udev: introduce udev net_id "naming schemes"

With this we can stabilize how naming works for network interfaces. A
user can request through a kernel cmdline option or an env var which
scheme to follow. The idea is that installers use this to set into stone
(a very soft stone though) the scheme used during installation so that
interface naming doesn't change afterwards anymore.

Why use env vars and kernel cmdline options, and not a config file of
its own?

Well, first of all there's no obvious existing one to use. But more
importantly: I have the feeling that this logic is kind of an incomplete
hack, and I simply don't want to do advertise this as a perfectly
working solution. So far we used env vars for the non-so-official
options and proper config files for the official stuff. Given how
incomplete this logic is (i.e. the big variable for naming remains the
kernel, which might expose sysfs attributes in newer versions that we
check for and didn't exist in older versions — and other problems like
this), I am simply not confident in giving this first-class exposure in
a primary configuration file.

Fixes: #10448

docs/ENVIRONMENT.md

@@ -91,6 +91,15 @@ systemd-logind:
   hibernation is available even if the swap devices do not provide enough room
   for it.
 
+* `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
+  v238, v239, v240 …) as parameter. If specified udev's net_id builtin will
+  follow the specified naming scheme when determining stable network interface
+  names. This may be used to revert to naming schemes of older udev versions,
+  in order to provide more stable naming across updates. This environment
+  variable takes precedence over the kernel command line option
+  `net.naming-scheme=`, except if the value is prefixed with `:` in which case
+  the kernel command line option takes precedence, if it is specified as well.
+
 installed systemd tests:
 
 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if

man/kernel-command-line.xml

@@ -268,6 +268,7 @@
         <term><varname>udev.event_timeout=</varname></term>
         <term><varname>rd.udev.event_timeout=</varname></term>
         <term><varname>net.ifnames=</varname></term>
+        <term><varname>net.naming-scheme=</varname></term>
 
         <listitem>
           <para>Parameters understood by the device event managing

man/systemd-udevd.service.xml

@@ -170,6 +170,22 @@
           when possible. It is enabled by default; specifying 0 disables it.</para>
         </listitem>
       </varlistentry>
+      <varlistentry>
+        <term><varname>net.naming-scheme=</varname></term>
+        <listitem>
+          <para>Network interfaces are renamed to give them predictable names when possible (unless
+          <varname>net.ifnames=0</varname> is specified, see above). The names are derived from various device metadata
+          fields. Newer versions of <filename>systemd-udevd.service</filename> take more of these fields into account,
+          improving (and thus possibly changing) the names used for the same devices. With this kernel command line
+          option it is possible to pick a specific version of this algorithm. It expects a naming scheme identifier as
+          argument. Currently the following identifiers are known: <literal>v238</literal>, <literal>v239</literal>,
+          <literal>v240</literal> which each implement the naming scheme that was the default in the indicated systemd
+          version. Note that selecting a specific scheme is not sufficient to fully stabilize interface naming: the
+          naming is generally derived from driver attributes exposed by the kernel. As the kernel is updated,
+          previously missing attributes <filename>systemd-udevd.service</filename> is checking might appear, which
+          affects older name derivation algorithms, too.</para>
+        </listitem>
+      </varlistentry>
     </variablelist>
     <!-- when adding entries here, consider also adding them
          in kernel-command-line.xml -->

src/udev/udev-builtin-net_id.c

@@ -107,6 +107,7 @@
 #include "fileio.h"
 #include "fs-util.h"
 #include "parse-util.h"
+#include "proc-cmdline.h"
 #include "stdio-util.h"
 #include "string-util.h"
 #include "strv.h"
@@ -115,6 +116,48 @@
 
 #define ONBOARD_INDEX_MAX (16*1024-1)
 
+/* So here's the deal: net_id is supposed to be an excercise in providing stable names for network devices. However, we
+ * also want to keep updating the naming scheme used in future versions of net_id. These two goals of course are
+ * contradictory: on one hand we want things to not change and on the other hand we want them to improve. Our way out
+ * of this dilemma is to introduce the "naming scheme" concept: each time we improve the naming logic we define a new
+ * flag for it. Then, we keep a list of schemes, each identified by a name associated with the flags it implements. Via
+ * a kernel command line and environment variable we then allow the user to pick the scheme they want us to follow:
+ * installers could "freeze" the used scheme at the moment of installation this way.
+ *
+ * Developers: each time you tweak the naming logic here, define a new flag below, and condition the tweak with
+ * it. Each time we do a release we'll then add a new scheme entry and include all newly defined flags.
+ *
+ * Note that this is only half a solution to the problem though: not only udev/net_id gets updated all the time, the
+ * kernel gets too. And thus a kernel that previously didn't expose some sysfs attribute we look for might eventually
+ * do, and thus affect our naming scheme too. Thus, enforcing a naming scheme will make interfacing more stable across
+ * OS versions, but not fully stabilize them. */
+typedef enum NamingSchemeFlags {
+        /* First, the individual features */
+        NAMING_SR_IOV_V        = 1 << 0, /* Use "v" suffix for SR-IOV, see 609948c7043a40008b8299529c978ed8e11de8f6*/
+        NAMING_NPAR_ARI        = 1 << 1, /* Use NPAR "ARI", see 6bc04997b6eab35d1cb9fa73889892702c27be09 */
+        NAMING_INFINIBAND      = 1 << 2, /* Use "ib" prefix for infiniband, see 938d30aa98df887797c9e05074a562ddacdcdf5e */
+        NAMING_ZERO_ACPI_INDEX = 1 << 3, /* Allow zero acpi_index field, see d81186ef4f6a888a70f20a1e73a812d6acb9e22f */
+
+        /* And now the masks that combine the features above */
+        NAMING_V238 = 0,
+        NAMING_V239 = NAMING_V238|NAMING_SR_IOV_V|NAMING_NPAR_ARI,
+        NAMING_V240 = NAMING_V239|NAMING_INFINIBAND|NAMING_ZERO_ACPI_INDEX,
+
+        _NAMING_SCHEME_FLAGS_INVALID = -1,
+} NamingSchemeFlags;
+
+typedef struct NamingScheme {
+        const char *name;
+        NamingSchemeFlags flags;
+} NamingScheme;
+
+static const NamingScheme naming_schemes[] = {
+        { "v238", NAMING_V238 },
+        { "v239", NAMING_V239 },
+        { "v240", NAMING_V240 },
+        /* … add more schemes here, as the logic to name devices is updated … */
+};
+
 enum netname_type{
         NET_UNDEF,
         NET_PCI,
@@ -150,6 +193,55 @@ struct virtfn_info {
         char suffix[IFNAMSIZ];
 };
 
