net: wwan: t7xx: Add __counted_by for struct t7xx_fsm_event and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wwan/t7xx/t7xx_state_monitor.c

@@ -445,7 +445,8 @@ int t7xx_fsm_append_event(struct t7xx_fsm_ctl *ctl, enum t7xx_fsm_event_state ev
 		return -EINVAL;
 	}
 
-	event = kmalloc(sizeof(*event) + length, in_interrupt() ? GFP_ATOMIC : GFP_KERNEL);
+	event = kmalloc(struct_size(event, data, length),
+			in_interrupt() ? GFP_ATOMIC : GFP_KERNEL);
 	if (!event)
 		return -ENOMEM;
 

drivers/net/wwan/t7xx/t7xx_state_monitor.h

@@ -102,7 +102,7 @@ struct t7xx_fsm_event {
 	struct list_head	entry;
 	enum t7xx_fsm_event_state event_id;
 	unsigned int		length;
-	unsigned char		data[];
+	unsigned char		data[] __counted_by(length);
 };
 
 struct t7xx_fsm_command {