switch-root would only mkdir the parents of the mount move directories.
With this patch, the mount move target directory is created to make
switch-root to an empty root directory work.
The unmount occurs after the pivot_root, so the path used to unmount
should take into account the new root directory. This will allow the
umount to actually succeed.
safe_close() automatically becomes a NOP when a negative fd is passed,
and returns -1 unconditionally. This makes it easy to write lines like
this:
fd = safe_close(fd);
Which will close an fd if it is open, and reset the fd variable
correctly.
By making use of this new scheme we can drop a > 200 lines of code that
was required to test for non-negative fds or to reset the closed fd
variable afterwards.
We should always try to umount the old root dir if possible, instead of
overmounting it -- if that's possible.
The initial ("first") kernel rootfs can never be umounted, hence
for the usual nitrd case we never bothered using pivot_root() and
hence with fully unmounting it. However, fedup now tranisitions twice
during boot, and in that case it is highly desirable that the "second"
root dir is entirely unmounted when we switch to the "third". This patch
makes that possible.
The pivot_root() needs a directory in the "third" root dir, to move the
"second" root dir to. We use /mnt for that, under the assumption that
this directory is likely to exist, and is not itself a mount point.
- don't use pivot_root() anymore, just reuse root hierarchy
- first create all mounts, then mark them read-only so that we get the
right behaviour when people want writable mounts inside of
read-only mounts
- don't pass invalid combinations of MS_ constants to the kernel
The kernel does not allow switching roots if things are mounted
MS_SHARED. As a work-around, remount things MS_PRIVATE before switching
roots.
This should be fixed in the kernel for good.
https://bugzilla.redhat.com/show_bug.cgi?id=847418
