mbuto has two ways of building the initramfs.  One is the typical approach
of staging its contents in a temporary directory, then building the
initramfs with cpio.  The other is to create an empty initramfs, mount
it with archivemount, and copy things into the mounted archive.

However, the archivemount approach is broken.  I'm not entirely sure why,
but it appears not to properly unmount the archive and retrieve the final.
filled version.  The upshot is that if archivemount is installed, then
mbuto generates an empty, gzip-compressed initramfs instead of whatever it
was supposed to.

It appears the archivemount approach was there to allow some later changes
to the archive without re-running mbuto.  However, it's essentially
impossible to use this safely - if archivemount still has the archive open
for writing, it's not safe to read it concurrently.  This appears to have
worked by accident with some earlier versions, but doesn't work now.

Just remove the archivemount approach entirely.

Reported-by: Laurent Vivier <lvivier(a)redhat.com>

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 mbuto | 41 +++++------------------------------------
 1 file changed, 5 insertions(+), 36 deletions(-)

diff --git a/mbuto b/mbuto
index a6f80ab..49d032c 100755
--- a/mbuto
+++ b/mbuto
@@ -368,7 +368,6 @@ cmd_check() {
 
 # cleanup() - Remove left-overs on exit, used from trap
 cleanup() {
-	[ -n "${ARCHIVEMOUNT}" ] && "${UMOUNT}" "${wd}"
2>/dev/null
 	"${RM}" -rf "${wd}"
 	[ -n "${pkg_tmp}" ] && "${RM}" -f "${pkg_tmp}"
 	[ -n "${compress_test1}" ] && "${RM}" -f
"${compress_test1}"
@@ -564,16 +563,6 @@ cpio_init() {
 		else
 			OUT="$("${REALPATH}" "${OUT}")"
 		fi
-
-		if [ -n "${ARCHIVEMOUNT}" ]; then
-			: | "${CPIO}" --create -H newc --quiet | \
-				"${GZIP}" > "${OUT}"
-		fi
-	fi
-
-	if [ -n "${ARCHIVEMOUNT}" ]; then
-		"${ARCHIVEMOUNT}" "${OUT}" "${wd}"
-		info "Mounted CPIO archive ${OUT} at ${wd}"
 	fi
 }
 
@@ -1099,26 +1088,17 @@ cmds() {
 
 	[ "${NOSTRIP}" != "y" ] && strip_all
 
-	if [ -z "${ARCHIVEMOUNT}" ]; then
-		( __out="$("${REALPATH}" "${OUT}")"
-		  "${CD}" "${wd}"
-		  "${FIND}" . | "${CPIO}" --create -H newc --quiet >
"${OUT}"
-		  cpio_compress "${__out}"
-		)
-	else
-		"${SYNC}"
-	fi
+	( __out="$("${REALPATH}" "${OUT}")"
+		"${CD}" "${wd}"
+		"${FIND}" . | "${CPIO}" --create -H newc --quiet >
"${OUT}"
+		cpio_compress "${__out}"
+	)
 
 	stats
 
 	printf "%s" "${OUTPUT}"						| \
 		sed 's,__INITRD__,'"$("${REALPATH}"
"${OUT}")"',g'	| \
 		sed 's,__KERNEL__,/boot/vmlinuz-'"${KERNEL}"',g'
-
-	if [ -n "${ARCHIVEMOUNT}" ]; then
-		trap - EXIT
-		notice "initramfs mounted at: ${wd}"
-	fi
 }
 
 # usage() - Print usage and exit
@@ -1271,17 +1251,6 @@ else
 	eval "profile_${PROFILE}" || err "profile ${PROFILE} not found"
 fi
 
-# Check if we can keep the CPIO mounted for convenience as we exit. This isn't
-# safe with fakeroot, as contents can't be touched before the environment
-# save-file is loaded again, so it needs root (and archivemount).
-if [ "${LD_PRELOAD}" = "libfakeroot-sysv.so" ]; then
-	if command -v archivemount >/dev/null 2>&1; then
-		notice "Not running as root, won't keep cpio mounted"
-	fi
-elif ! ARCHIVEMOUNT="$(command -v archivemount)"; then
-	warn "archivemount not available, won't keep cpio mounted"
-fi
-
 trap cleanup EXIT
 
 cmds "$@"
-- 
2.44.0

The cpio_init function is now only every called with the "discard" option.
But, moreover, what it does is create an initial mostly empty archive which
will just get overwritten by the final archive.

