Re: [PATCH v4][next] net/smc: Avoid -Wflex-array-member-not-at-end warnings

[Wen Gu <guwen@linux.alibaba.com>]

On 2024/3/29 05:52, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
> ready to enable it globally.
> 
> There are currently a couple of objects in `struct smc_clc_msg_proposal_area`
> that contain a couple of flexible structures:
> 
> struct smc_clc_msg_proposal_area {
> 	...
> 	struct smc_clc_v2_extension             pclc_v2_ext;
> 	...
> 	struct smc_clc_smcd_v2_extension        pclc_smcd_v2_ext;
> 	...
> };
> 
> So, in order to avoid ending up with a couple of flexible-array members
> in the middle of a struct, we use the `struct_group_tagged()` helper to
> separate the flexible array from the rest of the members in the flexible
> structure:
> 
> struct smc_clc_smcd_v2_extension {
>          struct_group_tagged(smc_clc_smcd_v2_extension_fixed, fixed,
>                              u8 system_eid[SMC_MAX_EID_LEN];
>                              u8 reserved[16];
>          );
>          struct smc_clc_smcd_gid_chid gidchid[];
> };
> 
> With the change described above, we now declare objects of the type of
> the tagged struct without embedding flexible arrays in the middle of
> another struct:
> 
> struct smc_clc_msg_proposal_area {
>          ...
>          struct smc_clc_v2_extension_fixed	pclc_v2_ext;
>          ...
>          struct smc_clc_smcd_v2_extension_fixed	pclc_smcd_v2_ext;
>          ...
> };
> 
> We also use `container_of()` when we need to retrieve a pointer to the
> flexible structures.
> 
> So, with these changes, fix the following warnings:
> 
> In file included from net/smc/af_smc.c:42:
> net/smc/smc_clc.h:186:49: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>    186 |         struct smc_clc_v2_extension             pclc_v2_ext;
>        |                                                 ^~~~~~~~~~~
> net/smc/smc_clc.h:188:49: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>    188 |         struct smc_clc_smcd_v2_extension        pclc_smcd_v2_ext;
>        |                                                 ^~~~~~~~~~~~~~~~
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thank you, Gustavo.

Reviewed-by: Wen Gu <guwen@linux.alibaba.com>

> ---
> Changes v4:
>   - Fix code comments (due to copy/paste error, aghhh).
> 
> Changes in v3:
>   - Adjust code to 80 columns. (Wen Gu)
>   - Add a couple of code comments.
>   - Link: https://lore.kernel.org/linux-hardening/ZgXlIADfiXSyRz8d@neat/
> 
> Changes in v2:
>   - Name the tagged struct *_fixed instead of *_hdr.
>   - Add Kees' RB tag.
>   - Link: https://lore.kernel.org/linux-hardening/ZfCXBykRw5XqBvf0@neat/
> 
> v1:
>   - Link: https://lore.kernel.org/linux-hardening/ZeIhOT44ON5rjPiP@neat/
> 
>   net/smc/smc_clc.c |  6 ++++--
>   net/smc/smc_clc.h | 26 ++++++++++++++++----------
>   2 files changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/net/smc/smc_clc.c b/net/smc/smc_clc.c
> index e55026c7529c..33fa787c28eb 100644
> --- a/net/smc/smc_clc.c
> +++ b/net/smc/smc_clc.c
> @@ -853,8 +853,10 @@ int smc_clc_send_proposal(struct smc_sock *smc, struct smc_init_info *ini)
>   	pclc_smcd = &pclc->pclc_smcd;
>   	pclc_prfx = &pclc->pclc_prfx;
>   	ipv6_prfx = pclc->pclc_prfx_ipv6;
> -	v2_ext = &pclc->pclc_v2_ext;
> -	smcd_v2_ext = &pclc->pclc_smcd_v2_ext;
> +	v2_ext = container_of(&pclc->pclc_v2_ext,
> +			      struct smc_clc_v2_extension, fixed);
> +	smcd_v2_ext = container_of(&pclc->pclc_smcd_v2_ext,
> +				   struct smc_clc_smcd_v2_extension, fixed);
>   	gidchids = pclc->pclc_gidchids;
>   	trl = &pclc->pclc_trl;
>   
> diff --git a/net/smc/smc_clc.h b/net/smc/smc_clc.h
> index 7cc7070b9772..467effb50cd6 100644
> --- a/net/smc/smc_clc.h
> +++ b/net/smc/smc_clc.h
> @@ -134,12 +134,15 @@ struct smc_clc_smcd_gid_chid {
>   			 */
>   
>   struct smc_clc_v2_extension {
> -	struct smc_clnt_opts_area_hdr hdr;
> -	u8 roce[16];		/* RoCEv2 GID */
> -	u8 max_conns;
> -	u8 max_links;
> -	__be16 feature_mask;
> -	u8 reserved[12];
> +	/* New members must be added within the struct_group() macro below. */
> +	struct_group_tagged(smc_clc_v2_extension_fixed, fixed,
> +		struct smc_clnt_opts_area_hdr hdr;
> +		u8 roce[16];		/* RoCEv2 GID */
> +		u8 max_conns;
> +		u8 max_links;
> +		__be16 feature_mask;
> +		u8 reserved[12];
> +	);
>   	u8 user_eids[][SMC_MAX_EID_LEN];
>   };
>   
> @@ -159,8 +162,11 @@ struct smc_clc_msg_smcd {	/* SMC-D GID information */
>   };
>   
>   struct smc_clc_smcd_v2_extension {
> -	u8 system_eid[SMC_MAX_EID_LEN];
> -	u8 reserved[16];
> +	/* New members must be added within the struct_group() macro below. */
> +	struct_group_tagged(smc_clc_smcd_v2_extension_fixed, fixed,
> +		u8 system_eid[SMC_MAX_EID_LEN];
> +		u8 reserved[16];
> +	);
>   	struct smc_clc_smcd_gid_chid gidchid[];
>   };
>   
> @@ -183,9 +189,9 @@ struct smc_clc_msg_proposal_area {
>   	struct smc_clc_msg_smcd			pclc_smcd;
>   	struct smc_clc_msg_proposal_prefix	pclc_prfx;
>   	struct smc_clc_ipv6_prefix	pclc_prfx_ipv6[SMC_CLC_MAX_V6_PREFIX];
> -	struct smc_clc_v2_extension		pclc_v2_ext;
> +	struct smc_clc_v2_extension_fixed	pclc_v2_ext;
>   	u8			user_eids[SMC_CLC_MAX_UEID][SMC_MAX_EID_LEN];
> -	struct smc_clc_smcd_v2_extension	pclc_smcd_v2_ext;
> +	struct smc_clc_smcd_v2_extension_fixed	pclc_smcd_v2_ext;
>   	struct smc_clc_smcd_gid_chid
>   				pclc_gidchids[SMCD_CLC_MAX_V2_GID_ENTRIES];
>   	struct smc_clc_msg_trail		pclc_trl;