[PATCH v6 1/2] Build verity images in rootfs Nix derivation

26 Nov 2025

Avoid redundant rebuilds of the rootfs verity superblock and roothash.
Remove duplicate code.  Clean up Makefile to avoid temporary files.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
---
Changes since v5:
- Use consistent naming scheme for Make variables: ROOT_FS is the
  directory containing the artifacts.  ROOT_FS_IMAGE is the root
  filesystem image, ROOT_FS_VERITY is the verity image, and
  ROOT_FS_VERITY_ROOTHASH is its root filesystem hash.
- Do not try to use a file as a directory.
- Use $@ where possible.
- Use consistent indentation.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
---
 host/initramfs/Makefile    | 26 +++++---------------------
 host/initramfs/default.nix |  1 +
 host/rootfs/Makefile       | 44 ++++++++++++++++++++++----------------------
 host/rootfs/default.nix    |  6 ++++--
 host/rootfs/shell.nix      |  2 +-
 lib/common.mk              |  4 ++++
 release/live/Makefile      | 26 +++++---------------------
 release/live/shell.nix     |  3 ++-
 8 files changed, 44 insertions(+), 68 deletions(-)

diff --git a/host/initramfs/Makefile b/host/initramfs/Makefile
index cb13fbb35f065b67d291d4a35591d6f12720060c..f27f5e07c4707914962197b4fea8f385729370aa 100644
--- a/host/initramfs/Makefile
+++ b/host/initramfs/Makefile
@@ -35,26 +35,10 @@ build/mountpoints:
 	cd build/mountpoints && mkdir -p $(MOUNTPOINTS)
 	find build/mountpoints -mindepth 1 -exec touch -d @0 {} ';'
 
-# veritysetup format produces two files, but Make only (portably)
-# supports one output per rule, so we combine the two outputs then
-# define two more rules to separate them again.
-build/rootfs.verity: $(ROOT_FS)
-	mkdir -p build
-	$(VERITYSETUP) format $(ROOT_FS) build/rootfs.verity.superblock.tmp \
-	    | awk -F ':[[:blank:]]*' '$$1 == "Root hash" {print $$2; exit}' \
-	    > build/rootfs.verity.roothash.tmp
-	cat build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp \
-	    > $@
-	rm build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp
-build/rootfs.verity.roothash: build/rootfs.verity
-	head -n 1 build/rootfs.verity > $@
-build/rootfs.verity.superblock: build/rootfs.verity
-	tail -n +2 build/rootfs.verity > $@
-
-build/live.img: ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk build/rootfs.verity.superblock build/rootfs.verity.roothash $(ROOT_FS)
+build/live.img: ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk $(ROOT_FS_IMAGES)
 	../../scripts/make-gpt.sh $@.tmp \
-	    build/rootfs.verity.superblock:verity:$$(../../scripts/format-uuid.sh "$$(dd if=build/rootfs.verity.roothash bs=32 skip=1 count=1 status=none)") \
-	    $(ROOT_FS):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 build/rootfs.verity.roothash)")
+	    $(ROOT_FS_VERITY):verity:$$(../../scripts/format-uuid.sh "$$(dd if=$(ROOT_FS_VERITY_ROOTHASH) bs=32 skip=1 count=1 status=none)") \
+	    $(ROOT_FS_IMAGE):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 $(ROOT_FS_VERITY_ROOTHASH))")
 	mv $@.tmp $@
 
 build/loop.tar: build/live.img
@@ -69,12 +53,12 @@ clean:
 	rm -rf build
 .PHONY: clean
 
-run: $(dest) build/rootfs.verity.roothash $(RUN_IMAGE)
+run: $(dest) $(RUN_IMAGE) $(ROOT_FS_VERITY_ROOTHASH)
 	@../../scripts/run-qemu.sh -m 4G \
 	    -machine virtualization=on \
 	    -kernel $(KERNEL) \
 	    -initrd $(dest) \
-	    -append "ro earlycon console=hvc0 intel_iommu=on roothash=$$(< build/rootfs.verity.roothash) nokaslr" \
+	    -append "ro earlycon console=hvc0 intel_iommu=on roothash=$$(< $(ROOT_FS_VERITY_ROOTHASH)) nokaslr" \
 	    -cpu max \
 	    -gdb unix:build/gdb.sock,server,nowait \
 	    -parallel none \
diff --git a/host/initramfs/default.nix b/host/initramfs/default.nix
index d35e1b514ec48015f5110e65e5ae944b28244c4f..16c932600c0ab54cf439dc7931fc12e863677e0e 100644
--- a/host/initramfs/default.nix
+++ b/host/initramfs/default.nix
@@ -105,6 +105,7 @@ stdenvNoCC.mkDerivation {
 
