* [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with flexible-array member
@ 2020-03-05 10:57 Gustavo A. R. Silva
2020-03-06 10:50 ` Daniel Vetter
0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-05 10:57 UTC (permalink / raw)
To: Eric Anholt, David Airlie, Daniel Vetter
Cc: dri-devel, linux-kernel, 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/vc4/vc4_drv.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
index f90c0d08e740..5ecb8b4a48a1 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 {
--
2.25.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with flexible-array member
2020-03-05 10:57 [PATCH][next] drm/vc4/vc4_drv.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-06 10:50 ` Daniel Vetter
0 siblings, 0 replies; 2+ messages in thread
From: Daniel Vetter @ 2020-03-06 10:50 UTC (permalink / raw)
To: Gustavo A. R. Silva
Cc: Eric Anholt, David Airlie, Daniel Vetter, dri-devel, linux-kernel
On Thu, Mar 05, 2020 at 04:57:07AM -0600, Gustavo A. R. Silva 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>
Applied to drm-misc-next, thanks for your patch.
-Daniel
> ---
> drivers/gpu/drm/vc4/vc4_drv.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_drv.h b/drivers/gpu/drm/vc4/vc4_drv.h
> index f90c0d08e740..5ecb8b4a48a1 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 {
> --
> 2.25.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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