[PATCH 1/1] rootfs-postcommands.bbclass: sort passwd entries

[Patrick Ohly <patrick.ohly@intel.com>]

The /etc passwd files in a rootfs consist of the default entries from
base-passwd plus anything that gets added by preinst scripts or
extrausers.bbclass.

The execution order of preinst scripts is not perfectly deterministic,
or at least unrelated changes caused it to change in a
non-deterministic way, resulting in irrelevant changes in the order of
passwd entries.

Such re-ordering is bad for reproducible builds and file-based update
mechanisms like swupd which work best if changes are as minimal as
possible.

To achieve that, the files get sorted in a post-processing command,
enabled by default. It would be slightly nicer to keep entries from
base-passwd at the beginning of the files, but that's harder to
implement.

The order of the entries should not matter, but in obscure cases where
it does (like having multiple entries for the same numeric ID) this
behavior can be disabled by setting SORT_PASSWD_POSTPROCESS_COMMAND to
an empty string.

Fixes: YOCTO #10520

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
---
 meta/classes/rootfs-postcommands.bbclass | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/meta/classes/rootfs-postcommands.bbclass b/meta/classes/rootfs-postcommands.bbclass
index 8d48a2d..4ca031f 100644
--- a/meta/classes/rootfs-postcommands.bbclass
+++ b/meta/classes/rootfs-postcommands.bbclass
@@ -30,6 +30,13 @@ ROOTFS_POSTPROCESS_COMMAND += 'empty_var_volatile;'
 SSH_DISABLE_DNS_LOOKUP ?= " ssh_disable_dns_lookup ; "
 ROOTFS_POSTPROCESS_COMMAND_append_qemuall = "${SSH_DISABLE_DNS_LOOKUP}"
 
+# Sort the user and group entries in /etc in order to make the content
+# deterministic. Package installs are not deterministic, causing the ordering
+# of entries to change between builds. In case that this isn't desired,
+# the command can be overridden.
+SORT_PASSWD_POSTPROCESS_COMMAND ??= "sort_passwd; "
+ROOTFS_POSTPROCESS_COMMAND += "${SORT_PASSWD_POSTPROCESS_COMMAND}"
+
 systemd_create_users () {
 	for conffile in ${IMAGE_ROOTFS}/usr/lib/sysusers.d/systemd.conf ${IMAGE_ROOTFS}/usr/lib/sysusers.d/systemd-remote.conf; do
 		[ -e $conffile ] || continue
@@ -146,6 +153,19 @@ ssh_disable_dns_lookup () {
 	fi
 }
 
+sort_passwd () {
+	for i in passwd shadow group gshadow; do
+		for suffix in "" "-"; do
+			file="${IMAGE_ROOTFS}/${sysconfdir}/$i$suffix"
+			if [ -f $file ]; then
+				sort $file >$file.tmp
+				cat $file.tmp >$file
+				rm $file.tmp
+			fi
+		done
+	done
+}
+
 #
 # Enable postinst logging if debug-tweaks is enabled
 #
-- 
2.1.4