+static const NamingScheme* naming_scheme(void) {
+        static const NamingScheme *cache = NULL;
+        _cleanup_free_ char *buffer = NULL;
+        const char *e, *k;
+
+        if (cache)
+                return cache;
+
+        /* Acquire setting from the kernel command line */
+        (void) proc_cmdline_get_key("net.naming-scheme", 0, &buffer);
+
+        /* Also acquire it from an env var */
+        e = getenv("NET_NAMING_SCHEME");
+        if (e) {
+                if (*e == ':') {
+                        /* If prefixed with ':' the kernel cmdline takes precedence */
+                        k = buffer ?: e + 1;
+                } else
+                        k = e; /* Otherwise the env var takes precedence */
+        } else
+                k = buffer;
+
+        if (k) {
+                size_t i;
+
+                for (i = 0; i < ELEMENTSOF(naming_schemes); i++)
+                        if (streq(naming_schemes[i].name, k)) {
+                                cache = naming_schemes + i;
+                                break;
+                        }
+
+                if (!cache)
+                        log_warning("Unknown interface naming scheme '%s' requested, ignoring.", k);
+        }
+
+        if (cache)
+                log_info("Using interface naming scheme '%s'.", cache->name);
+        else {
+                cache = naming_schemes + ELEMENTSOF(naming_schemes) - 1;
+                log_info("Using default interface naming scheme '%s'.", cache->name);
+        }
+
+        return cache;
+}
+
+static bool naming_scheme_has(NamingSchemeFlags flags) {
+        return FLAGS_SET(naming_scheme()->flags, flags);
+}
+
 /* skip intermediate virtio devices */
 static sd_device *skip_virtio(sd_device *dev) {
         sd_device *parent;
@@ -255,6 +347,8 @@ static int dev_pci_onboard(sd_device *dev, struct netnames *names) {
         r = safe_atolu(attr, &idx);
         if (r < 0)
                 return r;
+        if (idx == 0 && !naming_scheme_has(NAMING_ZERO_ACPI_INDEX))
+                return -EINVAL;
 
         /* Some BIOSes report rubbish indexes that are excessively high (2^24-1 is an index VMware likes to report for
          * example). Let's define a cut-off where we don't consider the index reliable anymore. We pick some arbitrary
@@ -335,7 +429,8 @@ static int dev_pci_slot(sd_device *dev, struct netnames *names) {
         if (sscanf(sysname, "%x:%x:%x.%u", &domain, &bus, &slot, &func) != 4)
                 return -ENOENT;
 
-        if (is_pci_ari_enabled(names->pcidev))
+        if (naming_scheme_has(NAMING_NPAR_ARI) &&
+            is_pci_ari_enabled(names->pcidev))
                 /* ARI devices support up to 256 functions on a single device ("slot"), and interpret the
                  * traditional 5-bit slot and 3-bit function number as a single 8-bit function number,
                  * where the slot makes up the upper 5 bits. */
@@ -565,7 +660,8 @@ static int names_pci(sd_device *dev, struct netnames *names) {
                         return r;
         }
 
-        if (get_virtfn_info(dev, names, &vf_info) >= 0) {
+        if (naming_scheme_has(NAMING_SR_IOV_V) &&
+            get_virtfn_info(dev, names, &vf_info) >= 0) {
                 /* If this is an SR-IOV virtual device, get base name using physical device and add virtfn suffix. */
                 vf_names.pcidev = vf_info.physfn_pcidev;
                 dev_pci_onboard(dev, &vf_names);
@@ -821,7 +917,10 @@ static int builtin_net_id(sd_device *dev, int argc, char *argv[], bool test) {
                 prefix = "en";
                 break;
         case ARPHRD_INFINIBAND:
+                if (naming_scheme_has(NAMING_INFINIBAND))
                         prefix = "ib";
+                else
+                        return 0;
                 break;
         case ARPHRD_SLIP:
                 prefix = "sl";
@@ -847,6 +946,8 @@ static int builtin_net_id(sd_device *dev, int argc, char *argv[], bool test) {
                         prefix = "ww";
         }
 
+        udev_builtin_add_property(dev, test, "ID_NET_NAMING_SCHEME", naming_scheme()->name);
+
         r = names_mac(dev, &names);
         if (r >= 0 && names.mac_valid) {
                 char str[IFNAMSIZ];