[PATCH] drm/vc4_validate: Remove VLA usage

[PATCH] drm/vc4_validate: Remove VLA usage

[Gustavo A. R. Silva]

In preparation to enabling -Wvla, remove VLA. In this particular
case use macro ARRAY_SIZE so the length of array _bo_ can be
computed at preprocessing time.

The use of stack Variable Length Arrays needs to be avoided, as they
can be a vector for stack exhaustion, which can be both a runtime bug
or a security flaw. Also, in general, as code evolves it is easy to
lose track of how big a VLA can get. Thus, we can end up having runtime
failures that are hard to debug.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 drivers/gpu/drm/vc4/vc4_validate.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/vc4/vc4_validate.c b/drivers/gpu/drm/vc4/vc4_validate.c
index 2db485a..eec76af 100644
--- a/drivers/gpu/drm/vc4/vc4_validate.c
+++ b/drivers/gpu/drm/vc4/vc4_validate.c
@@ -753,7 +753,7 @@ validate_gl_shader_rec(struct drm_device *dev,
 		28, /* cs */
 	};
 	uint32_t shader_reloc_count = ARRAY_SIZE(shader_reloc_offsets);
-	struct drm_gem_cma_object *bo[shader_reloc_count + 8];
+	struct drm_gem_cma_object *bo[ARRAY_SIZE(shader_reloc_offsets) + 8];
 	uint32_t nr_attributes, nr_relocs, packet_size;
 	int i;
 
-- 
2.7.4

Re: [PATCH] drm/vc4_validate: Remove VLA usage

[Eric Anholt]

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"Gustavo A. R. Silva" <gustavo@embeddedor.com> writes:

> In preparation to enabling -Wvla, remove VLA. In this particular
> case use macro ARRAY_SIZE so the length of array _bo_ can be
> computed at preprocessing time.
>
> The use of stack Variable Length Arrays needs to be avoided, as they
> can be a vector for stack exhaustion, which can be both a runtime bug
> or a security flaw. Also, in general, as code evolves it is easy to
> lose track of how big a VLA can get. Thus, we can end up having runtime
> failures that are hard to debug.
>
> Also, fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/gpu/drm/vc4/vc4_validate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_validate.c b/drivers/gpu/drm/vc4/vc4_validate.c
> index 2db485a..eec76af 100644
> --- a/drivers/gpu/drm/vc4/vc4_validate.c
> +++ b/drivers/gpu/drm/vc4/vc4_validate.c
> @@ -753,7 +753,7 @@ validate_gl_shader_rec(struct drm_device *dev,
>  		28, /* cs */
>  	};
>  	uint32_t shader_reloc_count = ARRAY_SIZE(shader_reloc_offsets);
> -	struct drm_gem_cma_object *bo[shader_reloc_count + 8];
> +	struct drm_gem_cma_object *bo[ARRAY_SIZE(shader_reloc_offsets) + 8];
>  	uint32_t nr_attributes, nr_relocs, packet_size;
>  	int i;
>  
> -- 
> 2.7.4

It's a shame that the compiler is warning about this when it's obviously
a compile-time constant.  Regardless, reviewed and applied.

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