drm/amd/amdgpu: Fix GPR read from debugfs (v2)

The offset into the array was specified in bytes but should
be in terms of 32-bit words. Also prevent large reads that
would also cause a buffer overread.

v2: Read from correct offset from internal storage buffer.

Signed-off-by: Tom St Denis <[e-mail]>
Acked-by: Christian König <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/amd/amdgpu/amdgpu_debugfs.c

···
 	ssize_t result = 0;
 	uint32_t offset, se, sh, cu, wave, simd, thread, bank, *data;
 
-	if (size & 3 || *pos & 3)
+	if (size > 4096 || size & 3 || *pos & 3)
 		return -EINVAL;
 
 	/* decode offset */
-	offset = *pos & GENMASK_ULL(11, 0);
+	offset = (*pos & GENMASK_ULL(11, 0)) >> 2;
 	se = (*pos & GENMASK_ULL(19, 12)) >> 12;
 	sh = (*pos & GENMASK_ULL(27, 20)) >> 20;
 	cu = (*pos & GENMASK_ULL(35, 28)) >> 28;
···
 	while (size) {
 		uint32_t value;
 
-		value = data[offset++];
+		value = data[result >> 2];
 		r = put_user(value, (uint32_t *)buf);
 		if (r) {
 			result = r;