Kernel: Don't memset() allocated memory twice in kcalloc()

This patch adds a way to ask the allocator to skip its internal
scrubbing memset operation. Before this change, kcalloc() would scrub
twice: once internally in kmalloc() and then again in kcalloc().

The same mechanism already existed in LibC malloc, and this patch
brings it over to the kernel heap allocator as well.

This solves one FIXME in kcalloc(). :^)

Kernel/Heap/Heap.h

@@ -14,6 +14,11 @@
 
 namespace Kernel {
 
+enum class CallerWillInitializeMemory {
+    No,
+    Yes,
+};
+
 template<size_t CHUNK_SIZE, unsigned HEAP_SCRUB_BYTE_ALLOC = 0, unsigned HEAP_SCRUB_BYTE_FREE = 0>
 class Heap {
     AK_MAKE_NONCOPYABLE(Heap);
@@ -63,7 +68,7 @@ public:
         return needed_chunks * CHUNK_SIZE + (needed_chunks + 7) / 8;
     }
 
-    void* allocate(size_t size)
+    void* allocate(size_t size, CallerWillInitializeMemory caller_will_initialize_memory)
     {
         // We need space for the AllocationHeader at the head of the block.
         size_t real_size = size + sizeof(AllocationHeader);
@@ -92,8 +97,10 @@ public:
         m_bitmap.set_range_and_verify_that_all_bits_flip(first_chunk.value(), chunks_needed, true);
 
         m_allocated_chunks += chunks_needed;
-        if constexpr (HEAP_SCRUB_BYTE_ALLOC != 0) {
+        if (caller_will_initialize_memory == CallerWillInitializeMemory::No) {
-            __builtin_memset(ptr, HEAP_SCRUB_BYTE_ALLOC, (chunks_needed * CHUNK_SIZE) - sizeof(AllocationHeader));
+            if constexpr (HEAP_SCRUB_BYTE_ALLOC != 0) {
+                __builtin_memset(ptr, HEAP_SCRUB_BYTE_ALLOC, (chunks_needed * CHUNK_SIZE) - sizeof(AllocationHeader));
+            }
         }
         return ptr;
     }

Kernel/Heap/kmalloc.cpp

@@ -123,7 +123,7 @@ public:
 
     size_t slab_size() const { return m_slab_size; }
 
-    void* allocate()
+    void* allocate(CallerWillInitializeMemory caller_will_initialize_memory)
     {
         if (m_usable_blocks.is_empty()) {
             // FIXME: This allocation wastes `block_size` bytes due to the implementation of kmalloc_aligned().
@@ -141,7 +141,9 @@ public:
         if (block->is_full())
             m_full_blocks.append(*block);
 
-        memset(ptr, KMALLOC_SCRUB_BYTE, m_slab_size);
+        if (caller_will_initialize_memory == CallerWillInitializeMemory::No) {
+            memset(ptr, KMALLOC_SCRUB_BYTE, m_slab_size);
+        }
         return ptr;
     }
 
@@ -220,17 +222,17 @@ struct KmallocGlobalData {
         subheaps.append(*subheap);
     }
 
-    void* allocate(size_t size)
+    void* allocate(size_t size, CallerWillInitializeMemory caller_will_initialize_memory)
     {
         VERIFY(!expansion_in_progress);
 
         for (auto& slabheap : slabheaps) {
             if (size <= slabheap.slab_size())
-                return slabheap.allocate();
+                return slabheap.allocate(caller_will_initialize_memory);
         }
 
         for (auto& subheap : subheaps) {
-            if (auto* ptr = subheap.allocator.allocate(size))
+            if (auto* ptr = subheap.allocator.allocate(size, caller_will_initialize_memory))
                 return ptr;
         }
 
@@ -247,14 +249,14 @@ struct KmallocGlobalData {
                 }
             }
             if (did_purge)
-                return allocate(size);
+                return allocate(size, caller_will_initialize_memory);
         }
 
         if (!try_expand(size)) {
             PANIC("OOM when trying to expand kmalloc heap.");
         }
 
-        return allocate(size);
+        return allocate(size, caller_will_initialize_memory);
     }
 
     void deallocate(void* ptr, size_t size)
@@ -419,7 +421,7 @@ UNMAP_AFTER_INIT void kmalloc_init()
     s_lock.initialize();
 }
 
-void* kmalloc(size_t size)
+static void* kmalloc_impl(size_t size, CallerWillInitializeMemory caller_will_initialize_memory)
 {
     // Catch bad callers allocating under spinlock.
     if constexpr (KMALLOC_VERIFY_NO_SPINLOCK_HELD) {
@@ -434,7 +436,7 @@ void* kmalloc(size_t size)
         Kernel::dump_backtrace();
     }
 
-    void* ptr = g_kmalloc_global->allocate(size);
+    void* ptr = g_kmalloc_global->allocate(size, caller_will_initialize_memory);
 
     Thread* current_thread = Thread::current();
     if (!current_thread)
@@ -449,13 +451,17 @@ void* kmalloc(size_t size)
     return ptr;
 }
 
+void* kmalloc(size_t size)
+{
+    return kmalloc_impl(size, CallerWillInitializeMemory::No);
+}
+
 void* kcalloc(size_t count, size_t size)
 {
     if (Checked<size_t>::multiplication_would_overflow(count, size))
         return nullptr;
     size_t new_size = count * size;
-    auto* ptr = kmalloc(new_size);
+    auto* ptr = kmalloc_impl(new_size, CallerWillInitializeMemory::Yes);
-    // FIXME: Avoid redundantly scrubbing the memory in kmalloc()
     if (ptr)
         memset(ptr, 0, new_size);
     return ptr;