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

Alyssa Ross
2025-09-08T08:47:12Z

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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.

> 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.

So I get why, given the overlayfs idea, it's important for /etc and /var
to not be user-writeable, but what I don't understand is: why aren't we
checking permissions for other directories, like /bin or /lib?

> +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"
>
> --
> 2.51.0

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