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[next] 6lowpan: Replace zero-length array with flexible-array member

Message ID 20200228134758.GA29760@embeddedor (mailing list archive)
State New, archived
Headers show
Series [next] 6lowpan: Replace zero-length array with flexible-array member | expand

Commit Message

Gustavo A. R. Silva Feb. 28, 2020, 1:47 p.m. UTC
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]

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 <gustavo@embeddedor.com>
 include/net/6lowpan.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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diff --git a/include/net/6lowpan.h b/include/net/6lowpan.h
index a71378007e61..c80539be1542 100644
--- a/include/net/6lowpan.h
+++ b/include/net/6lowpan.h
@@ -138,7 +138,7 @@  struct lowpan_dev {
 	struct lowpan_iphc_ctx_table ctx;
 	/* must be last */
-	u8 priv[0] __aligned(sizeof(void *));
+	u8 priv[] __aligned(sizeof(void *));
 struct lowpan_802154_neigh {