Bug#804944: Acknowledgement (dtach seems broken on arm64)

[David Bremner]

It seems this is not a problem on all arm64 boxes. On IRC suihkulokki
found that it worked on his self compiled 4.2 kernel.  Some poking about
with strace revealed that in the cases where dtach worked (whether
arm64, or my amd64 box), statfs("/dev/pts") returns DEVPTS_SUPER_MAGIC,
while in the case where it fails it returns TMPFS_MAGIC. Not that I know
what this signifies.