linux-kernel - [PATCH] drm/i915: Add flex arrays to struct i915

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Message-Id: <20240208181318.4259-1-erick.archer@gmx.com>
Date: Thu,  8 Feb 2024 19:13:18 +0100
From: Erick Archer <erick.archer@....com>
To: Jani Nikula <jani.nikula@...ux.intel.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Erick Archer <erick.archer@....com>,
	intel-gfx@...ts.freedesktop.org,
	dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: [PATCH] drm/i915: Add flex arrays to struct i915_syncmap

The "struct i915_syncmap" uses a dynamically sized set of trailing
elements. It can use an "u32" array or a "struct i915_syncmap *"
array.

So, use the preferred way in the kernel declaring flexible arrays [1].
Because there are two possibilities for the trailing arrays, it is
necessary to declare a union and use the DECLARE_FLEX_ARRAY macro.

The comment can be removed as the union is now clear enough.

Also, avoid the open-coded arithmetic in the memory allocator functions
[2] using the "struct_size" macro.

Moreover, refactor the "__sync_seqno" and "__sync_child" functions due
to now it is possible to use the union members added to the structure.
This way, it is also possible to avoid the open-coded arithmetic in
pointers.

Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [2]
Signed-off-by: Erick Archer <erick.archer@....com>
---
 drivers/gpu/drm/i915/i915_syncmap.c | 19 ++++++++-----------
 1 file changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_syncmap.c b/drivers/gpu/drm/i915/i915_syncmap.c
index 60404dbb2e9f..df6437c37373 100644
--- a/drivers/gpu/drm/i915/i915_syncmap.c
+++ b/drivers/gpu/drm/i915/i915_syncmap.c
@@ -75,13 +75,10 @@ struct i915_syncmap {
 	unsigned int height;
 	unsigned int bitmap;
 	struct i915_syncmap *parent;
-	/*
-	 * Following this header is an array of either seqno or child pointers:
-	 * union {
-	 *	u32 seqno[KSYNCMAP];
-	 *	struct i915_syncmap *child[KSYNCMAP];
-	 * };
-	 */
+	union {
+		DECLARE_FLEX_ARRAY(u32, seqno);
+		DECLARE_FLEX_ARRAY(struct i915_syncmap *, child);
+	};
 };

 /**
@@ -99,13 +96,13 @@ 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 +197,7 @@ __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 +279,7 @@ 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