[PATCH v2] ACPICA: Replace one-element array with flexible-array

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

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.

This issue was found with the help of Coccinelle and audited _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@kernel.org>
---
Changes in v2:
 - Don't use struct_size() for now.
 - Update subject line and changelog text.

 drivers/acpi/acpica/utids.c | 2 +-
 include/acpi/actypes.h      | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/acpica/utids.c b/drivers/acpi/acpica/utids.c
index 3bb06935a2ad3..225f3c60203c7 100644
--- a/drivers/acpi/acpica/utids.c
+++ b/drivers/acpi/acpica/utids.c
@@ -263,7 +263,7 @@ acpi_ut_execute_CID(struct acpi_namespace_node *device_node,
 	 * 3) Size of the actual CID strings
 	 */
 	cid_list_size = sizeof(struct acpi_pnp_device_id_list) +
-	    ((count - 1) * sizeof(struct acpi_pnp_device_id)) +
+	    count * sizeof(struct acpi_pnp_device_id) +
 	    string_area_size;
 
 	cid_list = ACPI_ALLOCATE_ZEROED(cid_list_size);
diff --git a/include/acpi/actypes.h b/include/acpi/actypes.h
index 4defed58ea338..c7bcda0ad366a 100644
--- a/include/acpi/actypes.h
+++ b/include/acpi/actypes.h
@@ -1145,7 +1145,7 @@ struct acpi_pnp_device_id {
 struct acpi_pnp_device_id_list {
 	u32 count;		/* Number of IDs in Ids array */
 	u32 list_size;		/* Size of list, including ID strings */
-	struct acpi_pnp_device_id ids[1];	/* ID array */
+	struct acpi_pnp_device_id ids[];	/* ID array */
 };
 
 /*
-- 
2.27.0