nixos: Add derivations for SD card installation images on ARM

The resulting image can be copied to a SD card with `dd` and is directly
bootable by a suitably configured U-Boot. Though depending on the board, some
extra steps are required for copying U-Boot itself to the SD card.

Inside the image is a partition table, with a FAT32 /boot and a normal
writable EXT4 rootfs. It's possible to directly reuse the SD image's
partition layout and "install" NixOS on the same SD card by replacing
the default configuration.nix and nixos-rebuild, and actually is the
preferred way to use these images. To assist in this installation
method, the boot scripts on the image automatically resize the rootfs
partition to fit the SD card on the first boot.

The SD images come in two flavors; one for the ARMv6 Raspberry Pi,
and one multiplatform image for all the boards supported by the
mainline kernel's multi_v7_defconfig config target. At the moment, these
have been tested on:
    - Raspberry Pi Model B (512MB model)
    - NVIDIA Jetson TK1
    - Linksprite pcDuino3 Nano

To build, run:

nix-build '<nixpkgs/nixos>' -A config.system.build.sdImage \
    -I nixos-config='<nixpkgs/nixos/modules/installer/cd-dvd/sd-image-armv7l-multiplatform.nix>'

1 files changed, 88 insertions, 0 deletions

diff --git a/nixos/lib/make-ext4-fs.nix b/nixos/lib/make-ext4-fs.nix
new file mode 100644
index 00000000000..23839ea487d
--- /dev/null
+++ b/nixos/lib/make-ext4-fs.nix
@@ -0,0 +1,88 @@
+# Builds an ext4 image containing a populated /nix/store with the closure
+# of store paths passed in the storePaths parameter. The generated image
+# is sized to only fit its contents, with the expectation that a script
+# resizes the filesystem at boot time.
+{ pkgs
+, storePaths
+, volumeLabel
+}:
+
+pkgs.stdenv.mkDerivation {
+  name = "ext4-fs.img";
+
+  buildInputs = with pkgs; [e2fsprogs libfaketime perl];
+
+  # For obtaining the closure of `storePaths'.
+  exportReferencesGraph =
+    map (x: [("closure-" + baseNameOf x) x]) storePaths;
+
+  buildCommand =
+    ''
+      # Add the closures of the top-level store objects.
+      storePaths=$(perl ${pkgs.pathsFromGraph} closure-*)
+
+      # Also include a manifest of the closures in a format suitable
+      # for nix-store --load-db.
+      printRegistration=1 perl ${pkgs.pathsFromGraph} closure-* > nix-path-registration
+
+      # Make a crude approximation of the size of the target image.
+      # If the script starts failing, increase the fudge factors here.
+      numInodes=$(find $storePaths | wc -l)
+      numDataBlocks=$(du -c -B 4096 --apparent-size $storePaths | awk '$2 == "total" { print int($1 * 1.03) }')
+      bytes=$((2 * 4096 * $numInodes + 4096 * $numDataBlocks))
+      echo "Creating an EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks)"
+
+      truncate -s $bytes $out
+      faketime "1970-01-01 00:00:00" mkfs.ext4 -L ${volumeLabel} -U 44444444-4444-4444-8888-888888888888 $out
+
+      # Populate the image contents by piping a bunch of commands to the `debugfs` tool from e2fsprogs.
+      # For example, to copy /nix/store/abcd...efg-coreutils-8.23/bin/sleep:
+      #   cd /nix/store/abcd...efg-coreutils-8.23/bin
+      #   write /nix/store/abcd...efg-coreutils-8.23/bin/sleep sleep
+      #   sif sleep mode 040555
+      #   sif sleep gid 30000
+      # In particular, debugfs doesn't handle absolute target paths; you have to 'cd' in the virtual
+      # filesystem first. Likewise the intermediate directories must already exist (using `find`
+      # handles that for us). And when setting the file's permissions, the inode type flags (__S_IFDIR,
+      # __S_IFREG) need to be set as well.
+      (
+        echo write nix-path-registration nix-path-registration
+        echo mkdir nix
+        echo cd /nix
+        echo mkdir store
+
+        # XXX: This explodes in exciting ways if anything in /nix/store has a space in it.
+        find $storePaths -printf '%y %f %h %m\n'| while read -r type file dir perms; do
+          # echo "TYPE=$type DIR=$dir FILE=$file PERMS=$perms" >&2
+
+          echo "cd $dir"
+          case $type in
+            d)
+              echo "mkdir $file"
+              echo sif $file mode $((040000 | 0$perms)) # magic constant is __S_IFDIR
+              ;;
+            f)
+              echo "write $dir/$file $file"
+              echo sif $file mode $((0100000 | 0$perms)) # magic constant is __S_IFREG
+              ;;
+            l)
+              echo "symlink $file $(readlink "$dir/$file")"
+              ;;
+            *)
+              echo "Unknown entry: $type $dir $file $perms" >&2
+              exit 1
+              ;;
+          esac
+
+          echo sif $file gid 30000 # chgrp to nixbld
+        done
+      ) | faketime "1970-01-01 00:00:00" debugfs -w $out -f /dev/stdin > errorlog 2>&1
+
+      # The debugfs tool doesn't terminate on error nor exit with a non-zero status. Check manually.
+      if egrep -q 'Could not allocate|File not found' errorlog; then
+        cat errorlog
+        echo "--- Failed to create EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks) ---"
+        return 1
+      fi
+    '';
+}