Trying to figure out why my schroot builds keep failing after
upgrading to jessie, I finally narrowed it down to broken behaviour with
mount on jessie:
} root@valiant:/mnt# find
} .
} ./tmp1
} ./tmp1/dev
} ./tmp0
} ./tmp0/dev
Two trees, with a dev directory each.
Let's mount /dev and /dev/pts there:
} root@valiant:/mnt# /bin/mount -v -t none -o rw,bind /dev tmp0/dev
} mount: /dev bound on /mnt/tmp0/dev.
} root@valiant:/mnt# /bin/mount -v -t none -o rw,bind /dev/pts tmp0/dev/pts
} mount: /dev/pts bound on /mnt/tmp0/dev/pts.
} root@valiant:/mnt# grep /mnt /proc/mounts
} udev /mnt/tmp0/dev devtmpfs
rw,relatime,size=10240k,nr_inodes=3087992,mode=755 0 0
} devpts /mnt/tmp0/dev/pts devpts
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
Seems to have worked. Yay.
Now the second tree:
} root@valiant:/mnt# /bin/mount -v -t none -o rw,bind /dev tmp1/dev
} mount: /dev bound on /mnt/tmp1/dev.
} root@valiant:/mnt# grep /mnt /proc/mounts
} udev /mnt/tmp0/dev devtmpfs
rw,relatime,size=10240k,nr_inodes=3087992,mode=755 0 0
} devpts /mnt/tmp0/dev/pts devpts
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
} udev /mnt/tmp1/dev devtmpfs
rw,relatime,size=10240k,nr_inodes=3087992,mode=755 0 0
So far, so good. Now for /dev/pts:
} root@valiant:/mnt# /bin/mount -v -t none -o rw,bind /dev/pts tmp1/dev/pts
} mount: /dev/pts bound on /mnt/tmp1/dev/pts.
} root@valiant:/mnt# grep /mnt /proc/mounts
} udev /mnt/tmp0/dev devtmpfs
rw,relatime,size=10240k,nr_inodes=3087992,mode=755 0 0
} devpts /mnt/tmp0/dev/pts devpts
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
} udev /mnt/tmp1/dev devtmpfs
rw,relatime,size=10240k,nr_inodes=3087992,mode=755 0 0
} devpts /mnt/tmp1/dev/pts devpts
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
} devpts /mnt/tmp0/dev/pts devpts
rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
Something's fishy here. Note how there is now a second mount on
tmp0/dev/pts.
This is a real problem as exhibited in these two use-cases:
o) you cannot unmount the tmp0 tree while the tmp1 tree is busy:
} root@valiant:/mnt# (cd tmp1/dev/pts ; sleep 10 &)
} root@valiant:/mnt# umount /mnt/tmp0/dev/pts
} umount: /mnt/tmp0/dev/pts: target is busy
} (In some cases useful info about processes that
} use the device is found by lsof(8) or fuser(1).)
o) if it's not busy, and you try to umount the tmp0 tree, you mess with
the tmp1 tree instead:
} root@valiant:/mnt# ls /mnt/tmp0/dev/pts
} 0 1 10 11 13 14 15 16 17 18 19 2 20 21 22 23 24 25 26 3 4
5 6 7 8 ptmx
} root@valiant:/mnt# ls /mnt/tmp1/dev/pts
} 0 1 10 11 13 14 15 16 17 18 19 2 20 21 22 23 24 25 26 3 4
5 6 7 8 ptmx
} root@valiant:/mnt# umount /mnt/tmp0/dev/pts
} root@valiant:/mnt# ls /mnt/tmp0/dev/pts
} 0 1 10 11 13 14 15 16 17 18 19 2 20 21 22 23 24 25 26 3 4
5 6 7 8 ptmx
} root@valiant:/mnt# ls /mnt/tmp1/dev/pts
} root@valiant:/mnt#
It has been suggested on IRC that this is due to systemd mounting
filesystems with O_SHARE. Regardless of why, these two things at the
bottom are horribly broken. We need to fix this somehow.
Cheers,
--
| .''`. ** Debian **
Peter Palfrader | : :' : The universal
https://www.palfrader.org/ | `. `' Operating System
| `- https://www.debian.org/
