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

Demi Marie Obenour
2025-09-08T17:16:33Z

[-- Attachment #1.1.1: Type: text/plain, Size: 2181 bytes --]

On 9/8/25 04:46, 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.
>
>> 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?

Other way around: /etc, /var, and /nix/store are skipped (via -prune -o)
and the rest are checked.

>> +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

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

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 7253 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]