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

Alyssa Ross
2025-09-21T12:23:32Z

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

> On 9/19/25 13:50, Alyssa Ross wrote:
>> 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?
>
> Yes, it is. I will fix this in v2.
>
>> 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.)
>
> systemd-udevd needs /var to be mounted read-write. Without that,
> its behavior (and that of all other systemd tools) is undefined
> past a certain point in early boot.

It does? That's surprising to me, since lots of initrds will run
systemd-udevd and I suspect not have /var (such as the NixOS one, I
think). Looking at systemd's build system, I only see three uses of
localstatedir: polkitpkladir, systemdstatedir, and randomseeddir. As
far as I can tell, none of these are used by systemd-udevd.

>> 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. 🤫)
>
> Per 'man 1 find', the find command I provided is POSIX except for
> -execdir. However, -execdir is also documented as being provided
> by BSD OSs. The documentation also warns against -exec, though
> the race that -execdir blocks is irrelevant here.

Yeah, exactly. Might as well use the POSIX one when it suffices.

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