Re: [PATCH] can: Replace zero-length array with flexible-array

From: Oliver Hartkopp
Date: Tue May 12 2020 - 09:34:18 EST




On 2020-05-07 20:51, Gustavo A. R. Silva wrote:
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 <gustavoars@xxxxxxxxxx>
---
include/linux/can/skb.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/can/skb.h b/include/linux/can/skb.h
index a954def26c0d..900b9f4e0605 100644
--- a/include/linux/can/skb.h
+++ b/include/linux/can/skb.h
@@ -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 <socketcan@xxxxxxxxxxxx>

@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?

Best,
Oliver