net: fddi: skfp: smt: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of flexible-array members in
smt_sif_operation structure, instead of one-element arrays. Also, make
use of the struct_size() helper instead of the open-coded version
to calculate the size of the struct-with-flex-array. Additionally, make
use of the typeof operator to properly determine the object type to be
passed to macro smtod().

Also, this helps the ongoing efforts to enable -Warray-bounds by fixing
the following warnings:

  CC [M]  drivers/net/fddi/skfp/smt.o
drivers/net/fddi/skfp/smt.c: In function ‘smt_send_sif_operation’:
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */
      |                   ^~~
drivers/net/fddi/skfp/smt.c:1084:30: warning: array subscript 1 is above array bounds of ‘struct smt_p_lem[1]’ [-Warray-bounds]
 1084 |    smt_fill_lem(smc,&sif->lem[i],i) ;
      |                      ~~~~~~~~^~~
In file included from drivers/net/fddi/skfp/h/smc.h:42,
                 from drivers/net/fddi/skfp/smt.c:15:
drivers/net/fddi/skfp/h/smt.h:767:19: note: while referencing ‘lem’
  767 |  struct smt_p_lem lem[1] ; /* phy lem status */

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/fddi/skfp/h/smt.h

@@ -764,10 +764,8 @@ struct smt_sif_operation {
 	struct smt_p_setcount	setcount ;	 /* Set Count mandatory */
 #endif
 	/* must be last */
-	struct smt_p_lem	lem[1] ;	/* phy lem status */
+	struct smt_p_lem	lem[];		/* phy lem status */
 } ;
-#define SIZEOF_SMT_SIF_OPERATION	(sizeof(struct smt_sif_operation)- \
-					 sizeof(struct smt_p_lem))
 
 /*
  * ECF : echo frame

drivers/net/fddi/skfp/smt.c

@@ -1063,9 +1063,9 @@ static void smt_send_sif_operation(struct s_smc *smc, struct fddi_addr *dest,
 #endif
 
 	if (!(mb = smt_build_frame(smc,SMT_SIF_OPER,SMT_REPLY,
-		SIZEOF_SMT_SIF_OPERATION+ports*sizeof(struct smt_p_lem))))
+				   struct_size(sif, lem, ports))))
 		return ;
-	sif = smtod(mb, struct smt_sif_operation *) ;
+	sif = smtod(mb, typeof(sif));
 	smt_fill_timestamp(smc,&sif->ts) ;	/* set time stamp */
 	smt_fill_mac_status(smc,&sif->status) ; /* set mac status */
 	smt_fill_mac_counter(smc,&sif->mc) ; /* set mac counter field */