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

[Gustavo A. R. Silva]

-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@xxxxxxxxxxxx>
Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx>
---
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;
-- 
2.34.1