net/ipv4: Replace one-element array with flexible-array member
authorGustavo A. R. Silva <gustavoars@kernel.org>
Sat, 31 Jul 2021 17:08:30 +0000 (12:08 -0500)
committerDavid S. Miller <davem@davemloft.net>
Mon, 2 Aug 2021 14:17:28 +0000 (15:17 +0100)
commit2d3e5caf96b9449af951e63476657acd759c1a30
treeb89b846db9bca30a8863daa2243ae0297e4f14ac
parent29a097b7747725da003245412dab61093d4e5976
net/ipv4: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Use an anonymous union with a couple of anonymous structs in order to
keep userspace unchanged:

$ pahole -C ip_msfilter net/ipv4/ip_sockglue.o
struct ip_msfilter {
union {
struct {
__be32     imsf_multiaddr_aux;   /*     0     4 */
__be32     imsf_interface_aux;   /*     4     4 */
__u32      imsf_fmode_aux;       /*     8     4 */
__u32      imsf_numsrc_aux;      /*    12     4 */
__be32     imsf_slist[1];        /*    16     4 */
};                                       /*     0    20 */
struct {
__be32     imsf_multiaddr;       /*     0     4 */
__be32     imsf_interface;       /*     4     4 */
__u32      imsf_fmode;           /*     8     4 */
__u32      imsf_numsrc;          /*    12     4 */
__be32     imsf_slist_flex[0];   /*    16     0 */
};                                       /*     0    16 */
};                                               /*     0    20 */

/* size: 20, cachelines: 1, members: 1 */
/* last cacheline: 20 bytes */
};

Also, refactor the code accordingly and make use of the struct_size()
and flex_array_size() helpers.

This helps with the ongoing efforts to globally enable -Warray-bounds
and get us closer to being able to tighten the FORTIFY_SOURCE routines
on memcpy().

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
include/uapi/linux/in.h
net/ipv4/igmp.c
net/ipv4/ip_sockglue.c