flex_array: allow 0 length elements

flex_arrays are supposed to be a replacement for:
kmalloc(num_elements * sizeof(element))

If kmalloc is given 0 num_elements or a 0 size element it will happily return
ZERO_SIZE_PTR.  Which looks like a valid allocation, but which will explode if
something actually try to use it.  The current flex_array code will return an
equivalent result if num_elements is 0, but will fail to work if
sizeof(element) is 0.  This patch allows allocation to work even for 0 size
elements.  It will cause flex_arrays to explode though if they are used.
Imitating the kmalloc behavior.
Based-on-patch-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>

lib/flex_array.c

@@ -88,7 +88,10 @@ struct flex_array *flex_array_alloc(int element_size, unsigned int total,
 					gfp_t flags)
 {
 	struct flex_array *ret;
-	int max_size = FLEX_ARRAY_NR_BASE_PTRS *
+	int max_size = 0;
+
+	if (element_size)
+		max_size = FLEX_ARRAY_NR_BASE_PTRS *
 			   FLEX_ARRAY_ELEMENTS_PER_PART(element_size);
 
 	/* max_size will end up 0 if element_size > PAGE_SIZE */
@@ -183,15 +186,18 @@ __fa_get_part(struct flex_array *fa, int part_nr, gfp_t flags)
 int flex_array_put(struct flex_array *fa, unsigned int element_nr, void *src,
 			gfp_t flags)
 {
-	int part_nr = fa_element_to_part_nr(fa, element_nr);
+	int part_nr;
 	struct flex_array_part *part;
 	void *dst;
 
 	if (element_nr >= fa->total_nr_elements)
 		return -ENOSPC;
+	if (!fa->element_size)
+		return 0;
 	if (elements_fit_in_base(fa))
 		part = (struct flex_array_part *)&fa->parts[0];
 	else {
+		part_nr = fa_element_to_part_nr(fa, element_nr);
 		part = __fa_get_part(fa, part_nr, flags);
 		if (!part)
 			return -ENOMEM;
@@ -211,15 +217,18 @@ EXPORT_SYMBOL(flex_array_put);
  */
 int flex_array_clear(struct flex_array *fa, unsigned int element_nr)
 {
-	int part_nr = fa_element_to_part_nr(fa, element_nr);
+	int part_nr;
 	struct flex_array_part *part;
 	void *dst;
 
 	if (element_nr >= fa->total_nr_elements)
 		return -ENOSPC;
+	if (!fa->element_size)
+		return 0;
 	if (elements_fit_in_base(fa))
 		part = (struct flex_array_part *)&fa->parts[0];
 	else {
+		part_nr = fa_element_to_part_nr(fa, element_nr);
 		part = fa->parts[part_nr];
 		if (!part)
 			return -EINVAL;
@@ -264,6 +273,8 @@ int flex_array_prealloc(struct flex_array *fa, unsigned int start,
 
 	if (end >= fa->total_nr_elements)
 		return -ENOSPC;
+	if (!fa->element_size)
+		return 0;
 	if (elements_fit_in_base(fa))
 		return 0;
 	start_part = fa_element_to_part_nr(fa, start);
@@ -291,14 +302,17 @@ EXPORT_SYMBOL(flex_array_prealloc);
  */
 void *flex_array_get(struct flex_array *fa, unsigned int element_nr)
 {
-	int part_nr = fa_element_to_part_nr(fa, element_nr);
+	int part_nr;
 	struct flex_array_part *part;
 
+	if (!fa->element_size)
+		return NULL;
 	if (element_nr >= fa->total_nr_elements)
 		return NULL;
 	if (elements_fit_in_base(fa))
 		part = (struct flex_array_part *)&fa->parts[0];
 	else {
+		part_nr = fa_element_to_part_nr(fa, element_nr);
 		part = fa->parts[part_nr];
 		if (!part)
 			return NULL;
@@ -353,7 +367,7 @@ int flex_array_shrink(struct flex_array *fa)
 	int part_nr;
 	int ret = 0;
 
-	if (!fa->total_nr_elements)
+	if (!fa->total_nr_elements || !fa->element_size)
 		return 0;
 	if (elements_fit_in_base(fa))
 		return ret;