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switchroot: Stop making /sysroot mount private
Back in 2b8d586, /sysroot was changed to be a private mount so that submounts of /var do not propagate back to the stateroot /var. That's laudible, but it makes /sysroot different than every other shared mount in the root namespace. In particular, it means that submounts of /sysroot do not propagate into separate mount namespaces. Rather than make /sysroot private, make /var a slave+shared mount so that it receives mount events from /sysroot but not vice versa. That achieves the same effect of preventing /var submount events from propagating back to /sysroot while allowing /sysroot mount events to propagate forward like every other system mount. See mount_namespaces(7)[1] and the linux shared subtrees[2] documentation for details on slave+shared mount propagation. When /var is mounted in the initramfs, this is accomplished with mount(2) syscalls. When /var is mounted after switching to the real root, the mount propagation flags are applied as options in the generated var.mount unit. This depends on a mount(8) feature that has been present since util-linux 2.23. That's available in RHEL 7 and every non-EOL Debian and Ubuntu release. Applying the propagation from var.mount fixes a small race, too. Previously, if a /var submount was added before /sysroot was made private, it would have propagated back into /sysroot. That was possible since ostree-remount.service orders itself after var.mount but not before any /var submounts. 1. https://man7.org/linux/man-pages/man7/mount_namespaces.7.html 2. https://docs.kernel.org/filesystems/sharedsubtree.html Fixes: #2086
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