Merge branch 'net-deduplicate-netdev-name-allocation'

Jakub Kicinski says:

====================
net: deduplicate netdev name allocation

After recent fixes we have even more duplicated code in netdev name
allocation helpers. There are two complications in this code.
First, __dev_alloc_name() clobbers its output arg even if allocation
fails, forcing callers to do extra copies. Second as our experience in
commit 55a5ec9b ("Revert "net: core: dev_get_valid_name is now the same as dev_alloc_name_ns"") and
commit 029b6d14 ("Revert "net: core: maybe return -EEXIST in __dev_alloc_name"")
taught us, user space is very sensitive to the exact error codes.

Align the callers of __dev_alloc_name(), and remove some of its
complexity.

v1: https://lore.kernel.org/all/20231020011856.3244410-1-kuba@kernel.org/
====================

Link: https://lore.kernel.org/r/20231023152346.3639749-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -1057,7 +1057,7 @@ EXPORT_SYMBOL(dev_valid_name);
  *	__dev_alloc_name - allocate a name for a device
  *	@net: network namespace to allocate the device name in
  *	@name: name format string
- *	@buf:  scratch buffer and result name string
+ *	@res: result name string
  *
  *	Passed a format string - eg "lt%d" it will try and find a suitable
  *	id. It scans list of devices to build up a free map, then chooses
@@ -1068,25 +1068,20 @@ EXPORT_SYMBOL(dev_valid_name);
  *	Returns the number of the unit assigned or a negative errno code.
  */
 
-static int __dev_alloc_name(struct net *net, const char *name, char *buf)
+static int __dev_alloc_name(struct net *net, const char *name, char *res)
 {
 	int i = 0;
 	const char *p;
 	const int max_netdevices = 8*PAGE_SIZE;
 	unsigned long *inuse;
 	struct net_device *d;
+	char buf[IFNAMSIZ];
 
-	if (!dev_valid_name(name))
-		return -EINVAL;
-
-	p = strchr(name, '%');
-	if (p) {
-		/*
-		 * Verify the string as this thing may have come from
-		 * the user.  There must be either one "%d" and no other "%"
-		 * characters.
+	/* Verify the string as this thing may have come from the user.
+	 * There must be one "%d" and no other "%" characters.
 	 */
-		if (p[1] != 'd' || strchr(p + 2, '%'))
+	p = strchr(name, '%');
+	if (!p || p[1] != 'd' || strchr(p + 2, '%'))
 		return -EINVAL;
 
 	/* Use one page as a bit array of possible slots */
@@ -1121,53 +1116,31 @@ static int __dev_alloc_name(struct net *net, const char *name, char *buf)
 
 	i = find_first_zero_bit(inuse, max_netdevices);
 	bitmap_free(inuse);
-	}
+	if (i == max_netdevices)
+		return -ENFILE;
 
-	snprintf(buf, IFNAMSIZ, name, i);
-	if (!netdev_name_in_use(net, buf))
+	snprintf(res, IFNAMSIZ, name, i);
 	return i;
-
-	/* It is possible to run out of possible slots
-	 * when the name is long and there isn't enough space left
-	 * for the digits, or if all bits are used.
-	 */
-	return -ENFILE;
 }
 
+/* Returns negative errno or allocated unit id (see __dev_alloc_name()) */
 static int dev_prep_valid_name(struct net *net, struct net_device *dev,
-			       const char *want_name, char *out_name)
+			       const char *want_name, char *out_name,
+			       int dup_errno)
 {
-	int ret;
-
 	if (!dev_valid_name(want_name))
 		return -EINVAL;
 
-	if (strchr(want_name, '%')) {
-		ret = __dev_alloc_name(net, want_name, out_name);
-		return ret < 0 ? ret : 0;
-	} else if (netdev_name_in_use(net, want_name)) {
-		return -EEXIST;
-	} else if (out_name != want_name) {
-		strscpy(out_name, want_name, IFNAMSIZ);
-	}
+	if (strchr(want_name, '%'))
+		return __dev_alloc_name(net, want_name, out_name);
 
+	if (netdev_name_in_use(net, want_name))
+		return -dup_errno;
+	if (out_name != want_name)
+		strscpy(out_name, want_name, IFNAMSIZ);
 	return 0;
 }
 
-static int dev_alloc_name_ns(struct net *net,
-			     struct net_device *dev,
-			     const char *name)
-{
-	char buf[IFNAMSIZ];
-	int ret;
-
-	BUG_ON(!net);
-	ret = __dev_alloc_name(net, name, buf);
-	if (ret >= 0)
-		strscpy(dev->name, buf, IFNAMSIZ);
-	return ret;
-}
-
 /**
  *	dev_alloc_name - allocate a name for a device
  *	@dev: device
@@ -1184,20 +1157,17 @@ static int dev_alloc_name_ns(struct net *net,
 
 int dev_alloc_name(struct net_device *dev, const char *name)
 {
-	return dev_alloc_name_ns(dev_net(dev), dev, name);
+	return dev_prep_valid_name(dev_net(dev), dev, name, dev->name, ENFILE);
 }
 EXPORT_SYMBOL(dev_alloc_name);
 
 static int dev_get_valid_name(struct net *net, struct net_device *dev,
 			      const char *name)
 {
-	char buf[IFNAMSIZ];
 	int ret;
 
-	ret = dev_prep_valid_name(net, dev, name, buf);
-	if (ret >= 0)
-		strscpy(dev->name, buf, IFNAMSIZ);
-	return ret;
+	ret = dev_prep_valid_name(net, dev, name, dev->name, EEXIST);
+	return ret < 0 ? ret : 0;
 }
 
 /**
@@ -11135,7 +11105,7 @@ int __dev_change_net_namespace(struct net_device *dev, struct net *net,
 		/* We get here if we can't use the current device name */
 		if (!pat)
 			goto out;
-		err = dev_prep_valid_name(net, dev, pat, new_name);
+		err = dev_prep_valid_name(net, dev, pat, new_name, EEXIST);
 		if (err < 0)
 			goto out;
 	}