this);
+      return (table_address << 9) | (index << 12);
+    }
+    // TODO: Implement behaviour for huge pages currently not done
+    return std::nullopt;
+  }
 }  // namespace teachos::arch::memory