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>

1 files changed, 8 insertions, 7 deletions

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index ec6036713e2c..bbe660b84a91 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -663,12 +663,11 @@ static int set_mcast_msfilter(struct sock *sk, int ifindex,
 			      struct sockaddr_storage *group,
 			      struct sockaddr_storage *list)
 {
-	int msize = IP_MSFILTER_SIZE(numsrc);
 	struct ip_msfilter *msf;
 	struct sockaddr_in *psin;
 	int err, i;
 
-	msf = kmalloc(msize, GFP_KERNEL);
+	msf = kmalloc(struct_size(msf, imsf_slist_flex, numsrc), GFP_KERNEL);
 	if (!msf)
 		return -ENOBUFS;
 
@@ -684,7 +683,7 @@ static int set_mcast_msfilter(struct sock *sk, int ifindex,
 
 		if (psin->sin_family != AF_INET)
 			goto Eaddrnotavail;
-		msf->imsf_slist[i] = psin->sin_addr.s_addr;
+		msf->imsf_slist_flex[i] = psin->sin_addr.s_addr;
 	}
 	err = ip_mc_msfilter(sk, msf, ifindex);
 	kfree(msf);
@@ -1229,7 +1228,7 @@ static int do_ip_setsockopt(struct sock *sk, int level, int optname,
 	{
 		struct ip_msfilter *msf;
 
-		if (optlen < IP_MSFILTER_SIZE(0))
+		if (optlen < struct_size(msf, imsf_slist_flex, 0))
 			goto e_inval;
 		if (optlen > sysctl_optmem_max) {
 			err = -ENOBUFS;
@@ -1247,7 +1246,8 @@ static int do_ip_setsockopt(struct sock *sk, int level, int optname,
 			err = -ENOBUFS;
 			break;
 		}
-		if (IP_MSFILTER_SIZE(msf->imsf_numsrc) > optlen) {
+		if (struct_size(msf, imsf_slist_flex, msf->imsf_numsrc) >
+		    optlen) {
 			kfree(msf);
 			err = -EINVAL;
 			break;
@@ -1660,11 +1660,12 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
 	{
 		struct ip_msfilter msf;
 
-		if (len < IP_MSFILTER_SIZE(0)) {
+		if (len < struct_size(&msf, imsf_slist_flex, 0)) {
 			err = -EINVAL;
 			goto out;
 		}
-		if (copy_from_user(&msf, optval, IP_MSFILTER_SIZE(0))) {
+		if (copy_from_user(&msf, optval,
+				   struct_size(&msf, imsf_slist_flex, 0))) {
 			err = -EFAULT;
 			goto out;
 		}