options/internal: Implement get_size in the debug allocator

Also document the option to enable it in README.

README.md

@@ -32,6 +32,7 @@ The following custom meson options are accepted, in addition to the [built-in op
 - `build_tests`: Build the test suite (see below).
 - `disable_x_option`: Disable `x` component of mlibc functionality. See `meson_options.txt` for a full list of possible values for `x`. This may be used to e.g disable POSIX and glibc extensions.
 - `linux_kernel_headers`: Allows for directing mlibc to installed linux headers. [These can be obtained easily](https://docs.kernel.org/kbuild/headers_install.html), placed in a directory and this option set to the corresponding path. This is required if the linux option is enabled, i.e. when the linux option is not disabled.
+- `debug_allocator`: Replace the normal allocator with a debug allocator (see `mlibc/options/internal/generic/allocator.cpp` for implementation details).
 - `use_freestnd_hdrs`: Use freestnd-c{,xx}-hdrs instead of looking for compiler headers. Useful if not using a compiler with the correct target triple.
 
 The type of library to be built (static, shared, or both) is controlled by meson's `default_library` option. Passing `-Ddefault_library=static` effectively disables the dynamic linker.

options/internal/generic/allocator.cpp

@@ -186,6 +186,19 @@ void *MemoryAllocator::reallocate(void *ptr, size_t size) {
 	return newArea;
 }
 
+size_t MemoryAllocator::get_size(void *ptr) {
+	if constexpr (logAllocations)
+		mlibc::infoLogger() << "MemoryAllocator::get_size(" << ptr << ")" << frg::endlog;
+
+	uintptr_t page_addr = reinterpret_cast<uintptr_t>(ptr) & ~size_t{pageSize - 1};
+	AllocatorMeta *meta = reinterpret_cast<AllocatorMeta *>(page_addr - pageSize);
+
+	if (meta->magic != allocatorMagic)
+		mlibc::panicLogger() << "Invalid allocator metadata magic in MemoryAllocator::get_size" << frg::endlog;
+
+	return meta->allocatedSize;
+}
+
 MemoryAllocator &getAllocator() {
 	// use frg::eternal to prevent a call to __cxa_atexit().
 	// this is necessary because __cxa_atexit() call this function.

options/internal/include/mlibc/allocator.hpp

@@ -28,6 +28,7 @@ struct MemoryAllocator {
 	void free(void *ptr);
 	void deallocate(void *ptr, size_t size);
 	void *reallocate(void *ptr, size_t size);
+	size_t get_size(void *ptr);
 };
 
 MemoryAllocator &getAllocator();