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net: mpls: Replace zero-length array with flexible-array member 

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 7649773293 ("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 07:26:15 -06:00 committed by David S. Miller
parent 95e486f551
commit 680a93166e
2 changed files with 3 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
include/net/mpls_iptunnel.h
View File

@ -11,7 +11,7 @@ struct mpls_iptunnel_encap {
u8 ttl_propagate; u8 ttl_propagate;
u8 default_ttl; u8 default_ttl;
u8 reserved1; u8 reserved1;
u32 label[0]; u32 label[];
}; };
static inline struct mpls_iptunnel_encap *mpls_lwtunnel_encap(struct lwtunnel_state *lwtstate) static inline struct mpls_iptunnel_encap *mpls_lwtunnel_encap(struct lwtunnel_state *lwtstate)

4
net/mpls/internal.h
View File

@ -98,7 +98,7 @@ struct mpls_nh { /* next hop label forwarding entry */
u8 nh_via_table; u8 nh_via_table;
u8 nh_reserved1; u8 nh_reserved1;
u32 nh_label[0]; u32 nh_label[];
}; };
/* offset of via from beginning of mpls_nh */ /* offset of via from beginning of mpls_nh */
@ -154,7 +154,7 @@ struct mpls_route { /* next hop label forwarding entry */
u8 rt_nh_size; u8 rt_nh_size;
u8 rt_via_offset; u8 rt_via_offset;
u8 rt_reserved1; u8 rt_reserved1;
struct mpls_nh rt_nh[0]; struct mpls_nh rt_nh[];
}; };
#define for_nexthops(rt) { \ #define for_nexthops(rt) { \