So, it's entirely unnecessary except for one subtlety.  Our use of realpath
when generating the final output requires that a file already exist in the
output location.  We can fix that by shuffling some things out of a
subshell, removing the need for realpath.

This removes functionality to modify an existing initramfs image.  However
that was already broken in a several ways.  Just remove it, and update the
help messages to match.

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 mbuto | 39 +++++++--------------------------------
 1 file changed, 7 insertions(+), 32 deletions(-)

diff --git a/mbuto b/mbuto
index eb559fb..247b63d 100755
--- a/mbuto
+++ b/mbuto
@@ -544,28 +544,6 @@ subopts_get() {
 
 ### CPIO #######################################################################
 
-# cpio_init() - Source existing CPIO archive, or create if needed
-# $1:	Path to CPIO archive, might exist, might be discarded if existing
-cpio_init() {
-	if [ -f "${OUT}" ] && [ "${1}" != "discard" ];
then
-		info "Sourcing CPIO archive from ${OUT}"
-
-		if ! "${GZIP}" -dfc "${OUT}" |
-		   "${CPIO}" --quiet -iD "${wd}"; then
-			err "Invalid CPIO archive ${OUT}"
-		fi
-	else
-		info "Creating new CPIO archive"
-
-		if [ -z "${OUT}" ]; then
-			OUT="$("${MKTEMP}")"
-			notice "Creating image: ${OUT}"
-		else
-			OUT="$("${REALPATH}" "${OUT}")"
-		fi
-	fi
-}
-
 # compress_select() - Try compressors and pick the fastest to decompress image
 # $1:	Existing CPIO archive
 compress_select() {
@@ -987,7 +965,6 @@ pkg_add() {
 
 # build() - Build a new image, sourcing contents
 build() {
-	cpio_init discard
 	kmod_init
 
 	for __d in ${DIRS}; do
@@ -1083,11 +1060,11 @@ cmds() {
 
 	[ "${NOSTRIP}" != "y" ] && strip_all
 
-	( __out="$("${REALPATH}" "${OUT}")"
+	(
 		"${CD}" "${wd}"
-		"${FIND}" . | "${CPIO}" --create -H newc --quiet >
"${OUT}"
-		cpio_compress "${__out}"
-	)
+		"${FIND}" . | "${CPIO}" --create -H newc --quiet
+	) > "${OUT}"
+	cpio_compress "${OUT}"
 
 	stats
 
@@ -1139,9 +1116,9 @@ usage() {
 	echo "	-v: verbose"
 	echo "	-h: show this help"
 	echo
-	echo "Build initramfs image unless an existing one is passed."
-	echo "Additional programs, kernel modules, device nodes, generic files"
-	echo "can be passed as a list of ADD_ON parameters."
+	echo "Build initramfs image.  Additional programs, kernel modules,"
+	echo "device nodes, generic files can be passed as a list of ADD_ON"
+	echo "parameters."
 	echo
 	echo "Distribution packages (deb and RPM currently supported) can be"
 	echo "added too, but they will simply be extracted, not installed."
@@ -1173,8 +1150,6 @@ usage() {
 	echo "		Build a base image as a temporary file"
 	echo "	${0} grep"
 	echo "		Build a new image including grep and needed libraries"
-	echo "	${0} -f kata.img zsh_5.6.2-3_amd64.deb"
-	echo "		Install zsh package to pre-existing kata.img"
 	echo "	${0} -v -f kata.img -p kata_debug -c lz4"
 	echo "		Use lz4 compression, run a shell before proceeding"
 	exit 1
-- 
2.44.0

On Fri, 26 Apr 2024 12:01:04 +1000
David Gibson <david(a)gibson.dropbear.id.au> wrote:

...
  While debugging a problem Laurent Vivier was having
with the passt
 test suite, I discovered mbuto's archivemount support is entirely
 broken: if archivemount is on the system, mbuto won't work.  While
 fixing that, I discovered some slightly related simplfications that
 can be made.  Here they all are.
 
 Changes since v1:
  * Updated commit message in 2/4 to clarify why the desired advantages
    of using archivemount can't be used in practice
  * Improve accuracy of comment on compress_select in 3/4
  * Clarify removal of update existing archive functionality.
 
 David Gibson (4):
   ${wd} is always set, no need to test for it
   Remove stale archivemount support
   Split "auto" compression mode into its own path
   Remove unnecessary cpio_init function 
Applied, thanks.

-- 
Stefano