linux-kernel - Re: [PATCH v2][next] Bluetooth: Avoid a couple dozen -Wflex-array-member-not-at-end warnings

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Message-ID: <bbf020fc-4567-4c12-8400-5077ae2a7718@molgen.mpg.de>
Date: Wed, 10 Sep 2025 13:11:52 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: Marcel Holtmann <marcel@...tmann.org>,
 Johan Hedberg <johan.hedberg@...il.com>,
 Luiz Augusto von Dentz <luiz.dentz@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, linux-bluetooth@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2][next] Bluetooth: Avoid a couple dozen
 -Wflex-array-member-not-at-end warnings

Dear Gustavo,


Thank you for your patch.

Am 09.09.25 um 14:13 schrieb Gustavo A. R. Silva:
> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> getting ready to enable it, globally.
> 
> Use the __struct_group() helper to fix 31 instances of the following
> type of warnings:
> 
> 30 net/bluetooth/mgmt_config.c:16:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> 1 net/bluetooth/mgmt_config.c:22:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

You could add an explanation, why the macro `__struct_group()` defined 
in `include/uapi/linux/stddef.h` fixes this, and why it is preferred 
over `TRAILING_OVERLAP()`. Also, the two underscores would suggest to 
me, it’s some kind of internal implementation.

> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
> Changes in v2:
>   - Use __struct_group() instead of TRAILING_OVERLAP().
> 
> v1:
>   - Link: https://lore.kernel.org/linux-hardening/aLSCu8U62Hve7Dau@kspp/
> 
>   include/net/bluetooth/mgmt.h | 9 +++++++--
>   net/bluetooth/mgmt_config.c  | 4 ++--
>   2 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/bluetooth/mgmt.h b/include/net/bluetooth/mgmt.h
> index 3575cd16049a..74edea06985b 100644
> --- a/include/net/bluetooth/mgmt.h
> +++ b/include/net/bluetooth/mgmt.h
> @@ -53,10 +53,15 @@ struct mgmt_hdr {
>   } __packed;
>   
>   struct mgmt_tlv {
> -	__le16 type;
> -	__u8   length;
> +	/* New members MUST be added within the __struct_group() macro below. */
> +	__struct_group(mgmt_tlv_hdr, __hdr, __packed,
> +		__le16 type;
> +		__u8   length;
> +	);
>   	__u8   value[];
>   } __packed;
> +static_assert(offsetof(struct mgmt_tlv, value) == sizeof(struct mgmt_tlv_hdr),
> +	      "struct member likely outside of __struct_group()");
>   
>   struct mgmt_addr_info {
>   	bdaddr_t	bdaddr;
> diff --git a/net/bluetooth/mgmt_config.c b/net/bluetooth/mgmt_config.c
> index 6ef701c27da4..c4063d200c0a 100644
> --- a/net/bluetooth/mgmt_config.c
> +++ b/net/bluetooth/mgmt_config.c
> @@ -13,13 +13,13 @@
>   
>   #define HDEV_PARAM_U16(_param_name_) \
>   	struct {\
> -		struct mgmt_tlv entry; \
> +		struct mgmt_tlv_hdr entry; \
>   		__le16 value; \
>   	} __packed _param_name_
>   
>   #define HDEV_PARAM_U8(_param_name_) \
>   	struct {\
> -		struct mgmt_tlv entry; \
> +		struct mgmt_tlv_hdr entry; \
>   		__u8 value; \
>   	} __packed _param_name_
>   


Kind regards,

Paul