lib/malloc: Implement a simple free() only for last malloc()

Implement a free() that supports only the last malloc(). Rewind
the heap to the last allocation point if the ptr to be freed is
matching the end of heap before last malloc(). With current situation,
since free() is no-op, every call to malloc() is a memory leak.

BUG=b:140124451
TEST=Wrote a test function to do malloc and free operations.

Change-Id: I6d43cf54b79e6897cf6882335730b2310e4eae45
Signed-off-by: Bora Guvendik <bora.guvendik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37919
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/stdlib.h

@@ -5,7 +5,6 @@
 
 void *memalign(size_t boundary, size_t size);
 void *malloc(size_t size);
-/* We never free memory */
-static inline void free(void *ptr) {}
+void free(void *ptr);
 
 #endif /* STDLIB_H */

src/lib/malloc.c

@@ -11,6 +11,8 @@
 extern unsigned char _heap, _eheap;
 static void *free_mem_ptr = &_heap;		/* Start of heap */
 static void *free_mem_end_ptr = &_eheap;	/* End of heap */
+static void *free_last_alloc_ptr = &_heap;	/* End of heap before
+						   last allocation */
 
 /* We don't restrict the boundary. This is firmware,
  * you are supposed to know what you are doing.
@@ -26,6 +28,12 @@ void *memalign(size_t boundary, size_t size)
 
 	p = free_mem_ptr;
 	free_mem_ptr += size;
+	/*
+	 * Store last allocation pointer after ALIGN, as malloc() will
+	 * return it. This may cause n bytes of gap between allocations
+	 * where n < boundary.
+	 */
+	free_last_alloc_ptr = p;
 
 	if (free_mem_ptr >= free_mem_end_ptr) {
 		printk(BIOS_ERR, "memalign(boundary=%zu, size=%zu): failed: ",
@@ -46,3 +54,24 @@ void *malloc(size_t size)
 {
 	return memalign(sizeof(u64), size);
 }
+
+void free(void *ptr)
+{
+	if (ptr == NULL)
+		return;
+
+	if (ptr < (void *)&_heap || ptr >= free_mem_end_ptr) {
+		printk(BIOS_WARNING, "Warning - Pointer passed to %s is not "
+					"pointing to the heap\n", __func__);
+		return;
+	}
+
+	/*
+	 * Rewind the heap pointer to the end of heap
+	 * before the last successful malloc().
+	 */
+	if (ptr == free_last_alloc_ptr) {
+		free_mem_ptr = free_last_alloc_ptr;
+		free_last_alloc_ptr = NULL;
+	}
+}