linux-kernel - Re: [PATCH][next] drm/i915: Replace zero-length array with flexible-array member

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Date:   Wed, 04 Mar 2020 11:16:47 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH][next] drm/i915: Replace zero-length array with flexible-array member

On Tue, 03 Mar 2020, "Gustavo A. R. Silva" <gustavo@...eddedor.com> wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
>         int stuff;
>         struct boo array[];
> };
>
> By making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>

Reviewed-by: Jani Nikula <jani.nikula@...el.com>

> ---
>  drivers/gpu/drm/i915/display/intel_vbt_defs.h | 4 ++--
>  drivers/gpu/drm/i915/gt/intel_lrc.c           | 2 +-
>  drivers/gpu/drm/i915/i915_gpu_error.h         | 2 +-
>  3 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> index 05c7cbe32eb4..aef7fe932d1a 100644
> --- a/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> +++ b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
> @@ -462,7 +462,7 @@ struct bdb_general_definitions {
>  	 * number = (block_size - sizeof(bdb_general_definitions))/
>  	 *	     defs->child_dev_size;
>  	 */
> -	u8 devices[0];
> +	u8 devices[];
>  } __packed;
>  
>  /*
> @@ -839,7 +839,7 @@ struct bdb_mipi_config {
>  
>  struct bdb_mipi_sequence {
>  	u8 version;
> -	u8 data[0]; /* up to 6 variable length blocks */
> +	u8 data[]; /* up to 6 variable length blocks */
>  } __packed;
>  
>  /*
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index b9b3f78f1324..a49ddda649b9 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -216,7 +216,7 @@ struct virtual_engine {
>  
>  	/* And finally, which physical engines this virtual engine maps onto. */
>  	unsigned int num_siblings;
> -	struct intel_engine_cs *siblings[0];
> +	struct intel_engine_cs *siblings[];
>  };
>  
>  static struct virtual_engine *to_virtual_engine(struct intel_engine_cs *engine)
> diff --git a/drivers/gpu/drm/i915/i915_gpu_error.h b/drivers/gpu/drm/i915/i915_gpu_error.h
> index 0d1f6c8ff355..5a6561f7a210 100644
> --- a/drivers/gpu/drm/i915/i915_gpu_error.h
> +++ b/drivers/gpu/drm/i915/i915_gpu_error.h
> @@ -42,7 +42,7 @@ struct i915_vma_coredump {
>  	int num_pages;
>  	int page_count;
>  	int unused;
> -	u32 *pages[0];
> +	u32 *pages[];
>  };
>  
>  struct i915_request_coredump {

-- 
Jani Nikula, Intel Open Source Graphics Center