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

[Demi Marie Obenour <demiobenour@gmail.com>]

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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.

> 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.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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