fs: find rootfs mount of the mount namespace

fs: find rootfs mount of the mount namespace

The method we used was predicated on the assumption that the mount
immediately following the root mount of the mount namespace would be the
rootfs mount of the namespace. That's not always the case though. For
example:

ID  PARENT ID
408 412       0:60 /containers/overlay-containers/bc391117192b32071b22ef2083ebe7735d5c390f87a5779e02faf79ba0746ceb/userdata/hosts /etc/hosts rw,nosuid,nodev,relatime - tmpfs tmpfs rw,size=954664k,nr_inodes=238666,mode=700,uid=1000,gid=1000,inode64
409 414       0:61 / /dev/shm rw,nosuid,nodev,noexec,relatime - tmpfs shm rw,size=64000k,uid=1000,gid=1000,inode64
410 412       0:60 /containers/overlay-containers/bc391117192b32071b22ef2083ebe7735d5c390f87a5779e02faf79ba0746ceb/userdata/.containerenv /run/.containerenv rw,nosuid,nodev,relatime - tmpfs tmpfs rw,size=954664k,nr_inodes=238666,mode=700,uid=1000,gid=1000,inode64
411 412       0:60 /containers/overlay-containers/bc391117192b32071b22ef2083ebe7735d5c390f87a5779e02faf79ba0746ceb/userdata/hostname /etc/hostname rw,nosuid,nodev,relatime - tmpfs tmpfs rw,size=954664k,nr_inodes=238666,mode=700,uid=1000,gid=1000,inode64
412 363       0:65 / / rw,relatime - overlay overlay rw,lowerdir=/home/user1/.local/share/containers/storage/overlay/l/JS65SUCGTPCP2EEBHLRP4UCFI5:/home/user1/.local/share/containers/storage/overlay/l/DLW22KVDWUNI4242D6SDJ5GKCL [...]
413 412       0:68 / /proc rw,nosuid,nodev,noexec,relatime - proc proc rw
414 412       0:69 / /dev rw,nosuid - tmpfs tmpfs rw,size=65536k,mode=755,uid=1000,gid=1000,inode64
415 412       0:70 / /sys ro,nosuid,nodev,noexec,relatime - sysfs sysfs rw
416 414       0:71 / /dev/pts rw,nosuid,noexec,relatime - devpts devpts rw,gid=100004,mode=620,ptmxmode=666
417 414       0:67 / /dev/mqueue rw,nosuid,nodev,noexec,relatime - mqueue mqueue rw
418 415       0:27 / /sys/fs/cgroup ro,nosuid,nodev,noexec,relatime - cgroup2 cgroup2 rw,nsdelegate,memory_recursiveprot
419 414       0:6 /null /dev/null rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
420 414       0:6 /zero /dev/zero rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
422 414       0:6 /full /dev/full rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
423 414       0:6 /tty /dev/tty rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
430 414       0:6 /random /dev/random rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
431 414       0:6 /urandom /dev/urandom rw,nosuid,noexec - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
433 413       0:72 / /proc/acpi ro,relatime - tmpfs tmpfs rw,size=0k,uid=1000,gid=1000,inode64
440 413       0:6 /null /proc/kcore ro,nosuid - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
441 413       0:6 /null /proc/keys ro,nosuid - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
442 413       0:6 /null /proc/timer_list ro,nosuid - devtmpfs devtmpfs rw,size=4096k,nr_inodes=1179282,mode=755,inode64
443 413       0:73 / /proc/scsi ro,relatime - tmpfs tmpfs rw,size=0k,uid=1000,gid=1000,inode64
444 415       0:74 / /sys/firmware ro,relatime - tmpfs tmpfs rw,size=0k,uid=1000,gid=1000,inode64
445 415       0:75 / /sys/dev/block ro,relatime - tmpfs tmpfs rw,size=0k,uid=1000,gid=1000,inode64
446 413       0:68 /bus /proc/bus ro,nosuid,nodev,noexec,relatime - proc proc rw
447 413       0:68 /fs /proc/fs ro,nosuid,nodev,noexec,relatime - proc proc rw
448 413       0:68 /irq /proc/irq ro,nosuid,nodev,noexec,relatime - proc proc rw
449 413       0:68 /sys /proc/sys ro,nosuid,nodev,noexec,relatime - proc proc rw
450 413       0:68 /sysrq-trigger /proc/sysrq-trigger ro,nosuid,nodev,noexec,relatime - proc proc rw
364 414       0:71 /0 /dev/console rw,relatime - devpts devpts rw,gid=100004,mode=620,ptmxmode=666

In this mount table the root mount of the mount namespace is the mount
with id 363 (It isn't visible because it's literally just what the
rootfs mount is mounted upon and usually it's just a copy of the real
rootfs).

The rootfs mount that's mounted on the root mount of the mount namespace
is the mount with id 412. But the mount namespace contains mounts that
were created before the rootfs mount and thus have earlier mount ids. So
the first call to listmnt_next() would return the mount with the mount
id 408 and not the rootfs mount.

So we need to find the actual rootfs mount mounted on the root mount of
the mount namespace. This logic is also present in mntns_install() where
vfs_path_lookup() is used. We can't use this though as we're holding the
namespace semaphore. We could look at the children of the root mount of
the mount namespace directly but that also seems a bit out of place
while we have the rbtree. So let's just iterate through the rbtree
starting from the root mount of the mount namespace and find the mount
whose parent is the root mount of the mount namespace. That mount will
usually appear very early in the rbtree and afaik there can only be one.
IOW, it would be very strange if we ended up with a root mount of a
mount namespace that has shadow mounts.

Fixes: 0a3deb11858a ("fs: Allow listmount() in foreign mount namespace") # mainline only
Signed-off-by: Christian Brauner <brauner@kernel.org>