Re: [PATCH v2] drm/radeon/dpm: Replace one-element array and use struct_size() helper

[Alex Deucher <alexdeucher@gmail.com>]

On Fri, May 22, 2020 at 1:29 PM Gustavo A. R. Silva
<gustavoars@kernel.org> wrote:
>
> 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://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Reviewed-by: Christian König <christian.koenig@amd.com>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Applied.  Thanks!

Alex

> ---
> Changes in v2:
>  - Use type size_t instead of u16 for state_size variable.
>
>  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..abb6345bfae32 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;
> +       size_t state_size = struct_size(smc_state, levels,
> +                       NISLANDS_MAX_SMC_PERFORMANCE_LEVELS_PER_SWSTATE);
>         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;
>
> --
> 2.26.2
>
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