[PATCH][next] net: ipv6: mld: Replace zero-length array with flexible-array member

Gustavo A. R. Silva
2020-02-29T00:48:51Z

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/mld.h | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/mld.h b/include/net/mld.h
index b0f5b3105ef0..496bddb59942 100644
--- a/include/net/mld.h
+++ b/include/net/mld.h
@@ -24,12 +24,12 @@ struct mld2_grec {
__u8 grec_auxwords;
__be16 grec_nsrcs;
struct in6_addr grec_mca;
- struct in6_addr grec_src[0];
+ struct in6_addr grec_src[];
};

struct mld2_report {
struct icmp6hdr mld2r_hdr;
- struct mld2_grec mld2r_grec[0];
+ struct mld2_grec mld2r_grec[];
};

#define mld2r_type mld2r_hdr.icmp6_type
@@ -55,7 +55,7 @@ struct mld2_query {
#endif
__u8 mld2q_qqic;
__be16 mld2q_nsrcs;
- struct in6_addr mld2q_srcs[0];
+ struct in6_addr mld2q_srcs[];
};

#define mld2q_type mld2q_hdr.icmp6_type
--
2.25.0