   env = {
     PACKAGES_CPIO = packagesCpio;
+    ROOT_FS = rootfs;
   } // lib.optionalAttrs stdenvNoCC.hostPlatform.isx86_64 {
     MICROCODE = microcode;
   };
diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile
index 4067be0c45cc83ce4670ed76e956db58f8e93e02..4cb048ac08b3265b0435d6ff6fc612a58c169ce9 100644
--- a/host/rootfs/Makefile
+++ b/host/rootfs/Makefile
@@ -1,12 +1,13 @@
 # SPDX-License-Identifier: EUPL-1.2+
 # SPDX-FileCopyrightText: 2021-2024 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>
 
 .POSIX:
 
 include ../../lib/common.mk
 include file-list.mk
 
-dest = build/rootfs.erofs
+ROOT_FS = build
 
 DIRS = \
 	dev \
@@ -40,8 +41,22 @@ FIFOS = etc/s6-linux-init/run-image/service/s6-svscan-log/fifo
 
 BUILD_FILES = build/etc/s6-rc
 
-$(dest): ../../scripts/make-erofs.sh $(PACKAGES_FILE) $(FILES) $(BUILD_FILES) build/empty build/fifo file-list.mk
-	set -euo pipefail; \
+# This rule produces three files but Make only (portably)
+# supports one output per rule.  Instead of resorting to temporary
+# files, a timestamp file is created as the last step.  The actual
+# outputs are produced as side-effects.
+build/verity-timestamp: $(ROOT_FS_IMAGE)
+	$(VERITYSETUP) format \
+		--root-hash-file $(ROOT_FS_VERITY_ROOTHASH) \
+		-- $(ROOT_FS_IMAGE) $(ROOT_FS_VERITY)
+# Add trailing newline so that the read < $(ROOT_FS_VERITY_ROOTHASH) succeeds.
+# Without the trailing newline it assumes a premature EOF happened and returns
+# a nonzero status.
+	echo >> $(ROOT_FS_VERITY_ROOTHASH)
+	touch -- $@
+
+$(ROOT_FS_IMAGE): ../../scripts/make-erofs.sh $(PACKAGES_FILE) $(FILES) $(BUILD_FILES) build/empty build/fifo file-list.mk
+	mkdir -p $(ROOT_FS) && \
 	{ \
 	    cat $(PACKAGES_FILE) ;\
 	    for file in $(FILES) $(LINKS); do printf '%s\n%s\n' $$file "$${file#image/}"; done ;\
@@ -77,25 +92,10 @@ clean:
 	rm -rf build
 .PHONY: clean
 
-# veritysetup format produces two files, but Make only (portably)
-# supports one output per rule, so we combine the two outputs then
-# define two more rules to separate them again.
-build/rootfs.verity: $(dest)
-	$(VERITYSETUP) format $(dest) build/rootfs.verity.superblock.tmp \
-	    | awk -F ':[[:blank:]]*' '$$1 == "Root hash" {print $$2; exit}' \
-	    > build/rootfs.verity.roothash.tmp
-	cat build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp \
-	    > $@
-	rm build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp
-build/rootfs.verity.roothash: build/rootfs.verity
-	head -n 1 build/rootfs.verity > $@
-build/rootfs.verity.superblock: build/rootfs.verity
-	tail -n +2 build/rootfs.verity > $@
-
-build/live.img: ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk build/rootfs.verity.superblock build/rootfs.verity.roothash $(dest)
+build/live.img: ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk build/verity-timestamp $(ROOT_FS_IMAGES)
 	../../scripts/make-gpt.sh $@.tmp \
-	    build/rootfs.verity.superblock:verity:$$(../../scripts/format-uuid.sh "$$(dd if=build/rootfs.verity.roothash bs=32 skip=1 count=1 status=none)") \
-	    $(dest):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 build/rootfs.verity.roothash)")
+	    $(ROOT_FS_VERITY):verity:$$(../../scripts/format-uuid.sh "$$(dd if=$(ROOT_FS_VERITY_ROOTHASH) bs=32 skip=1 count=1 status=none)") \
+	    $(ROOT_FS_IMAGE):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 $(ROOT_FS_VERITY_ROOTHASH))")
 	mv $@.tmp $@
 
