fix/kernel: Fixed kfree to be more safe

kernel/GNUmakefile

@@ -57,6 +57,7 @@ IMPLICIT_SRCS :=
 ifneq ($(MAKECMDGOALS),menuconfig)
 ifeq ($(CONFIG_KERNEL_HEAP_FF),y)
     IMPLICIT_SRCS += src/mm/heap/ff.c
+    CFLAGS += -DFF_POOL_SIZE=$(CONFIG_KERNEL_HEAP_POOL_SIZE)
 else
     $(error Error: No heap algorithm was defined. Please run "make menuconfig" and select one.)
 endif

kernel/Kconfig

@@ -1,5 +1,6 @@
 mainmenu "EMK Build Configuration"
 
+menu "Build"
     choice
         prompt "Build Mode"
         default BUILD_MODE_DEBUG
@@ -15,7 +16,9 @@ config BUILD_MODE_RELEASE
           Build with optimizations and no debug symbols.
 
     endchoice
+endmenu
 
+menu "Memory"
     choice
         prompt "Kernel Heap Algorithm"
         default KERNEL_HEAP_FF
@@ -27,13 +30,25 @@ config KERNEL_HEAP_FF
 
     endchoice
 
-config ENABLE_FLANTERM
+    config KERNEL_HEAP_POOL_SIZE
-    bool "Enable Flanterm Support"
+        int "Pool Size (Pages)"
+        default 512
+        depends on KERNEL_HEAP_FF
         help
-      Includes support for the Flanterm terminal emulator. Useful for debugging real hardware.
+          Set the pool size in pages for the First-Fit heap algorithm.
+endmenu
 
+menu "Toolchain"
     config CC
         string "C Compiler"
         default "gcc"
         help
           Path to the C compiler to use for building.
+endmenu
+
+menu "Extras"
+    config ENABLE_FLANTERM
+        bool "Enable Flanterm Support"
+        help
+          Includes support for the Flanterm terminal emulator. Useful for debugging real hardware.
+endmenu

kernel/src/mm/heap/ff.c

@@ -6,6 +6,7 @@
 #include <boot/emk.h>
 #include <util/align.h>
 #include <sys/kpanic.h>
+#include <util/log.h>
 
 void *pool;
 block_t *freelist = NULL;
@@ -18,6 +19,7 @@ void heap_init()
     freelist = (block_t *)pool;
     freelist->size = (FF_POOL_SIZE * PAGE_SIZE) - sizeof(block_t);
     freelist->next = NULL;
+    log_early("Initialized heap with a pool of %d pages (~%dMB)", FF_POOL_SIZE, DIV_ROUND_UP(FF_POOL_SIZE * PAGE_SIZE, 1024 * 1024));
 }
 
 void *kmalloc(size_t size)
@@ -73,10 +75,35 @@ void kfree(void *ptr)
     if (!ptr)
         return;
 
-    /* get block header */
     block_t *block = (block_t *)((uint8_t *)ptr - sizeof(block_t));
 
-    /* insert freed block at head of freelist */
+    /* insert block sorted by address in freelist */
-    block->next = freelist;
+    block_t **cur = &freelist;
-    freelist = block;
+    while (*cur && *cur < block)
+    {
+        cur = &(*cur)->next;
+    }
+
+    block->next = *cur;
+    *cur = block;
+
+    /* coalesce with next block if adjacent */
+    if (block->next && (uint8_t *)block + sizeof(block_t) + block->size == (uint8_t *)block->next)
+    {
+        block->size += sizeof(block_t) + block->next->size;
+        block->next = block->next->next;
+    }
+
+    /* coalesce with previous block if adjacent */
+    if (cur != &freelist)
+    {
+        block_t *prev = freelist;
+        while (prev->next != block)
+            prev = prev->next;
+        if ((uint8_t *)prev + sizeof(block_t) + prev->size == (uint8_t *)block)
+        {
+            prev->size += sizeof(block_t) + block->size;
+            prev->next = block->next;
+        }
+    }
 }

kernel/src/mm/heap/ff.h

@@ -7,7 +7,9 @@
 #include <stdint.h>
 #include <stddef.h>
 
-#define FF_POOL_SIZE 512 // 2MB, in pages
+#ifndef FF_POOL_SIZE
+#define FF_POOL_SIZE 512
+#endif // FF_POOL_SIZE
 
 typedef struct block
 {