can: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language extension to the C90 standard, but the preferred mechanism to declare variable-length types such as these ones is a flexible array member[1][2], introduced in C99:

struct foo {
         int stuff;
         struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in case the flexible array does not occur last in the structure, which will help us prevent some kind of undefined behavior bugs from being inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this change:

"Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array members have incomplete type[1]. There are some instances of code in which the sizeof operator is being incorrectly/erroneously applied to zero-length arrays and the result is zero. Such instances may be hiding some bugs. So, this work (flexible-array member conversions) will also help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
  include/linux/can/skb.h |    2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

include/linux/can/skb.h +0 -0

index a954def26c0d..900b9f4e0605 100644

Quoted 45 lines · Gustavo A. R. Silva

The current codebase makes use of the zero-length array language extension to the C90 standard, but the preferred mechanism to declare variable-length types such as these ones is a flexible array member[1][2], introduced in C99:

struct foo {
         int stuff;
         struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in case the flexible array does not occur last in the structure, which will help us prevent some kind of undefined behavior bugs from being inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this change:

"Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array members have incomplete type[1]. There are some instances of code in which the sizeof operator is being incorrectly/erroneously applied to zero-length arrays and the result is zero. Such instances may be hiding some bugs. So, this work (flexible-array member conversions) will also help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
  include/linux/can/skb.h |    2 +-

  1 file changed, 1 insertion(+), 1 deletion(-)

include/linux/can/skb.h +0 -0

index a954def26c0d..900b9f4e0605 100644

include/linux/can/skb.h line 40

Quoted 8 lines

include/linux/can/skb.h +1 -1

@@ -34,7 +34,7 @@
   struct can_skb_priv {
       int ifindex;
       int skbcnt;
-    struct can_frame cf[0];
+    struct can_frame cf[];
   };
     static inline struct can_skb_priv *can_skb_prv(struct sk_buff *skb)
 

Acked-by Oliver Hartkopp <[email protected]>

@Gustavo: Just to be sure:

From the referenced URLs I got the information that the sizeof() operator causes problems when applied to e.g. cf[0].

We don't have this case in our code - but one question remains to me:

We are using the above construct to ensure the padding between the two 'int' values and the struct can_frame which enforces a 64 bit alignment.

This intention is not affected by the patch, right?

Sorry for the late reply. I totally lost track of this thread. :/

pahole shows exactly the same output either if cf is a zero-length array or a flexible-array member:

$ pahole -C 'can_skb_priv' drivers/net/can/dev.o

struct can_skb_priv {
	int                        ifindex;              /*     0     4 */
	int                        skbcnt;               /*     4     4 */
	struct can_frame           cf[] __attribute__((__aligned__(8))); /*     8     0 */

	/* size: 8, cachelines: 1, members: 3 */
	/* forced alignments: 1 */
	/* last cacheline: 8 bytes */
} __attribute__((__aligned__(8)));

So, it seems everything should fine. :)

Great! Thanks for the info - and how I could have checked it on my own, too.