autofs: use AUTOFS_DEV_IOCTL_SIZE

Use a macro which defines misc-dev ioctl parameter size (excluding a path
beyond &path[0]) since it's been used to initialize and copy this
structure ever since it first appeared in 8d7b48e0 in 2008.

(or simply get rid of this if this is just unnecessary abstraction when
all it needs is sizeof(struct autofs_dev_ioctl))

Edit: raven@themaw.net
That's a good point but I'd prefer to keep the macro define.
End edit: raven@themaw.net

Link: http://lkml.kernel.org/r/150285068577.4670.2599968823770600622.stgit@pluto.themaw.net
Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/dev-ioctl.c

@@ -97,13 +97,13 @@ static struct autofs_dev_ioctl *
 {
 	struct autofs_dev_ioctl tmp, *res;
 
-	if (copy_from_user(&tmp, in, sizeof(tmp)))
+	if (copy_from_user(&tmp, in, AUTOFS_DEV_IOCTL_SIZE))
 		return ERR_PTR(-EFAULT);
 
-	if (tmp.size < sizeof(tmp))
+	if (tmp.size < AUTOFS_DEV_IOCTL_SIZE)
 		return ERR_PTR(-EINVAL);
 
-	if (tmp.size > (PATH_MAX + sizeof(tmp)))
+	if (tmp.size > AUTOFS_DEV_IOCTL_SIZE + PATH_MAX)
 		return ERR_PTR(-ENAMETOOLONG);
 
 	res = memdup_user(in, tmp.size);
@@ -133,8 +133,8 @@ static int validate_dev_ioctl(int cmd, struct autofs_dev_ioctl *param)
 		goto out;
 	}
 
-	if (param->size > sizeof(*param)) {
-		err = invalid_str(param->path, param->size - sizeof(*param));
+	if (param->size > AUTOFS_DEV_IOCTL_SIZE) {
+		err = invalid_str(param->path, param->size - AUTOFS_DEV_IOCTL_SIZE);
 		if (err) {
 			pr_warn(
 			  "path string terminator missing for cmd(0x%08x)\n",
@@ -451,7 +451,7 @@ static int autofs_dev_ioctl_requester(struct file *fp,
 	dev_t devid;
 	int err = -ENOENT;
 
-	if (param->size <= sizeof(*param)) {
+	if (param->size <= AUTOFS_DEV_IOCTL_SIZE) {
 		err = -EINVAL;
 		goto out;
 	}
@@ -539,7 +539,7 @@ static int autofs_dev_ioctl_ismountpoint(struct file *fp,
 	unsigned int devid, magic;
 	int err = -ENOENT;
 
-	if (param->size <= sizeof(*param)) {
+	if (param->size <= AUTOFS_DEV_IOCTL_SIZE) {
 		err = -EINVAL;
 		goto out;
 	}