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

[PATCH][next] drm: 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/etnaviv/etnaviv_gem.h         | 2 +-
 drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
 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 +-
 drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
 drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
 drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
 drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
 drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
 drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
 include/drm/bridge/mhl.h                      | 4 ++--
 include/drm/drm_displayid.h                   | 2 +-
 include/uapi/drm/i915_drm.h                   | 4 ++--
 14 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.h b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
index 6b68fe16041b..98e60df882b6 100644
--- a/drivers/gpu/drm/etnaviv/etnaviv_gem.h
+++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.h
@@ -105,7 +105,7 @@ struct etnaviv_gem_submit {
 	unsigned int nr_pmrs;
 	struct etnaviv_perfmon_request *pmrs;
 	unsigned int nr_bos;
-	struct etnaviv_gem_submit_bo bos[0];
+	struct etnaviv_gem_submit_bo bos[];
 	/* No new members here, the previous one is variable-length! */
 };
 
diff --git a/drivers/gpu/drm/gma500/intel_bios.h b/drivers/gpu/drm/gma500/intel_bios.h
index a1f9ce9465a5..0e6facf21e33 100644
--- a/drivers/gpu/drm/gma500/intel_bios.h
+++ b/drivers/gpu/drm/gma500/intel_bios.h
@@ -227,7 +227,7 @@ struct bdb_general_definitions {
 	 * number = (block_size - sizeof(bdb_general_definitions))/
 	 *	     sizeof(child_device_config);
 	 */
-	struct child_device_config devices[0];
+	struct child_device_config devices[];
 };
 
 struct bdb_lvds_options {
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 47561dc29304..5cec79152f17 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 {
diff --git a/drivers/gpu/drm/msm/msm_gem.h b/drivers/gpu/drm/msm/msm_gem.h
index 9e0953c2b7ce..37aa556c5f92 100644
--- a/drivers/gpu/drm/msm/msm_gem.h
+++ b/drivers/gpu/drm/msm/msm_gem.h
@@ -157,7 +157,7 @@ struct msm_gem_submit {
 			uint32_t handle;
 		};
 		uint64_t iova;
-	} bos[0];
+	} bos[];
 };
 
 #endif /* __MSM_GEM_H__ */
diff --git a/drivers/gpu/drm/qxl/qxl_cmd.c b/drivers/gpu/drm/qxl/qxl_cmd.c
index ef09dc6bc635..d1086b2a6892 100644
--- a/drivers/gpu/drm/qxl/qxl_cmd.c
+++ b/drivers/gpu/drm/qxl/qxl_cmd.c
@@ -36,7 +36,7 @@ static int qxl_reap_surface_id(struct qxl_device *qdev, int max_to_reap);
 
 struct ring {
 	struct qxl_ring_header      header;
-	uint8_t                     elements[0];
+	uint8_t                     elements[];
 };
 
 struct qxl_ring {
diff --git a/drivers/gpu/drm/vboxvideo/vboxvideo.h b/drivers/gpu/drm/vboxvideo/vboxvideo.h
index 0592004f71aa..a5de40fe1a76 100644
--- a/drivers/gpu/drm/vboxvideo/vboxvideo.h
+++ b/drivers/gpu/drm/vboxvideo/vboxvideo.h
@@ -138,7 +138,7 @@ struct vbva_buffer {
 
 	u32 data_len;
 	/* variable size for the rest of the vbva_buffer area in VRAM. */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 #define VBVA_MAX_RECORD_SIZE (128 * 1024 * 1024)
diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
index 6627b20c99e9..282293e6f751 100644
--- a/drivers/gpu/drm/vc4/vc4_drv.h
+++ b/drivers/gpu/drm/vc4/vc4_drv.h
@@ -65,7 +65,7 @@ struct vc4_perfmon {
 	 * Note that counter values can't be reset, but you can fake a reset by
 	 * destroying the perfmon and creating a new one.
 	 */
-	u64 counters[0];
+	u64 counters[];
 };
 
 struct vc4_dev {
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
index f07aa857587c..60cfbfadd3f2 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c
@@ -69,7 +69,7 @@ struct vmw_bo_dirty {
 	unsigned int ref_count;
 	unsigned long bitmap_size;
 	size_t size;
-	unsigned long bitmap[0];
+	unsigned long bitmap[];
 };
 
 /**
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 3ce630aa4fde..ec893cd17b50 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -79,7 +79,7 @@ struct vmw_surface_dirty {
 	struct svga3dsurface_cache cache;
 	size_t size;
 	u32 num_subres;
-	SVGA3dBox boxes[0];
+	SVGA3dBox boxes[];
 };
 
 static void vmw_user_surface_free(struct vmw_resource *res);
diff --git a/include/drm/bridge/mhl.h b/include/drm/bridge/mhl.h
index 1cc77bf38324..d96626a0e3fa 100644
--- a/include/drm/bridge/mhl.h
+++ b/include/drm/bridge/mhl.h
@@ -327,13 +327,13 @@ struct mhl_burst_bits_per_pixel_fmt {
 	struct {
 		u8 stream_id;
 		u8 pixel_format;
-	} __packed desc[0];
+	} __packed desc[];
 } __packed;
 
 struct mhl_burst_emsc_support {
 	struct mhl3_burst_header hdr;
 	u8 num_entries;
-	__be16 burst_id[0];
+	__be16 burst_id[];
 } __packed;
 
 struct mhl_burst_audio_descr {
diff --git a/include/drm/drm_displayid.h b/include/drm/drm_displayid.h
index 9d3b745c3107..94b4390bf990 100644
--- a/include/drm/drm_displayid.h
+++ b/include/drm/drm_displayid.h
@@ -89,7 +89,7 @@ struct displayid_detailed_timings_1 {
 
 struct displayid_detailed_timing_block {
 	struct displayid_block base;
-	struct displayid_detailed_timings_1 timings[0];
+	struct displayid_detailed_timings_1 timings[];
 };
 
