udev: link_setup - respect kernel name assign policy

Newer kernels export meta-information about the origin of an ifname. Respect this
from the ifname rename logic. We do not rename any interfaces that was originally
named by userspace, nor once which have already been renamed from userspace.

Moreover, we optionally do not (the default) rename interfaces which the kernel
claims to have named in a predictable way.

man/systemd.link.xml

@@ -236,10 +236,20 @@
                                         not set directly, but is exported to udev as
                                         the property <literal>ID_NET_NAME</literal>,
                                         which is, by default, used by a udev rule to set
-                                        <literal>NAME</literal>. The available policies
-                                        are:</para>
+                                        <literal>NAME</literal>. If the name has already
+                                        been set by userspace, no renaming is performed.
+                                        The available policies are:</para>
 
                                         <variablelist>
+                                                <varlistentry>
+                                                        <term><literal>kernel</literal></term>
+                                                        <listitem>
+                                                                <para>If the kernel claims that the name it
+                                                                has set for a device is predictable, then
+                                                                no renaming is performed.
+                                                                </para>
+                                                        </listitem>
+                                                </varlistentry>
                                                 <varlistentry>
                                                         <term><literal>database</literal></term>
                                                         <listitem>

network/99-default.link

@@ -1,3 +1,3 @@
 [Link]
-NamePolicy=database onboard slot path
+NamePolicy=kernel database onboard slot path
 MACAddressPolicy=persistent

src/shared/missing.h

@@ -525,3 +525,23 @@ static inline int setns(int fd, int nstype) {
 #ifndef BOND_XMIT_POLICY_ENCAP34
 #define BOND_XMIT_POLICY_ENCAP34 4
 #endif
+
+#ifndef NET_ADDR_RANDOM
+#  define NET_ADDR_RANDOM 1
+#endif
+
+#ifndef NET_NAME_ENUM
+#  define NET_NAME_ENUM 1
+#endif
+
+#ifndef NET_NAME_PREDICTABLE
+#  define NET_NAME_PREDICTABLE 2
+#endif
+
+#ifndef NET_NAME_USER
+#  define NET_NAME_USER 3
+#endif
+
+#ifndef NET_NAME_RENAMED
+#  define NET_NAME_RENAMED 4
+#endif

src/udev/net/link-config.c

@@ -20,10 +20,11 @@
 ***/
 
 #include <netinet/ether.h>
-#include <net/if.h>
+#include <linux/netdevice.h>
 
 #include "sd-id128.h"
 
+#include "missing.h"
 #include "link-config.h"
 #include "ethtool-util.h"
 
@@ -283,8 +284,33 @@ static bool mac_is_random(struct udev_device *device) {
         if (r < 0)
                 return false;
 
-        /* check for NET_ADDR_RANDOM */
-        return type == 1;
+        return type == NET_ADDR_RANDOM;
+}
+
+static bool should_rename(struct udev_device *device, bool respect_predictable) {
+        const char *s;
+        unsigned type;
+        int r;
+
+        s = udev_device_get_sysattr_value(device, "name_assign_type");
+        if (!s)
+                return true; /* if we don't know, assume we should rename */
+        r = safe_atou(s, &type);
+        if (r < 0)
+                return true;
+
+        switch (type) {
+        case NET_NAME_USER:
+        case NET_NAME_RENAMED:
+                return false; /* these were already named by userspace, do not touch again */
+        case NET_NAME_PREDICTABLE:
+                if (respect_predictable)
+                        return false; /* the kernel claims to have given a predictable name */
+                /* fall through */
+        case NET_NAME_ENUM:
+        default:
+                return true; /* the name is known to be bad, or of an unknown type */
+        }
 }
 
 static int get_mac(struct udev_device *device, bool want_random, struct ether_addr *mac) {
@@ -315,6 +341,7 @@ int link_config_apply(link_config_ctx *ctx, link_config *config, struct udev_dev
         const char *new_name = NULL;
         struct ether_addr generated_mac;
         struct ether_addr *mac = NULL;
+        bool respect_predictable = false;
         int r, ifindex;
 
         assert(ctx);
@@ -350,8 +377,12 @@ int link_config_apply(link_config_ctx *ctx, link_config *config, struct udev_dev
         if (ctx->enable_name_policy && config->name_policy) {
                 NamePolicy *policy;
 
-                for (policy = config->name_policy; !new_name && *policy != _NAMEPOLICY_INVALID; policy++) {
+                for (policy = config->name_policy; !respect_predictable && !new_name &&
+                                                   *policy != _NAMEPOLICY_INVALID; policy++) {
                         switch (*policy) {
+                                case NAMEPOLICY_KERNEL:
+                                        respect_predictable = true;
+                                        break;
                                 case NAMEPOLICY_DATABASE:
                                         new_name = udev_device_get_property_value(device, "ID_NET_NAME_FROM_DATABASE");
                                         break;
@@ -373,12 +404,14 @@ int link_config_apply(link_config_ctx *ctx, link_config *config, struct udev_dev
                 }
         }
 
-        if (new_name)
-                *name = new_name; /* a name was set by a policy */
-        else if (config->name)
-                *name = config->name; /* a name was set manually in the config */
-        else
-                *name = NULL;
+        if (should_rename(device, respect_predictable)) {
+                if (!new_name)
+                        /* if not set by policy, fall back manually set name */
+                        new_name = config->name;
+        } else
+                new_name = NULL;
+
+        *name = new_name;
 
         switch (config->mac_policy) {
                 case MACPOLICY_PERSISTENT:
@@ -444,6 +477,7 @@ DEFINE_STRING_TABLE_LOOKUP(mac_policy, MACPolicy);
 DEFINE_CONFIG_PARSE_ENUM(config_parse_mac_policy, mac_policy, MACPolicy, "Failed to parse MAC address policy");
 
 static const char* const name_policy_table[_NAMEPOLICY_MAX] = {
+        [NAMEPOLICY_KERNEL] = "kernel",
         [NAMEPOLICY_DATABASE] = "database",
         [NAMEPOLICY_ONBOARD] = "onboard",
         [NAMEPOLICY_SLOT] = "slot",

src/udev/net/link-config.h

@@ -39,6 +39,7 @@ typedef enum MACPolicy {
 } MACPolicy;
 
 typedef enum NamePolicy {
+        NAMEPOLICY_KERNEL,
         NAMEPOLICY_DATABASE,
         NAMEPOLICY_ONBOARD,
         NAMEPOLICY_SLOT,