Any trivial usage of hostfs seems to be broken since commit cd140ce9 ("hostfs: convert hostfs to use the new mount API") - that's what it bisected down to.
Steps to reproduce;
The following assumes that the ARCH=um kernel has already been compiled (and the 'vmlinux' executable is in the local directory, as is the case when building from the top directory of a source tree). I built mine from a fresh clone using 'defconfig'. The uml_run.sh script creates a bootable root FS image (from debian, via docker) and then boots it with a hostfs mount to demonstrate the regression. This should be observable with any other bootable image though, simply pass "hostfs=<hostpath>" to the ./vmlinux kernel and then try to mount it from within the booted VM ("mount -t hostfs none <guestpath>").
The following 3 text files are used, and as they're small enough for copy-n-paste I figured (hoped) it was best to inline them rather than post attachments.
uml_run.sh: #!/bin/bash set -ex cat Dockerfile | docker build -t foobar:foobar - docker export -o foobar.tar \ `docker run -d foobar:foobar /bin/true` dd if=/dev/zero of=rootfs.img \ bs=$(expr 2048 * 1024 * 1024 / 512) count=512 mkfs.ext4 rootfs.img sudo ./uml_root.sh cp rootfs.img temp.img dd if=/dev/zero of=swapfile bs=1M count=1024 chmod 600 swapfile mkswap swapfile ./vmlinux mem=4G ubd0=temp.img rw ubd1=swapfile \ hostfs=$(pwd)
uml_root.sh: #!/bin/bash set -ex losetup -D LOOPDEVICE=$(losetup -f) losetup ${LOOPDEVICE} rootfs.img mkdir -p tmpmnt mount -t auto ${LOOPDEVICE} tmpmnt/ (cd tmpmnt && tar xf ../foobar.tar) umount tmpmnt losetup -D
Dockerfile: FROM debian:trixie RUN echo 'debconf debconf/frontend select Noninteractive' | \ debconf-set-selections RUN apt-get update RUN apt-get install -y apt-utils RUN apt-get -y full-upgrade RUN echo "US/Eastern" > /etc/timezone RUN chmod 644 /etc/timezone RUN cd /etc && rm -f localtime && \ ln -s /usr/share/zoneinfo/US/Eastern localtime RUN apt-get install -y systemd-sysv kmod RUN echo "root:root" | chpasswd RUN echo "/dev/ubdb swap swap defaults 0 0" >> /etc/fstab RUN mkdir /hosthack RUN echo "none /hosthack hostfs defaults 0 0" >> /etc/fstab RUN systemctl set-default multi-user.target
Execute ./uml_run.sh to build the rootfs image and boot the VM. This requires a system with docker, and will also require a sudo password when creating the rootfs. The boot output indicates whether the hostfs mount succeeds or not - the boot should degrade to emergency mode if the mount fails, otherwise a login prompt indicates success. (Login is root:root, e.g. if you prefer to go in and shutdown the VM gracefully.)
Please let me know if I can/should provide anything else.
Cheers, Geoff