Commit 680a9316 authored Feb 28, 2020 by Gustavo A. R. Silva Committed by David S. Miller Feb 28, 2020

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 95e486f5

include/net/mpls_iptunnel.h

+1 −1

Original line number	Original line	Diff line number	Diff line
	@@ -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)

net/mpls/internal.h

+2 −2

Original line number	Original line	Diff line number	Diff line
	@@ -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) { \

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