 #define for_each_displayid_db(displayid, block, idx, length) \
diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
index 829c0a48577f..33de5dfadaf5 100644
--- a/include/uapi/drm/i915_drm.h
+++ b/include/uapi/drm/i915_drm.h
@@ -1708,7 +1708,7 @@ struct i915_context_engines_load_balance {
 
 	__u64 mbz64; /* reserved for future use; must be zero */
 
-	struct i915_engine_class_instance engines[0];
+	struct i915_engine_class_instance engines[];
 } __attribute__((packed));
 
 #define I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(name__, N__) struct { \
@@ -1746,7 +1746,7 @@ struct i915_context_engines_bond {
 	__u64 flags; /* all undefined flags must be zero */
 	__u64 mbz64[4]; /* reserved for future use; must be zero */
 
-	struct i915_engine_class_instance engines[0];
+	struct i915_engine_class_instance engines[];
 } __attribute__((packed));
 
 #define I915_DEFINE_CONTEXT_ENGINES_BOND(name__, N__) struct { \
-- 
2.25.0

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

[Jani Nikula]

On Tue, 25 Feb 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>
> ---
>  drivers/gpu/drm/etnaviv/etnaviv_gem.h         | 2 +-
>  drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
>  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 +-

Please split out the i915 changes to a separate patch.

>  drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
>  drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
>  drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
>  drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
>  drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
>  drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
>  include/drm/bridge/mhl.h                      | 4 ++--
>  include/drm/drm_displayid.h                   | 2 +-
>  include/uapi/drm/i915_drm.h                   | 4 ++--

Not sure it's worth touching uapi headers. They're full of both [0] and
[]. Again, please at least split it to a separate patch to be decided
separately.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center

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

[Chris Wilson]

Quoting Gustavo A. R. Silva (2020-02-25 14:03:47)
> 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:

I remember when gcc didn't support []. For the record, it appears
support for flexible arrays landed in gcc-3.0. So passes the minimum
compiler spec. That would be useful to mention for old farts with
forgetful memories.
-Chris

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

[Gustavo A. R. Silva]

On 2/25/20 08:17, Jani Nikula wrote:
> On Tue, 25 Feb 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>
>> ---
>>  drivers/gpu/drm/etnaviv/etnaviv_gem.h         | 2 +-
>>  drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
>>  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 +-
> 
> Please split out the i915 changes to a separate patch.
> 

Sure thing. I can do that.

>>  drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
>>  drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
>>  drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
>>  drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
>>  drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
>>  drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
>>  include/drm/bridge/mhl.h                      | 4 ++--
>>  include/drm/drm_displayid.h                   | 2 +-
>>  include/uapi/drm/i915_drm.h                   | 4 ++--
> 
> Not sure it's worth touching uapi headers. They're full of both [0] and
> []. Again, please at least split it to a separate patch to be decided
> separately.
> 

Yeah, it's worth it; the purpose of these patches is to replace [0] with [] across
the whole tree.

Thanks
--
Gustavo

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

[Thomas Zimmermann]

[-- Attachment #1.1: Type: text/plain, Size: 3000 bytes --]

Hi Gustavo

Am 03.03.20 um 19:20 schrieb Gustavo A. R. Silva:
> 
> 
> On 2/25/20 08:17, Jani Nikula wrote:
>> On Tue, 25 Feb 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>
>>> ---
>>>  drivers/gpu/drm/etnaviv/etnaviv_gem.h         | 2 +-
>>>  drivers/gpu/drm/gma500/intel_bios.h           | 2 +-
>>>  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 +-
>>
>> Please split out the i915 changes to a separate patch.
>>
> 
> Sure thing. I can do that.

I think each driver deserves it's own patch. Makes backporting easier.

Best regards
Thomas

> 
>>>  drivers/gpu/drm/msm/msm_gem.h                 | 2 +-
>>>  drivers/gpu/drm/qxl/qxl_cmd.c                 | 2 +-
>>>  drivers/gpu/drm/vboxvideo/vboxvideo.h         | 2 +-
>>>  drivers/gpu/drm/vc4/vc4_drv.h                 | 2 +-
>>>  drivers/gpu/drm/vmwgfx/vmwgfx_page_dirty.c    | 2 +-
>>>  drivers/gpu/drm/vmwgfx/vmwgfx_surface.c       | 2 +-
>>>  include/drm/bridge/mhl.h                      | 4 ++--
>>>  include/drm/drm_displayid.h                   | 2 +-
>>>  include/uapi/drm/i915_drm.h                   | 4 ++--
>>
>> Not sure it's worth touching uapi headers. They're full of both [0] and
>> []. Again, please at least split it to a separate patch to be decided
>> separately.
>>
> 
> Yeah, it's worth it; the purpose of these patches is to replace [0] with [] across
> the whole tree.
> 
> Thanks
> --
> Gustavo
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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