[PATCH] drm/i915: Prefer struct_size over open coded arithmetic

[Len Baker <len.baker@gmx.com>]

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.

In this case these are not actually dynamic sizes: all the operands
involved in the calculation are constant values. However it is better to
refactor them anyway, just to keep the open-coded math idiom out of
code.

So, add at the end of the struct i915_syncmap a union with two flexible
array members (these arrays share the same memory layout). This is
possible using the new DECLARE_FLEX_ARRAY macro. And then, 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 __sync_seqno and __sync_child
making them more readable.

This code was detected with the help of Coccinelle and audited and fixed
manually.

[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>
---
 drivers/gpu/drm/i915/i915_syncmap.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_syncmap.c b/drivers/gpu/drm/i915/i915_syncmap.c
index 60404dbb2e9f..a8d35491d05a 100644
--- a/drivers/gpu/drm/i915/i915_syncmap.c
+++ b/drivers/gpu/drm/i915/i915_syncmap.c
@@ -82,6 +82,10 @@ struct i915_syncmap {
 	 *	struct i915_syncmap *child[KSYNCMAP];
 	 * };
 	 */
+	union {
+		DECLARE_FLEX_ARRAY(u32, seqno);
+		DECLARE_FLEX_ARRAY(struct i915_syncmap *, child);
+	};
 };

 /**
@@ -99,13 +103,13 @@ void i915_syncmap_init(struct i915_syncmap **root)
 static inline u32 *__sync_seqno(struct i915_syncmap *p)
 {
 	GEM_BUG_ON(p->height);
-	return (u32 *)(p + 1);
+	return p->seqno;
 }

 static inline struct i915_syncmap **__sync_child(struct i915_syncmap *p)
 {
 	GEM_BUG_ON(!p->height);
-	return (struct i915_syncmap **)(p + 1);
+	return p->child;
 }

 static inline unsigned int
@@ -200,7 +204,7 @@ __sync_alloc_leaf(struct i915_syncmap *parent, u64 id)
 {
 	struct i915_syncmap *p;

-	p = kmalloc(sizeof(*p) + KSYNCMAP * sizeof(u32), GFP_KERNEL);
+	p = kmalloc(struct_size(p, seqno, KSYNCMAP), GFP_KERNEL);
 	if (unlikely(!p))
 		return NULL;

@@ -282,7 +286,7 @@ static noinline int __sync_set(struct i915_syncmap **root, u64 id, u32 seqno)
 			unsigned int above;

 			/* Insert a join above the current layer */
-			next = kzalloc(sizeof(*next) + KSYNCMAP * sizeof(next),
+			next = kzalloc(struct_size(next, child, KSYNCMAP),
 				       GFP_KERNEL);
 			if (unlikely(!next))
 				return -ENOMEM;
--
2.25.1