[PATCH] selftests/iommu: Fix array-bounds warning in get_hw_info

[Nirbhay Sharma <nirbhay.lkd@gmail.com>]

GCC warns about potential out-of-bounds access when the test provides
a buffer smaller than struct iommu_test_hw_info:

iommufd_utils.h:817:37: warning: array subscript 'struct
iommu_test_hw_info[0]' is partly outside array bounds of 'struct
iommu_test_hw_info_buffer_smaller[1]'
[-Warray-bounds=]
  817 |                         assert(!info->flags);
      |                                 ~~~~^~~~~~~

The warning occurs because 'info' is cast to a pointer to the full
8-byte struct at the top of the function, but the buffer_smaller test
case passes only a 4-byte buffer. While the code correctly checks
data_len before accessing each field, GCC's flow analysis with inlining
doesn't recognize that the size check protects the access.

Fix this by accessing fields through appropriately-typed pointers that
match the actual field sizes (__u32), declared only after the bounds
check. This makes the relationship between the size check and memory
access explicit to the compiler.

Signed-off-by: Nirbhay Sharma <nirbhay.lkd@gmail.com>
---
 tools/testing/selftests/iommu/iommufd_utils.h | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/iommu/iommufd_utils.h b/tools/testing/selftests/iommu/iommufd_utils.h
index 9f472c20c190..37c1b994008c 100644
--- a/tools/testing/selftests/iommu/iommufd_utils.h
+++ b/tools/testing/selftests/iommu/iommufd_utils.h
@@ -770,7 +770,6 @@ static int _test_cmd_get_hw_info(int fd, __u32 device_id, __u32 data_type,
 				 void *data, size_t data_len,
 				 uint32_t *capabilities, uint8_t *max_pasid)
 {
-	struct iommu_test_hw_info *info = (struct iommu_test_hw_info *)data;
 	struct iommu_hw_info cmd = {
 		.size = sizeof(cmd),
 		.dev_id = device_id,
@@ -810,11 +809,19 @@ static int _test_cmd_get_hw_info(int fd, __u32 device_id, __u32 data_type,
 		}
 	}
 
-	if (info) {
-		if (data_len >= offsetofend(struct iommu_test_hw_info, test_reg))
-			assert(info->test_reg == IOMMU_HW_INFO_SELFTEST_REGVAL);
-		if (data_len >= offsetofend(struct iommu_test_hw_info, flags))
-			assert(!info->flags);
+	if (data) {
+		if (data_len >= offsetofend(struct iommu_test_hw_info,
+					    test_reg)) {
+			__u32 *test_reg = (__u32 *)data + 1;
+
+			assert(*test_reg == IOMMU_HW_INFO_SELFTEST_REGVAL);
+		}
+		if (data_len >= offsetofend(struct iommu_test_hw_info,
+					    flags)) {
+			__u32 *flags = data;
+
+			assert(!*flags);
+		}
 	}
 
 	if (max_pasid)
-- 
2.48.1