[PATCH][next] net: wwan: t7xx: Add __counted_by for struct t7xx_fsm_event and use struct_size()

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

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>
---
 drivers/net/wwan/t7xx/t7xx_state_monitor.c | 3 ++-
 drivers/net/wwan/t7xx/t7xx_state_monitor.h | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wwan/t7xx/t7xx_state_monitor.c b/drivers/net/wwan/t7xx/t7xx_state_monitor.c
index 80edb8e75a6a..0bc97430211b 100644
--- a/drivers/net/wwan/t7xx/t7xx_state_monitor.c
+++ b/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;
 
diff --git a/drivers/net/wwan/t7xx/t7xx_state_monitor.h b/drivers/net/wwan/t7xx/t7xx_state_monitor.h
index b6e76f3903c8..b0b3662ae6d7 100644
--- a/drivers/net/wwan/t7xx/t7xx_state_monitor.h
+++ b/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 {
-- 
2.34.1