diff options
Diffstat (limited to 'nixos/lib')
-rw-r--r-- | nixos/lib/make-ext4-fs.nix | 88 |
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 000000000000..23839ea487db --- /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 + ''; +} |