linux-kernel - Re: [PATCH] drm/radeon/dpm: Replace one-element array and use struct

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Message-ID: <753faa45-247c-536c-d85a-5c1385ed0049@amd.com>
Date:   Fri, 22 May 2020 09:00:09 +0200
From:   Christian König <christian.koenig@....com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH] drm/radeon/dpm: Replace one-element array and use
 struct_size() helper

Am 22.05.20 um 03:25 schrieb Gustavo A. R. Silva:
> The current codebase makes use of one-element arrays in the following
> form:
>
> struct something {
>      int length;
>      u8 data[1];
> };
>
> struct something *instance;
>
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
>
> 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. So, replace
> the one-element array with a flexible-array member.
>
> Also, make use of the new struct_size() helper to properly calculate the
> size of struct NISLANDS_SMC_SWSTATE.
>
> This issue was found with the help of Coccinelle and, audited and fixed
> _manually_.
>
> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc%2FZero-Length.html&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C7dd54e944eff4d17178008d7fdee62d7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637257072615411745&amp;sdata=fQHnbXZpsgiMjHOr%2By0Uq12jpCYEYbdX5K26iNkwyeM%3D&amp;reserved=0
> [2] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FKSPP%2Flinux%2Fissues%2F21&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C7dd54e944eff4d17178008d7fdee62d7%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637257072615411745&amp;sdata=gg20YupmqsaW%2Bg3VyJL%2BkjE3kFwnWF1RD1D2QP04OLk%3D&amp;reserved=0
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>   drivers/gpu/drm/amd/amdgpu/si_dpm.h | 2 +-
>   drivers/gpu/drm/radeon/ni_dpm.c     | 7 ++++---
>   2 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/si_dpm.h b/drivers/gpu/drm/amd/amdgpu/si_dpm.h
> index 6b7d292b919f3..bc0be6818e218 100644
> --- a/drivers/gpu/drm/amd/amdgpu/si_dpm.h
> +++ b/drivers/gpu/drm/amd/amdgpu/si_dpm.h
> @@ -781,7 +781,7 @@ struct NISLANDS_SMC_SWSTATE
>       uint8_t                             levelCount;
>       uint8_t                             padding2;
>       uint8_t                             padding3;
> -    NISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[1];
> +    NISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[];
>   };
>   
>   typedef struct NISLANDS_SMC_SWSTATE NISLANDS_SMC_SWSTATE;
> diff --git a/drivers/gpu/drm/radeon/ni_dpm.c b/drivers/gpu/drm/radeon/ni_dpm.c
> index b57c37ddd164c..7d81dde509dc9 100644
> --- a/drivers/gpu/drm/radeon/ni_dpm.c
> +++ b/drivers/gpu/drm/radeon/ni_dpm.c
> @@ -2685,11 +2685,12 @@ static int ni_upload_sw_state(struct radeon_device *rdev,
>   	struct rv7xx_power_info *pi = rv770_get_pi(rdev);
>   	u16 address = pi->state_table_start +
>   		offsetof(NISLANDS_SMC_STATETABLE, driverState);
> -	u16 state_size = sizeof(NISLANDS_SMC_SWSTATE) +
> -		((NISLANDS_MAX_SMC_PERFORMANCE_LEVELS_PER_SWSTATE - 1) * sizeof(NISLANDS_SMC_HW_PERFORMANCE_LEVEL));
> +	NISLANDS_SMC_SWSTATE *smc_state;
> +	u16 state_size = struct_size(smc_state, levels,
> +			NISLANDS_MAX_SMC_PERFORMANCE_LEVELS_PER_SWSTATE);

Probably better to use size_t instead of u16 here. With that fixed feel 
free to stick my rb on the patch.

Regards,
Christian.

>   	int ret;
> -	NISLANDS_SMC_SWSTATE *smc_state = kzalloc(state_size, GFP_KERNEL);
>   
> +	smc_state = kzalloc(state_size, GFP_KERNEL);
>   	if (smc_state == NULL)
>   		return -ENOMEM;
>