Merge ~smoser/cloud-initramfs-tools:fix/1890803-overlayroot-no-fstab into cloud-initramfs-tools:master

[Scott Moser]

overlayroot: initialize /etc/fstab if root does not have one and read-only

A root filesystem may not have an /etc/fstab. That could be for either:
a.) not needed - the kernel command line has root mount options and no
  other filesystems are mounted.
b.) system has systemd.mounts - this is currently not supported by
overlayroot.

If the kernel command line contains 'ro', then overlayroot will
correctly leave root mounted 'ro'. The problem was that without an
/etc/fstab, systemd would not remount the root filesystem rw, and
thus failure would ensue.

The change here is to initialize an fstab in the case that the root
filesystem did not contain one, *and* 'ro' is present on the cmdline.
In cases where 'ro' is not present, then overlayroot will leave /
mounted as rw, so systemd does not need to remount rw.

LP: #1890803

see commit message

[Scott Moser (smoser)]

Changes compared to https://code.launchpad.net/~raharper/cloud-initramfs-tools/+git/cloud-initramfs-tools/+merge/389177

a.) only generate "source_fstab" if read-only on kernel cmdline.
b.) debug messages and better hint left in /etc/fstab that this happened.
c.) do not read /proc/mounts, but read initramfs-provided globals and /dev/root for fstab

I'm not opposed to reading /proc/mounts, it just seems like unneccessary work.

[Ryan Harper (raharper)]

Thanks, this is great improvement. I'm +1 on this as-is, I left some in-line comments/suggestions.

review: Approve

[Scott Moser (smoser)]

This is untested...
so before landing, it'd be good to have someone test it.

[Ryan Harper (raharper)]

Thanks for the update. I can test.

[Ryan Harper (raharper)]

OK, so rooturl, modifies ROOTFLAGS for the mount command.

{
    echo 'ROOTFSTYPE="root_url"'
    echo "ROOTFLAGS=\"-o move\""
    echo "ROOT=\"$rootmnt.tmp\""
}

and then '-o move' ends up in source_fstab and then is carried through. systemd-fstab-generator won't parse it so; with the ro option this leaves the rootfs in ro mode. Even if we didn't have the remount,ro any mount commands that attempt to parse fstab will fail.

/proc/mount parsing may be better here since it wont contain anything that fstab doesn't expect.

review: Needs Fixing

[Scott Moser (smoser)]

force-pushed over the top with parsing of /proc/mounts

probably just using '/dev/root / auto defaults 0 0' would have been sufficient.

I'm happy to go back tot hat if you want.

[Ryan Harper (raharper)]

Yes, either work. I don't yet see the update here.

[Ryan Harper (raharper)]

There we go; let me test

[Ryan Harper (raharper)]

Yeah, this works correctly with a local and an VMtest (rooturl).

review: Approve

diff --git a/overlayroot/scripts/init-bottom/overlayroot b/overlayroot/scripts/init-bottom/overlayroot
index c13c02b..43d9a83 100755
--- a/overlayroot/scripts/init-bottom/overlayroot
+++ b/overlayroot/scripts/init-bottom/overlayroot
@@ -299,6 +299,32 @@ get_workdir() {
 	clean_path "${root_rw}/${dir_prefix%/}-workdir/${file}"
 }
 
+gen_fstab() {
+	# gen_fstab(mp) - generate a fabricated /etc/fstab for mount point $mp
+	local mp="$1" m_spec="" m_opts="" m_fstype=""
+	local spec file vfstype opts pass freq
+
+	# just remove any trailing /
+	[ "$mp" != "/" ] && mp=${mp%/}
+	while read spec file vfstype opts pass freq; do
+		[ "$file" = "$mp" ] && {
+			m_spec="$spec"
+			m_opts="$opts"
+			m_fstype="$vfstype"
+			break
+		}
+	done </proc/mounts
+	[ -z "$m_spec" ] && {
+		log_warn "did not find root mount point $mp in /proc/mounts"
+		m_opts="/dev/root"
+		m_opts="defaults"
+		m_fstype="auto"
+	}
+
+	echo "# fabricated by overlayroot, rootfs did not contain /etc/fstab"
+	echo "${m_spec} / ${m_fstype} ${m_opts} 0 0 "
+}
+
 overlayrootify_fstab() {
 	# overlayrootify_fstab(input, root_ro, root_rw, dir_prefix, recurse, swap)
 	# read input fstab file, write an overlayroot version to stdout
@@ -312,6 +338,18 @@ overlayrootify_fstab() {
 	
