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

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

[Gustavo A. R. Silva]

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@embeddedor.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 {
-- 
2.25.0

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

[Jani Nikula]

On Tue, 03 Mar 2020, "Gustavo A. R. Silva" <gustavo@embeddedor.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@embeddedor.com>

Reviewed-by: Jani Nikula <jani.nikula@intel.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