Re: [PATCH 07/20] scripts/make-erofs.sh: Standardize file modes in images

[Alyssa Ross <hi@alyssa.is>]

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

> Enforce that anything under /var or /etc is 0755 for directories and
> executable files and 0644 for anything else.  Enforce that anything else
> is 0555 for directories and executable files and 0444 for anything else.
> This avoids depending on factors that may depend on the build
> environment, such as the user's umask.
>
> This requires that /var always exist, so add it to img/app/Makefile.
>
> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
> ---
>  host/rootfs/Makefile  |  3 ++-
>  img/app/Makefile      |  2 +-
>  scripts/make-erofs.sh | 21 +++++++++++++++++++++
>  3 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile
> index f677fe580f2e2be58113457e63468d97f49a49f6..dce78e60bc1a8c18f5f448aaa9aeed2c8a7da04e 100644
> --- a/host/rootfs/Makefile
> +++ b/host/rootfs/Makefile
> @@ -97,7 +97,8 @@ DIRS = \
>  	ext \
>  	run \
>  	proc \
> -	sys
> +	sys \
> +	var
>  
>  FIFOS = etc/s6-linux-init/run-image/service/s6-svscan-log/fifo
>  
> diff --git a/img/app/Makefile b/img/app/Makefile
> index 9665a6b7158f2d8b183831202a4559ae06d53d16..c6b9a23ce8796582d6e2f5121c30c2269975aa2d 100644
> --- a/img/app/Makefile
> +++ b/img/app/Makefile
> @@ -57,7 +57,7 @@ VM_FILES = \
>  	etc/wireplumber/wireplumber.conf.d/99_spectrum.conf \
>  	etc/xdg/xdg-desktop-portal/portals.conf
>  
> -VM_DIRS = dev run proc sys tmp \
> +VM_DIRS = dev run proc sys tmp var \
>  	etc/s6-linux-init/run-image/service \
>  	etc/s6-linux-init/run-image/user \
>  	etc/s6-linux-init/run-image/wait
> diff --git a/scripts/make-erofs.sh b/scripts/make-erofs.sh
> index 66abd1f388524c19cd3a1113415892d0d72e3f82..d566a4ac7b30f55338fe9b8b6a94702686f6ddd1 100755
> --- a/scripts/make-erofs.sh
> +++ b/scripts/make-erofs.sh
> @@ -95,4 +95,25 @@ while read -r arg1; do
>  	cp -RT -- "$arg1" "$root/$arg2"
>  done
>  
> +# Ensure that the permissions in the image are independent
> +# of those in the git repository or Nix store, except for
> +# the executable bit.  In particular, the mode of those
> +# outside the Nix store might depend on the user's umask.
> +# While the image itself is strictly read-only, it makes
> +# sense to populate an overlayfs over /etc and /var, and
> +# this overlayfs should be writable by root and readable
> +# by all users.  The remaining paths should not be writable
> +# by anyone, but should be world-readable.
> +find "$root" \
> +  -path "$root/nix/store" -prune -o \
> +  -path "$root/etc" -prune -o \
> +  -path "$root/var" -prune -o \
> +  -type l -o \
> +  -type d -a -perm 0555 -o \
> +  -type f -a -perm 0444 -o \
> +  -execdir chmod ugo-w,ugo+rX -- '{}' +
> +find "$root/etc" "$root/var" ! -type l -execdir chmod u+w,go-w,ugo+rX -- '{}' +
> +chmod 0755 "$root"
> +
> +# Make the erofs image.
>  mkfs.erofs -x-1 -b4096 --all-root "$@" "$root"

The idea here is reproducibility, right?  Can the body mention that?
And can we limit it to just doing r-Xr-Xr-X for now, and then worry
about the overlayfs stuff later if we need to?  (This also means we
don't have to add /var until we need it.)

I'd also like to stick to POSIX features for standard utilities where
possible, which it should be here.  (I know cp -T isn't POSIX. 🤫)

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