zfs: add option to restore kernel_neon for linux 6.2 support on aarch64

Introduced in aaeca98456
with the usual disdain for ZFS.

We have been there in the past with
<https://www.phoronix.com/news/NixOS-Linux-5.0-ZFS-FPU-Drop> /
https://github.com/NixOS/nixpkgs/pull/61076.

This fixes ZFS on aarch64 until the next breakage.

See https://github.com/openzfs/zfs/issues/14555 for original upstream
issue.

nixos/modules/tasks/filesystems/zfs.nix

@@ -323,6 +323,30 @@ in
           Defaults to 0, which waits forever.
         '';
       };
+
+      removeLinuxDRM = lib.mkOption {
+        type = types.bool;
+        default = false;
+        description = lib.mdDoc ''
+          Linux 6.2 dropped some kernel symbols required on aarch64 required by zfs.
+          Enabling this option will bring them back to allow this kernel version.
+          Note that in some jurisdictions this may be illegal as it might be considered
+          removing copyright protection from the code.
+          See https://www.ifross.org/?q=en/artikel/ongoing-dispute-over-value-exportsymbolgpl-function for further information.
+
+          If configure your kernel package with `zfs.latestCompatibleLinuxPackages`, you will need to also pass removeLinuxDRM to that package like this:
+
+          ```
+          { pkgs, ... }: {
+            boot.kernelPackages = (pkgs.zfs.override {
+              removeLinuxDRM = pkgs.hostPlatform.isAarch64;
+            }).latestCompatibleLinuxPackages;
+
+            boot.zfs.removeLinuxDRM = true;
+          }
+          ```
+        '';
+      };
     };
 
     services.zfs.autoSnapshot = {
@@ -532,11 +556,13 @@ in
         # https://github.com/NixOS/nixpkgs/issues/106093
         kernelParams = lib.optionals (!config.boot.zfs.allowHibernation) [ "nohibernate" ];
 
-        extraModulePackages = [
-          (if config.boot.zfs.enableUnstable then
+        extraModulePackages = let
+          kernelPkg = if config.boot.zfs.enableUnstable then
             config.boot.kernelPackages.zfsUnstable
            else
-            config.boot.kernelPackages.zfs)
+            config.boot.kernelPackages.zfs;
+        in [
+          (kernelPkg.override { inherit (cfgZfs) removeLinuxDRM; })
         ];
       };
 
@@ -654,6 +680,21 @@ in
       services.udev.packages = [ cfgZfs.package ]; # to hook zvol naming, etc.
       systemd.packages = [ cfgZfs.package ];
 
+      # Export kernel_neon_* symbols again.
+      # This change is necessary until ZFS figures out a solution
+      # with upstream or in their build system to fill the gap for
+      # this symbol.
+      # In the meantime, we restore what was once a working piece of code
+      # in the kernel.
+      boot.kernelPatches = lib.optional (cfgZfs.removeLinuxDRM && pkgs.stdenv.hostPlatform.system == "aarch64-linux") {
+        name = "export-neon-symbols-as-gpl";
+        patch = pkgs.fetchpatch {
+          url = "https://github.com/torvalds/linux/commit/aaeca98456431a8d9382ecf48ac4843e252c07b3.patch";
+          hash = "sha256-L2g4G1tlWPIi/QRckMuHDcdWBcKpObSWSRTvbHRIwIk=";
+          revert = true;
+        };
+      };
+
       systemd.services = let
         createImportService' = pool: createImportService {
           inherit pool;

pkgs/os-specific/linux/zfs/stable.nix

@@ -2,6 +2,7 @@
 , kernel ? null
 , stdenv
 , linuxKernel
+, removeLinuxDRM ? false
 , ...
 } @ args:
 
@@ -11,10 +12,13 @@ in
 callPackage ./generic.nix args {
   # check the release notes for compatible kernels
   kernelCompatible =
-    if stdenv'.isx86_64
+    if stdenv'.isx86_64 || removeLinuxDRM
     then kernel.kernelOlder "6.4"
     else kernel.kernelOlder "6.2";
-  latestCompatibleLinuxPackages = linuxKernel.packages.linux_6_3;
+  latestCompatibleLinuxPackages = if stdenv'.isx86_64 || removeLinuxDRM then
+    linuxKernel.packages.linux_6_3
+  else
+    linuxKernel.packages.linux_6_1;
 
   # this package should point to the latest release.
   version = "2.1.12";

pkgs/os-specific/linux/zfs/unstable.nix

@@ -2,6 +2,7 @@
 , kernel ? null
 , stdenv
 , linuxKernel
+, removeLinuxDRM ? false
 , ...
 } @ args:
 
@@ -13,19 +14,22 @@ callPackage ./generic.nix args {
   # NOTE:
   #   zfs-2.1.9<=x<=2.1.10 is broken with aarch64-linux-6.2
   #   for future releases, please delete this condition.
-  kernelCompatible = if stdenv'.isx86_64
-    then kernel.kernelOlder "6.3"
+  kernelCompatible = if stdenv'.isx86_64 || removeLinuxDRM
+    then kernel.kernelOlder "6.4"
     else kernel.kernelOlder "6.2";
-  latestCompatibleLinuxPackages = linuxKernel.packages.linux_6_1;
+
+  latestCompatibleLinuxPackages = if stdenv'.isx86_64 || removeLinuxDRM then
+    linuxKernel.packages.linux_6_3
+  else
+    linuxKernel.packages.linux_6_1;
 
   # this package should point to a version / git revision compatible with the latest kernel release
   # IMPORTANT: Always use a tagged release candidate or commits from the
   # zfs-<version>-staging branch, because this is tested by the OpenZFS
   # maintainers.
-  version = "2.1.12-staging-2023-04-18";
-  rev = "e25f9131d679692704c11dc0c1df6d4585b70c35";
+  version = "2.1.12";
 
-  sha256 = "tJLwyqUj1l5F0WKZDeMGrEFa8fc/axKqm31xtN51a5M=";
+  sha256 = "eYUR5d4gpTrlFu6j1uL83DWL9uPGgAUDRdSEb73V5i4=";
 
   isUnstable = true;
 }