net: Fix corruption in /proc/*/net/dev_mcast

I just hit this during my testing. Isn't there another bug lurking?

BUG kmalloc-8: Redzone overwritten

INFO: 0xc0000000de9dec48-0xc0000000de9dec4b. First byte 0x0 instead of 0xcc
INFO: Allocated in .__seq_open_private+0x30/0xa0 age=0 cpu=5 pid=3896
	.__kmalloc+0x1e0/0x2d0
	.__seq_open_private+0x30/0xa0
	.seq_open_net+0x60/0xe0
	.dev_mc_seq_open+0x4c/0x70
	.proc_reg_open+0xd8/0x260
	.__dentry_open.clone.11+0x2b8/0x400
	.do_last+0xf4/0x950
	.path_openat+0xf8/0x480
	.do_filp_open+0x48/0xc0
	.do_sys_open+0x140/0x250
	syscall_exit+0x0/0x40

dev_mc_seq_ops uses dev_seq_start/next/stop but only allocates
sizeof(struct seq_net_private) of private data, whereas it expects
sizeof(struct dev_iter_state):

struct dev_iter_state {
	struct seq_net_private p;
	unsigned int pos; /* bucket << BUCKET_SPACE + offset */
};

Create dev_seq_open_ops and use it so we don't have to expose
struct dev_iter_state.

[ Problem added by commit f04565dd (dev: use name hash for
  dev_seq_ops) -Eric ]
Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2536,6 +2536,8 @@ extern void		net_disable_timestamp(void);
 extern void *dev_seq_start(struct seq_file *seq, loff_t *pos);
 extern void *dev_seq_next(struct seq_file *seq, void *v, loff_t *pos);
 extern void dev_seq_stop(struct seq_file *seq, void *v);
+extern int dev_seq_open_ops(struct inode *inode, struct file *file,
+			    const struct seq_operations *ops);
 #endif
 
 extern int netdev_class_create_file(struct class_attribute *class_attr);

net/core/dev.c

@@ -4282,6 +4282,12 @@ static int dev_seq_open(struct inode *inode, struct file *file)
 			    sizeof(struct dev_iter_state));
 }
 
+int dev_seq_open_ops(struct inode *inode, struct file *file,
+		     const struct seq_operations *ops)
+{
+	return seq_open_net(inode, file, ops, sizeof(struct dev_iter_state));
+}
+
 static const struct file_operations dev_seq_fops = {
 	.owner	 = THIS_MODULE,
 	.open    = dev_seq_open,

net/core/dev_addr_lists.c

@@ -696,8 +696,7 @@ static const struct seq_operations dev_mc_seq_ops = {
 
 static int dev_mc_seq_open(struct inode *inode, struct file *file)
 {
-	return seq_open_net(inode, file, &dev_mc_seq_ops,
-			    sizeof(struct seq_net_private));
+	return dev_seq_open_ops(inode, file, &dev_mc_seq_ops);
 }
 
 static const struct file_operations dev_mc_seq_fops = {