 debug:
@@ -126,7 +126,7 @@ run: build/live.img build/rootfs.verity.roothash
 	    -device virtconsole,chardev=virtiocon0 \
 	    -drive file=build/live.img,if=virtio,format=raw,readonly=on \
 	    -drive file=/proc/self/fd/3,if=virtio,format=raw \
-	    -append "earlycon console=hvc0 roothash=$$(< build/rootfs.verity.roothash) intel_iommu=on nokaslr" \
+	    -append "earlycon console=hvc0 roothash=$$(< $(ROOT_FS_VERITY_ROOTHASH)) intel_iommu=on nokaslr" \
 	    -device virtio-keyboard \
 	    -device virtio-mouse \
 	    -device virtio-gpu \
diff --git a/host/rootfs/default.nix b/host/rootfs/default.nix
index f7974a41052468f9782214f32355dda97af3cd60..941c04e619baa7652d1812f4eb50445c607d5884 100644
--- a/host/rootfs/default.nix
+++ b/host/rootfs/default.nix
@@ -118,7 +118,7 @@ stdenvNoCC.mkDerivation {
   };
   sourceRoot = "source/host/rootfs";
 
-  nativeBuildInputs = [ erofs-utils spectrum-build-tools s6-rc ];
+  nativeBuildInputs = [ cryptsetup erofs-utils spectrum-build-tools s6-rc ];
 
   env = {
     PACKAGES = runCommand "packages" {} ''
@@ -127,7 +127,9 @@ stdenvNoCC.mkDerivation {
     '';
   };
 
-  makeFlags = [ "dest=$(out)" ];
+  # The Makefile uses $(ROOT_FS), not $(dest), so it can share code
+  # with other Makefiles that also use this variable.
+  makeFlags = [ "ROOT_FS=$(out)" ];
 
   dontInstall = true;
 
diff --git a/host/rootfs/shell.nix b/host/rootfs/shell.nix
index 1bf61bebf418333624e799cc8ca231f5783206f4..6df2f575fdfc7cdf8067ccfdb5fecaad9f6ea5e6 100644
--- a/host/rootfs/shell.nix
+++ b/host/rootfs/shell.nix
@@ -12,7 +12,7 @@ rootfs.overrideAttrs (
 
 {
   nativeBuildInputs = nativeBuildInputs ++ [
-    btrfs-progs cryptsetup jq netcat qemu_kvm reuse util-linux
+    btrfs-progs jq netcat qemu_kvm reuse util-linux
   ];
 
   env = env // {
diff --git a/lib/common.mk b/lib/common.mk
index 277c3544036d9a9057f8ba4ad37fe2207548cc59..84091a8dfd615b2aaa021c7477f34c4dff88ccde 100644
--- a/lib/common.mk
+++ b/lib/common.mk
@@ -11,6 +11,10 @@ GDB = gdb
 MCOPY = mcopy
 MKFS_FAT = mkfs.fat
 MMD = mmd
+ROOT_FS_IMAGE = $(ROOT_FS)/rootfs
+ROOT_FS_IMAGES = $(ROOT_FS_IMAGE) $(ROOT_FS_VERITY_ROOTHASH) $(ROOT_FS_VERITY)
+ROOT_FS_VERITY = $(ROOT_FS)/rootfs.verity.superblock
+ROOT_FS_VERITY_ROOTHASH = $(ROOT_FS)/rootfs.verity.roothash
 S6_IPCSERVER_SOCKETBINDER = s6-ipcserver-socketbinder
 TAR = tar
 TRUNCATE = truncate
diff --git a/release/live/Makefile b/release/live/Makefile
index 6dcbdeedda5d6ccf293f60dc62043f46c81ecf83..ba81c7e679429e045b24c1591a9f0b72f016cfab 100644
--- a/release/live/Makefile
+++ b/release/live/Makefile
@@ -9,17 +9,17 @@ DTBS ?= build/empty
 
 dest = build/live.img
 
-$(dest): ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk build/boot.fat build/rootfs.verity.superblock build/rootfs.verity.roothash $(ROOT_FS)
+$(dest): ../../scripts/format-uuid.sh ../../scripts/make-gpt.sh ../../scripts/sfdisk-field.awk build/boot.fat $(ROOT_FS_IMAGES)
 	../../scripts/make-gpt.sh $@.tmp \
 	    build/boot.fat:c12a7328-f81f-11d2-ba4b-00a0c93ec93b \
-	    build/rootfs.verity.superblock:verity:$$(../../scripts/format-uuid.sh "$$(dd if=build/rootfs.verity.roothash bs=32 skip=1 count=1 status=none)") \
-	    $(ROOT_FS):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 build/rootfs.verity.roothash)")
+	    $(ROOT_FS_VERITY):verity:$$(../../scripts/format-uuid.sh "$$(dd if=$(ROOT_FS_VERITY_ROOTHASH) bs=32 skip=1 count=1 status=none)") \
+	    $(ROOT_FS_IMAGE):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 $(ROOT_FS_VERITY_ROOTHASH))")
 	mv $@.tmp $@
 
 build/empty:
 	mkdir -p $@
 
-build/spectrum.efi: build/rootfs.verity.roothash $(DTBS) $(KERNEL) $(INITRAMFS)
+build/spectrum.efi: $(DTBS) $(KERNEL) $(INITRAMFS) $(ROOT_FS_VERITY_ROOTHASH)
 	{ \
 	    printf "[UKI]\nDeviceTreeAuto=" && \
 	    find $(DTBS) -name '*.dtb' -print0 | tr '\0' ' ' ;\
@@ -29,7 +29,7 @@ build/spectrum.efi: build/rootfs.verity.roothash $(DTBS) $(KERNEL) $(INITRAMFS)
 	    --linux $(KERNEL) \
 	    --initrd $(INITRAMFS) \
 	    --os-release $$'NAME="Spectrum"\n' \
-	    --cmdline "ro intel_iommu=on roothash=$$(cat build/rootfs.verity.roothash)"
+	    --cmdline "ro intel_iommu=on roothash=$$(cat $(ROOT_FS_VERITY_ROOTHASH))"
 
 build/boot.fat: $(SYSTEMD_BOOT_EFI) build/spectrum.efi
 	$(TRUNCATE) -s 440401920 $@
@@ -38,22 +38,6 @@ build/boot.fat: $(SYSTEMD_BOOT_EFI) build/spectrum.efi
 	$(MCOPY) -i $@ build/spectrum.efi ::/EFI/Linux
 	$(MCOPY) -i $@ $(SYSTEMD_BOOT_EFI) ::/EFI/BOOT/$(EFINAME)
 
-# veritysetup format produces two files, but Make only (portably)
-# supports one output per rule, so we combine the two outputs then
-# define two more rules to separate them again.
-build/rootfs.verity: $(ROOT_FS)
-	mkdir -p build
-	$(VERITYSETUP) format $(ROOT_FS) build/rootfs.verity.superblock.tmp \
-	    | awk -F ':[[:blank:]]*' '$$1 == "Root hash" {print $$2; exit}' \
-	    > build/rootfs.verity.roothash.tmp
-	cat build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp \
-	    > $@
-	rm build/rootfs.verity.roothash.tmp build/rootfs.verity.superblock.tmp
-build/rootfs.verity.roothash: build/rootfs.verity
-	head -n 1 build/rootfs.verity > $@
-build/rootfs.verity.superblock: build/rootfs.verity
-	tail -n +2 build/rootfs.verity > $@
-
 clean:
 	rm -rf build
 .PHONY: clean
diff --git a/release/live/shell.nix b/release/live/shell.nix
index 5acaa8c5b113fd2789aaea9268487b193bab37af..799d575b4f6e6f3dad8ac1a0a8bc2e3fc46aab44 100644
--- a/release/live/shell.nix
+++ b/release/live/shell.nix
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: MIT
 # SPDX-FileCopyrightText: 2021-2024 Alyssa Ross <hi@alyssa.is>
 
-import ../../lib/call-package.nix ({ callSpectrumPackage, stdenv, qemu_kvm }:
+import ../../lib/call-package.nix ({ callSpectrumPackage, stdenv, qemu_kvm, rootfs }:
 
 (callSpectrumPackage ./. {}).overrideAttrs (
   { nativeBuildInputs ? [], env ? {}, ... }:
@@ -10,6 +10,7 @@ import ../../lib/call-package.nix ({ callSpectrumPackage, stdenv, qemu_kvm }:
 
     env = env // {
       OVMF_CODE = "${qemu_kvm}/share/qemu/edk2-${stdenv.hostPlatform.qemuArch}-code.fd";
+      ROOT_FS = rootfs;
     };
   }
 )) (_: {})

-- 
2.52.0

    