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>

diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index d1b3270..193b7cf 100644 (file)
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -188,11 +188,22 @@ struct ip_mreq_source {
 };
 
 struct ip_msfilter {
-       __be32          imsf_multiaddr;
-       __be32          imsf_interface;
-       __u32           imsf_fmode;
-       __u32           imsf_numsrc;
-       __be32          imsf_slist[1];
+       union {
+               struct {
+                       __be32          imsf_multiaddr_aux;
+                       __be32          imsf_interface_aux;
+                       __u32           imsf_fmode_aux;
+                       __u32           imsf_numsrc_aux;
+                       __be32          imsf_slist[1];
+               };
+               struct {
+                       __be32          imsf_multiaddr;
+                       __be32          imsf_interface;
+                       __u32           imsf_fmode;
+                       __u32           imsf_numsrc;
+                       __be32          imsf_slist_flex[];
+               };
+       };
 };
 
 #define IP_MSFILTER_SIZE(numsrc) \

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index 03589a0..a5f4ecb 100644 (file)
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -2475,8 +2475,8 @@ int ip_mc_msfilter(struct sock *sk, struct ip_msfilter *msf, int ifindex)
                        goto done;
                }
                newpsl->sl_max = newpsl->sl_count = msf->imsf_numsrc;
-               memcpy(newpsl->sl_addr, msf->imsf_slist,
-                       msf->imsf_numsrc * sizeof(msf->imsf_slist[0]));
+               memcpy(newpsl->sl_addr, msf->imsf_slist_flex,
+                      flex_array_size(msf, imsf_slist_flex, msf->imsf_numsrc));
                err = ip_mc_add_src(in_dev, &msf->imsf_multiaddr,
                        msf->imsf_fmode, newpsl->sl_count, newpsl->sl_addr, 0);
                if (err) {
@@ -2551,14 +2551,14 @@ int ip_mc_msfget(struct sock *sk, struct ip_msfilter *msf,
                count = psl->sl_count;
        }
        copycount = count < msf->imsf_numsrc ? count : msf->imsf_numsrc;
-       len = copycount * sizeof(psl->sl_addr[0]);
+       len = flex_array_size(psl, sl_addr, copycount);
        msf->imsf_numsrc = count;
-       if (put_user(IP_MSFILTER_SIZE(copycount), optlen) ||
-           copy_to_user(optval, msf, IP_MSFILTER_SIZE(0))) {
+       if (put_user(struct_size(optval, imsf_slist_flex, copycount), optlen) ||
+           copy_to_user(optval, msf, struct_size(optval, imsf_slist_flex, 0))) {
                return -EFAULT;
        }
        if (len &&
-           copy_to_user(&optval->imsf_slist[0], psl->sl_addr, len))
+           copy_to_user(&optval->imsf_slist_flex[0], psl->sl_addr, len))
                return -EFAULT;
        return 0;
 done:

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index ec60367..bbe660b 100644 (file)
--- 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;
                }