Re: [PATCH] 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(-)

This doesn't seem to have addressed the review comments from last
time[1] — the special handling for directories that might want to use
overlayfs in future is still there even though it's uncertain we'll ever
do that, and so is -execdir.

To save us both time, I've just gone ahead and made the changes, and
pushed a simplified version of this.  Thanks for fixing it — I'm glad I
didn't have to work out that find command myself, and now we can move
forward with running things as non-root.

[1]: https://spectrum-os.org/lists/archives/spectrum-devel/877bxs570x.fsf@alyssa.is

> diff --git a/host/rootfs/Makefile b/host/rootfs/Makefile
> index aa45ca1d5c18d0dfb78d19267f263cc4222e8e84..ba1beddabb46afa6b20e66177107fbe6b6f42bd2 100644
> --- a/host/rootfs/Makefile
> +++ b/host/rootfs/Makefile
> @@ -40,7 +40,8 @@ DIRS = \
>  	ext \
>  	proc \
>  	run \
> -	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 981889ebe55d9ba03228977f3dc0ea3f26d5c4fb..2540075fbb2cdcbcde29853cb0ffe676de0b9063 100644
> --- a/img/app/Makefile
> +++ b/img/app/Makefile
> @@ -30,7 +30,7 @@ $(imgdir)/appvm/blk/root.img: ../../scripts/make-gpt.sh ../../scripts/sfdisk-fie
>  	    build/rootfs.erofs:root:5460386f-2203-4911-8694-91400125c604:root
>  	mv $@.tmp $@
>  
> -DIRS = dev run proc sys tmp \
> +DIRS = dev run proc sys tmp var \
>  	etc/s6-linux-init/run-image/service \
>  	etc/s6-linux-init/run-image/user \
>  	etc/s6-linux-init/run-image/wait
> diff --git a/scripts/make-erofs.sh b/scripts/make-erofs.sh
> index ad04844387c880047a79f2f05e1e985d8bd4229c..5e283a380dbdae3dbfb83d43915e5015a2ae6f04 100755
> --- a/scripts/make-erofs.sh
> +++ b/scripts/make-erofs.sh
> @@ -68,4 +68,25 @@ while read -r arg1; do
>  	cp -RT -- "$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"
>
> ---
> base-commit: c5d5786d3dc938af0b279c542d1e43bce381b4b9
> change-id: 20251021-fix-permissions-4549d0653368
>
> -- 
> Sincerely,
> Demi Marie Obenour (she/her/hers)

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