Kernel: Add helpers to validate memory addresses

This helper returns true if the Result contains a value, or false if it doesn't.
Additionally, if it has a value, it sets the passed reference to it.

kernel/CMakeLists.txt

@@ -21,6 +21,7 @@ add_executable(moon ${SOURCES})
 if("${ARCH}" MATCHES "x86_64")
     set(ASM_SOURCES
         src/arch/x86_64/CPU.asm
+        src/arch/x86_64/Entry.asm
     )
     add_library(moon-asm STATIC ${ASM_SOURCES})
     target_link_libraries(moon moon-asm)

kernel/moon.ld

@@ -1,4 +1,4 @@
-ENTRY(_start)
+ENTRY(_main)
 OUTPUT_FORMAT(elf64-x86-64)
 
 mmio        = 0xfffffffff8000000;              /* these are configurable for level 2 loaders */

kernel/src/arch/x86_64/Entry.asm

@@ -0,0 +1,6 @@
+global _main
+extern _start
+
+_main:
+    xor rbp, rbp
+    call _start

kernel/src/memory/MemoryManager.cpp

@@ -184,6 +184,21 @@ namespace MemoryManager
         return {};
     }
 
+    bool validate_readable_page(u64 address)
+    {
+        auto rc = MMU::get_flags(address);
+        if (rc.has_error()) return false;
+        return true;
+    }
+
+    bool validate_writable_page(u64 address)
+    {
+        auto rc = MMU::get_flags(address);
+        if (rc.has_error()) return false;
+        if (rc.release_value() & MMU::ReadWrite) return true;
+        return false;
+    }
+
     u64 free()
     {
         return free_mem;

kernel/src/memory/MemoryManager.h

@@ -17,6 +17,9 @@ namespace MemoryManager
     Result<void> remap(u64 address, usize count, int flags);
     Result<void> remap_unaligned(u64 address, usize count, int flags);
 
+    bool validate_readable_page(u64 address);
+    bool validate_writable_page(u64 address);
+
     u64 free();
     u64 used();
     u64 reserved();

luna/Result.h

@@ -104,6 +104,13 @@ template <typename T> class Result
         return other;
     }
 
+    bool try_set_value(T& ref)
+    {
+        if (!has_value()) return false;
+        ref = m_storage.fetch_reference();
+        return true;
+    }
+
     T release_value()
     {
         check(has_value());