assoc_array: Avoid open coded arithmetic in allocator arguments

As noted in the "Deprecated Interfaces, Language Features, Attributes,
and Conventions" documentation [1], size calculations (especially
multiplication) should not be performed in memory allocator (or similar)
function arguments due to the risk of them overflowing. This could lead
to values wrapping around and a smaller allocation being made than the
caller was expecting. Using those allocations could lead to linear
overflows of heap memory and other misbehaviors.

So, use the struct_size() helper to do the arithmetic instead of the
argument "size + count * size" in the kmalloc() and kzalloc() functions.

Also, take the opportunity to refactor the memcpy() calls to use the
struct_size() and flex_array_size() helpers.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments

Signed-off-by: Len Baker <len.baker@gmx.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

lib/assoc_array.c

@@ -741,8 +741,7 @@ all_leaves_cluster_together:
 	keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
 	keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
 
-	new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
-			 keylen * sizeof(unsigned long), GFP_KERNEL);
+	new_s0 = kzalloc(struct_size(new_s0, index_key, keylen), GFP_KERNEL);
 	if (!new_s0)
 		return false;
 	edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s0);
@@ -849,8 +848,8 @@ static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
 		keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
 		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
 
-		new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
-				 keylen * sizeof(unsigned long), GFP_KERNEL);
+		new_s0 = kzalloc(struct_size(new_s0, index_key, keylen),
+				 GFP_KERNEL);
 		if (!new_s0)
 			return false;
 		edit->new_meta[1] = assoc_array_shortcut_to_ptr(new_s0);
@@ -864,7 +863,7 @@ static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
 		new_n0->parent_slot = 0;
 
 		memcpy(new_s0->index_key, shortcut->index_key,
-		       keylen * sizeof(unsigned long));
+		       flex_array_size(new_s0, index_key, keylen));
 
 		blank = ULONG_MAX << (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
 		pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, diff, blank);
@@ -899,8 +898,8 @@ static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
 		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
 		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
 
-		new_s1 = kzalloc(sizeof(struct assoc_array_shortcut) +
-				 keylen * sizeof(unsigned long), GFP_KERNEL);
+		new_s1 = kzalloc(struct_size(new_s1, index_key, keylen),
+				 GFP_KERNEL);
 		if (!new_s1)
 			return false;
 		edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s1);
@@ -913,7 +912,7 @@ static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
 		new_n0->slots[sc_slot] = assoc_array_shortcut_to_ptr(new_s1);
 
 		memcpy(new_s1->index_key, shortcut->index_key,
-		       keylen * sizeof(unsigned long));
+		       flex_array_size(new_s1, index_key, keylen));
 
 		edit->set[1].ptr = &side->back_pointer;
 		edit->set[1].to = assoc_array_shortcut_to_ptr(new_s1);
@@ -1490,13 +1489,12 @@ descend:
 		shortcut = assoc_array_ptr_to_shortcut(cursor);
 		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
 		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
-		new_s = kmalloc(sizeof(struct assoc_array_shortcut) +
-				keylen * sizeof(unsigned long), GFP_KERNEL);
+		new_s = kmalloc(struct_size(new_s, index_key, keylen),
+				GFP_KERNEL);
 		if (!new_s)
 			goto enomem;
 		pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
-		memcpy(new_s, shortcut, (sizeof(struct assoc_array_shortcut) +
-					 keylen * sizeof(unsigned long)));
+		memcpy(new_s, shortcut, struct_size(new_s, index_key, keylen));
 		new_s->back_pointer = new_parent;
 		new_s->parent_slot = shortcut->parent_slot;
 		*new_ptr_pp = new_parent = assoc_array_shortcut_to_ptr(new_s);