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

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Message-ID: <1d87b6b5-04cc-48a3-ac5d-3b176814a0e6@embeddedor.com>
Date: Mon, 7 Apr 2025 13:19:32 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] w1: Avoid -Wflex-array-member-not-at-end warnings



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 <gustavoars@...nel.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
>>
>