linux: Disable bond0 creation by bonding driver

Linux defaults to creating a `bond0` interface when modprobing the
`bonding` driver, because the `max_bonds` option defaults to `1`¹.

This `bond0` interface is created with the default bond mode
`balance-rr`, which is most often undesirable.
Networkd in turn is unable to change this mode, since it does not
implement netdev recreation on parameter mismatch².

If we instead default `max_bonds=0` the driver will be loaded, but no
default interface will be created, leaving networkd in a better position
to set up the network configuration.

[1] https://www.kernel.org/doc/Documentation/networking/bonding.txt
[2] systemd/systemd#9627

pkgs/os-specific/linux/kernel/bonding-dont-create-bond0.patch

@@ -0,0 +1,18 @@
+Prevent the bonding driver to create a bond0 interface by default when
+loaded, as that conflicts with declarative configuration management and
+instead feels much like a side-effect.
+
+
+diff --git a/include/uapi/linux/if_bonding.h b/include/uapi/linux/if_bonding.h
+index d174914a837d..bf8e2af101a3 100644
+--- a/include/uapi/linux/if_bonding.h
++++ b/include/uapi/linux/if_bonding.h
+@@ -82,7 +82,7 @@
+ #define BOND_STATE_ACTIVE       0   /* link is active */
+ #define BOND_STATE_BACKUP       1   /* link is backup */
+
+-#define BOND_DEFAULT_MAX_BONDS  1   /* Default maximum number of devices to support */
++#define BOND_DEFAULT_MAX_BONDS  0   /* Default maximum number of devices to support */
+
+ #define BOND_DEFAULT_TX_QUEUES 16   /* Default number of tx queues per device */
+

pkgs/os-specific/linux/kernel/patches.nix

@@ -14,6 +14,11 @@
     };
   };
 
+  bonding_dont_create_bond0 =
+    { name = "bonding-dont-create-bond0";
+      patch = ./bonding-dont-create-bond0.patch;
+    };
+
   bridge_stp_helper =
     { name = "bridge-stp-helper";
       patch = ./bridge-stp-helper.patch;

pkgs/top-level/linux-kernels.nix

@@ -131,6 +131,7 @@ in {
 
     linux_5_4 = callPackage ../os-specific/linux/kernel/linux-5.4.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
         kernelPatches.rtl8761b_support
@@ -139,20 +140,23 @@ in {
 
     linux_rt_5_4 = callPackage ../os-specific/linux/kernel/linux-rt-5.4.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
       ];
     };
 
     linux_5_10 = callPackage ../os-specific/linux/kernel/linux-5.10.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
       ];
     };
 
     linux_rt_5_10 = callPackage ../os-specific/linux/kernel/linux-rt-5.10.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
         kernelPatches.export-rt-sched-migrate
@@ -161,13 +165,15 @@ in {
 
     linux_5_15 = callPackage ../os-specific/linux/kernel/linux-5.15.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
       ];
     };
 
     linux_5_16 = callPackage ../os-specific/linux/kernel/linux-5.16.nix {
       kernelPatches = [
+        kernelPatches.bonding_dont_create_bond0
         kernelPatches.bridge_stp_helper
         kernelPatches.request_key_helper
       ];
@@ -176,6 +182,7 @@ in {
     linux_testing = let
       testing = callPackage ../os-specific/linux/kernel/linux-testing.nix {
         kernelPatches = [
+          kernelPatches.bonding_dont_create_bond0
           kernelPatches.bridge_stp_helper
           kernelPatches.request_key_helper
         ];