 	[ -f "$input" ] || return 1
 
+    cat <<EOF
+#
+#  This fstab is for overlayroot. The real one can be found at
+#  ${root_ro}/etc/fstab
+#  The original entry for '/' and other mounts have been updated to be placed
+#  under $root_ro.
+#  To permanently modify this (or any other file), you should change-root into
+#  a writable view of the underlying filesystem using:
+#      sudo overlayroot-chroot
+#
+EOF
+
 	needs_workdir && needs_workdir=true || needs_workdir=false
 	while read spec file vfstype opts pass freq; do
 		[ "$file" = "/" ] && noauto="noauto" || noauto=""
@@ -660,6 +698,10 @@ OVERLAYROOT_DEBUG=${_RET_common_debug:-${OVERLAYROOT_DEBUG}}
 dir_prefix=${_RET_common_dir:-"/overlay"}
 overlayroot_driver=${_RET_common_driver}
 
+read cmdline </proc/cmdline
+cmdline=" $cmdline "
+[ "${cmdline#* ro }" != "$cmdline" ] && cmdline_ro=true || cmdline_ro=false
+
 if [ "${overlayroot_driver:-auto}" = "auto" ]; then
 	search_fs_driver overlay overlayfs ||
 		fail "Unable to find a driver. searched: overlay overlayfs"
@@ -791,23 +833,23 @@ mount --move "${root_rw}" "${ROOTMNT}${root_rw}" ||
 # Remember we are still on the initramfs root fs here, so we have to work
 # on ${ROOTMNT}/etc/fstab. The original fstab is
 # ${ROOTMNT}${root_ro}/etc/fstab.
-cat <<EOF >${ROOTMNT}/etc/fstab
-#
-#  This fstab is in an ${overlayroot_driver}. The real one can be found at
-#  ${root_ro}/etc/fstab
-#  The original entry for '/' and other mounts have been updated to be placed
-#  under $root_ro.
-#  To permanently modify this (or any other file), you should change-root into
-#  a writable view of the underlying filesystem using:
-#      sudo overlayroot-chroot
-#
-EOF
+source_fstab="$ROOTMNT/${root_ro}/etc/fstab"
+if [ ! -f "$source_fstab" ] && [ "$cmdline_ro" = "true" ]; then
+	debug "rootfs did not contain /etc/fstab, creating" \
+		"based on kernel cmdline"
+	source_fstab="${TEMP_D}/fstab"
+	gen_fstab "${ROOTMNT}${root_ro}" > "$source_fstab" ||
+		log_fail "gen_fstab failed for $ROOTMNT/$root_ro"
+fi
 
-[ $? -eq 0 ] || log_fail "failed to modify /etc/fstab (step 1)"
-overlayrootify_fstab "${ROOTMNT}${root_ro}/etc/fstab" "$root_ro" \
-	"$root_rw" "$dir_prefix" "$recurse" "$swap" "${overlayroot_driver}" \
-	>> "${ROOTMNT}/etc/fstab" ||
-	log_fail "failed to modify /etc/fstab (step 2)"
+if [ -f "$source_fstab" ]; then
+	overlayrootify_fstab "$source_fstab" "$root_ro" \
+		"$root_rw" "$dir_prefix" "$recurse" "$swap" "${overlayroot_driver}" \
+		>> "${ROOTMNT}/etc/fstab" ||
+		log_fail "failed to modify /etc/fstab"
+else
+	debug "rootfs did not contain /etc/fstab."
+fi
 
 # we have to make the directories in ${root_rw} because if they do not
 # exist, then the 'upper=' argument to overlayfs will fail.
@@ -822,9 +864,7 @@ fi
 
 # if / is supposed to be mounted read-only (cmdline with 'ro')
 # then mount our overlayfs as read-only just to be more normal
-read cmdline < /proc/cmdline
-cmdline=" $cmdline "
-if [ "${cmdline#* ro }" != "$cmdline" ]; then
+if [ "${cmdline_ro}" = "true" ]; then
 	mount -o remount,ro "$ROOTMNT" ||
 		log_fail "failed to remount overlayroot read-only"
 	debug "mounted $ROOTMNT read-only per kernel cmdline"