Re: [PATCH] drm/i915: Add flex arrays to struct i915_syncmap

[Kees Cook]

On Thu, Feb 08, 2024 at 07:13:18PM +0100, Erick Archer wrote:
> 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@xxxxxxx>
> ---
>  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);
> +	};

This is a new code pattern for me! Trailing arrays of different element
sizes but with a fixed element count. :)

I hope when __counted_by is expanded to take expressions we can add a
literal. :)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook