Re: [PATCH 1/2] Create Nix derivation for building verity images

Alyssa Ross
2025-11-06T10:20:59Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> This gets rid of a lot of duplicated code and allows building the verity
> roothash and superblock only when needed. It also removes a hack used
> to work around make limitations. Furthermore,
> 'veritysetup --root-hash-file' is used to avoid an awk script.
>
> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
> ---
> nix-shell --pure --run 'make run' in host/initramfs fails. This is a
> preexisting bug and I will send a separate patch for it.
> ---
> host/initramfs/Makefile | 25 +++++--------------------
> host/initramfs/shell.nix | 4 +++-
> host/rootfs/Makefile | 24 +++++-------------------
> host/rootfs/shell.nix | 3 +++
> host/verity.nix | 19 +++++++++++++++++++
> lib/common.mk | 1 -
> pkgs/default.nix | 1 +
> release/live/Makefile | 26 +++++---------------------
> release/live/default.nix | 4 +++-
> 9 files changed, 44 insertions(+), 63 deletions(-)

> diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile
> index 00d125774bb7b98736d0928c69cb307740cee034..bb602e2745fb5873204f453b35fc529c5c96f64a 100644
> --- a/host/rootfs/Makefile
> +++ b/host/rootfs/Makefile
> @@ -82,25 +82,11 @@ 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 $(ROOT_FS_VERITY) $(ROOT_FS_VERITY_ROOTHASH) $(dest)
> ../../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)")" \
> + $(dest):root:$$(../../scripts/format-uuid.sh "$$(head -c 32 "$$ROOT_FS_VERITY_ROOTHASH")")
> mv $@.tmp $@
>
> debug:
> @@ -110,7 +96,7 @@ debug:
> $(VMLINUX)
> .PHONY: debug
>
> -run: build/live.img $(EXT_FS) build/rootfs.verity.roothash
> +run: build/live.img $(EXT_FS) $(ROOT_FS_VERITY_ROOTHASH)
> @set -x && \
> ext="$$(mktemp build/spectrum-rootfs-extfs.XXXXXXXXXX.img)" && \
> truncate -s 10G "$$ext" && \
> @@ -131,7 +117,7 @@ run: build/live.img $(EXT_FS) build/rootfs.verity.roothash
> -device virtconsole,chardev=virtiocon0 \
> -drive file=build/live.img,if=virtio,format=raw,readonly=on \
> -drive file=/proc/self/fd/3,if=virtio,format=raw \
> - -append "earlycon console=hvc0 roothash=$$(< 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/shell.nix b/host/rootfs/shell.nix
> index 1bf61bebf418333624e799cc8ca231f5783206f4..f16e4905adfbc8faebde19d0a1364ad9df90219b 100644
> --- a/host/rootfs/shell.nix
> +++ b/host/rootfs/shell.nix
> @@ -5,6 +5,7 @@
> import ../../lib/call-package.nix (
> { callSpectrumPackage, rootfs, pkgsStatic, srcOnly, stdenv
> , btrfs-progs, cryptsetup, jq, netcat, qemu_kvm, reuse, util-linux
> +, verity
> }:
>
> rootfs.overrideAttrs (
> @@ -20,5 +21,7 @@ rootfs.overrideAttrs (
> KERNEL = "${passthru.kernel}/${stdenv.hostPlatform.linux-kernel.target}";
> LINUX_SRC = srcOnly passthru.kernel.configfile;
> VMLINUX = "${passthru.kernel.dev}/vmlinux";
> + ROOT_FS_VERITY = "${verity}/rootfs.verity.superblock";
> + ROOT_FS_VERITY_ROOTHASH = "${verity}/rootfs.verity.roothash";
> };
> })) (_: {})

Surely this would break interactive development of the rootfs?
If I'm in a Nix shell, and make a change to any part of the rootfs, the
verity data in the environment will be out of date. I'd have to leave
and re-enter the Nix shell after /any/ change, waiting for an evaluation
each time, as opposed to the current situation where that's only
necessary when modifying Nix code or other Spectrum components.

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