Re: [PATCH][next] w1: Avoid -Wflex-array-member-not-at-end warnings

[Gustavo A. R. Silva]

On 07/04/25 13:09, Kees Cook wrote:

On Thu, Mar 27, 2025 at 10:59:04AM -0600, Gustavo A. R. Silva wrote:

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for on-stack definitions of
a flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warnings:

drivers/w1/w1_netlink.c:198:31: warning: structure containing a flexible array 
member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/w1/w1_netlink.c:219:31: warning: structure containing a flexible array 
member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavo...@kernel.org>
---
  drivers/w1/w1_netlink.c | 41 +++++++++++++++++++----------------------
  1 file changed, 19 insertions(+), 22 deletions(-)

diff --git a/drivers/w1/w1_netlink.c b/drivers/w1/w1_netlink.c
index 691978cddab7..845d66ab7e89 100644
--- a/drivers/w1/w1_netlink.c
+++ b/drivers/w1/w1_netlink.c
@@ -194,16 +194,16 @@ static void w1_netlink_queue_status(struct w1_cb_block 
*block,
  static void w1_netlink_send_error(struct cn_msg *cn, struct w1_netlink_msg 
*msg,
        int portid, int error)
  {
-       struct {
-               struct cn_msg cn;
-               struct w1_netlink_msg msg;
-       } packet;
-       memcpy(&packet.cn, cn, sizeof(packet.cn));
-       memcpy(&packet.msg, msg, sizeof(packet.msg));
-       packet.cn.len = sizeof(packet.msg);
-       packet.msg.len = 0;
-       packet.msg.status = (u8)-error;
-       cn_netlink_send(&packet.cn, portid, 0, GFP_KERNEL);
+       DEFINE_RAW_FLEX(struct cn_msg, packet, data,
+                       sizeof(struct w1_netlink_msg));
+       struct w1_netlink_msg *pkt_msg = (struct w1_netlink_msg *)packet->data;

I'm starting to think we need a helper for "DEFINE_RAW_FLEX with a
trailing structure" for these. :)

Anyway, conversion looks good... structs are packed, so alignment issues
are unchanged.

+
+       memcpy(packet, cn, sizeof(*packet));
+       memcpy(pkt_msg, msg, sizeof(*pkt_msg));

These could just be:

        *packet = *cn;
        *pkg_msg = *msg;

But that was always true. The memcpy() style is retained. But it would
catch type mismatches (like is accidentally introduced below).

+       packet->len = sizeof(*pkt_msg);
+       pkt_msg->len = 0;
+       pkt_msg->status = (u8)-error;
+       cn_netlink_send(packet, portid, 0, GFP_KERNEL);
  }

/**

@@ -215,22 +215,19 @@ static void w1_netlink_send_error(struct cn_msg *cn, 
struct w1_netlink_msg *msg,
   */
  void w1_netlink_send(struct w1_master *dev, struct w1_netlink_msg *msg)
  {
-       struct {
-               struct cn_msg cn;
-               struct w1_netlink_msg msg;
-       } packet;
-       memset(&packet, 0, sizeof(packet));
+       DEFINE_RAW_FLEX(struct cn_msg, packet, data,
+                       sizeof(struct w1_netlink_msg));

- packet.cn.id.idx = CN_W1_IDX;

-       packet.cn.id.val = CN_W1_VAL;
+       packet->id.idx = CN_W1_IDX;
+       packet->id.val = CN_W1_VAL;

- packet.cn.seq = dev->seq++;

-       packet.cn.len = sizeof(*msg);
+       packet->seq = dev->seq++;
+       packet->len = sizeof(*msg);

- memcpy(&packet.msg, msg, sizeof(*msg));

-       packet.msg.len = 0;
+       memcpy(packet, msg, sizeof(*msg));

This memcpy() is wrong. It should be targeting packet->data.

Ah yes! Other similar instances of this (in other subsystems) use `msg`
as an alias for the flexible structure, so it seems I instinctively
thought `packet == msg` for a moment. Thanks for catching this! :)

+       ((struct w1_netlink_msg *)packet->data)->len = 0;

And since you need it again here, I'd recommend defining a struct
w1_netlink_msg pointer similar to the first hunk.

Right!

Thanks
--
Gustavo

-Kees

- cn_netlink_send(&packet.cn, 0, 0, GFP_KERNEL);

+       cn_netlink_send(packet, 0, 0, GFP_KERNEL);
  }

static void w1_send_slave(struct w1_master *dev, u64 rn)

--
2.43.0