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net: ipv6: mld: Replace zero-length array with flexible-array member

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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>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Gustavo A. R. Silva 2020-02-28 18:51:41 -06:00 committed by David S. Miller
parent e8316026d5
commit c61a2a76e5
1 changed files with 3 additions and 3 deletions
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6
include/net/mld.h
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@ -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