pyrox.dev
1# Builds an ext4 image containing a populated /nix/store with the closure
2# of store paths passed in the storePaths parameter. The generated image
3# is sized to only fit its contents, with the expectation that a script
4# resizes the filesystem at boot time.
5{ pkgs
6, storePaths
7, volumeLabel
8}:
9
10pkgs.stdenv.mkDerivation {
11 name = "ext4-fs.img";
12
13 buildInputs = with pkgs; [e2fsprogs libfaketime perl];
14
15 # For obtaining the closure of `storePaths'.
16 exportReferencesGraph =
17 map (x: [("closure-" + baseNameOf x) x]) storePaths;
18
19 buildCommand =
20 ''
21 # Add the closures of the top-level store objects.
22 storePaths=$(perl ${pkgs.pathsFromGraph} closure-*)
23
24 # Also include a manifest of the closures in a format suitable
25 # for nix-store --load-db.
26 printRegistration=1 perl ${pkgs.pathsFromGraph} closure-* > nix-path-registration
27
28 # Make a crude approximation of the size of the target image.
29 # If the script starts failing, increase the fudge factors here.
30 numInodes=$(find $storePaths | wc -l)
31 numDataBlocks=$(du -c -B 4096 --apparent-size $storePaths | awk '$2 == "total" { print int($1 * 1.03) }')
32 bytes=$((2 * 4096 * $numInodes + 4096 * $numDataBlocks))
33 echo "Creating an EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks)"
34
35 truncate -s $bytes $out
36 faketime "1970-01-01 00:00:00" mkfs.ext4 -L ${volumeLabel} -U 44444444-4444-4444-8888-888888888888 $out
37
38 # Populate the image contents by piping a bunch of commands to the `debugfs` tool from e2fsprogs.
39 # For example, to copy /nix/store/abcd...efg-coreutils-8.23/bin/sleep:
40 # cd /nix/store/abcd...efg-coreutils-8.23/bin
41 # write /nix/store/abcd...efg-coreutils-8.23/bin/sleep sleep
42 # sif sleep mode 040555
43 # sif sleep gid 30000
44 # In particular, debugfs doesn't handle absolute target paths; you have to 'cd' in the virtual
45 # filesystem first. Likewise the intermediate directories must already exist (using `find`
46 # handles that for us). And when setting the file's permissions, the inode type flags (__S_IFDIR,
47 # __S_IFREG) need to be set as well.
48 (
49 echo write nix-path-registration nix-path-registration
50 echo mkdir nix
51 echo cd /nix
52 echo mkdir store
53
54 # XXX: This explodes in exciting ways if anything in /nix/store has a space in it.
55 find $storePaths -printf '%y %f %h %m\n'| while read -r type file dir perms; do
56 # echo "TYPE=$type DIR=$dir FILE=$file PERMS=$perms" >&2
57
58 echo "cd $dir"
59 case $type in
60 d)
61 echo "mkdir $file"
62 echo sif $file mode $((040000 | 0$perms)) # magic constant is __S_IFDIR
63 ;;
64 f)
65 echo "write $dir/$file $file"
66 echo sif $file mode $((0100000 | 0$perms)) # magic constant is __S_IFREG
67 ;;
68 l)
69 echo "symlink $file $(readlink "$dir/$file")"
70 ;;
71 *)
72 echo "Unknown entry: $type $dir $file $perms" >&2
73 exit 1
74 ;;
75 esac
76
77 echo sif $file gid 30000 # chgrp to nixbld
78 done
79 ) | faketime "1970-01-01 00:00:00" debugfs -w $out -f /dev/stdin > errorlog 2>&1
80
81 # The debugfs tool doesn't terminate on error nor exit with a non-zero status. Check manually.
82 if egrep -q 'Could not allocate|File not found' errorlog; then
83 cat errorlog
84 echo "--- Failed to create EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks) ---"
85 return 1
86 fi
87 '';
88}