[PATCH] drm/[radeon|amdgpu]: Replace one-element array and use struct_size() helper

[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 SISLANDS_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")

Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
 drivers/gpu/drm/amd/amdgpu/si_dpm.c       | 5 ++---
 drivers/gpu/drm/amd/amdgpu/sislands_smc.h | 2 +-
 drivers/gpu/drm/radeon/si_dpm.c           | 5 ++---
 3 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/si_dpm.c b/drivers/gpu/drm/amd/amdgpu/si_dpm.c
index c00ba4b23c9a6..0fc56c5bac080 100644
--- a/drivers/gpu/drm/amd/amdgpu/si_dpm.c
+++ b/drivers/gpu/drm/amd/amdgpu/si_dpm.c
@@ -5715,10 +5715,9 @@ static int si_upload_sw_state(struct amdgpu_device *adev,
 	int ret;
 	u32 address = si_pi->state_table_start +
 		offsetof(SISLANDS_SMC_STATETABLE, driverState);
-	u32 state_size = sizeof(SISLANDS_SMC_SWSTATE) +
-		((new_state->performance_level_count - 1) *
-		 sizeof(SISLANDS_SMC_HW_PERFORMANCE_LEVEL));
 	SISLANDS_SMC_SWSTATE *smc_state = &si_pi->smc_statetable.driverState;
+	size_t state_size = struct_size(smc_state, levels,
+					new_state->performance_level_count);
 
 	memset(smc_state, 0, state_size);
 
diff --git a/drivers/gpu/drm/amd/amdgpu/sislands_smc.h b/drivers/gpu/drm/amd/amdgpu/sislands_smc.h
index d2930eceaf3c8..a089dbf8f7a93 100644
--- a/drivers/gpu/drm/amd/amdgpu/sislands_smc.h
+++ b/drivers/gpu/drm/amd/amdgpu/sislands_smc.h
@@ -186,7 +186,7 @@ struct SISLANDS_SMC_SWSTATE
     uint8_t                             levelCount;
     uint8_t                             padding2;
     uint8_t                             padding3;
-    SISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[1];
+    SISLANDS_SMC_HW_PERFORMANCE_LEVEL   levels[];
 };
 
 typedef struct SISLANDS_SMC_SWSTATE SISLANDS_SMC_SWSTATE;
diff --git a/drivers/gpu/drm/radeon/si_dpm.c b/drivers/gpu/drm/radeon/si_dpm.c
index a167e1c36d243..bab01ca864c63 100644
--- a/drivers/gpu/drm/radeon/si_dpm.c
+++ b/drivers/gpu/drm/radeon/si_dpm.c
@@ -5253,10 +5253,9 @@ static int si_upload_sw_state(struct radeon_device *rdev,
 	int ret;
 	u32 address = si_pi->state_table_start +
 		offsetof(SISLANDS_SMC_STATETABLE, driverState);
-	u32 state_size = sizeof(SISLANDS_SMC_SWSTATE) +
-		((new_state->performance_level_count - 1) *
-		 sizeof(SISLANDS_SMC_HW_PERFORMANCE_LEVEL));
 	SISLANDS_SMC_SWSTATE *smc_state = &si_pi->smc_statetable.driverState;
+	size_t state_size = struct_size(smc_state, levels,
+					new_state->performance_level_count);
 
 	memset(smc_state, 0, state_size);
 
-- 
2.26.2