On Fri, Jun 07, 2024 at 04:55:33PM +0200, Christian Brauner wrote: > util-linux is about to implement listmount() and statmount() support. > Karel requested the ability to scan the mount table in backwards order > because that's what libmount currently does in order to get the latest > mount first. We currently don't support this in listmount(). Add a new > LISTMOUNT_RESERVE flag to allow listing mounts in reverse order. For > example, listing all child mounts of /sys without LISTMOUNT_REVERSE > gives: > > /sys/kernel/security @ mnt_id: 4294968369 > /sys/fs/cgroup @ mnt_id: 4294968370 > /sys/firmware/efi/efivars @ mnt_id: 4294968371 > /sys/fs/bpf @ mnt_id: 4294968372 > /sys/kernel/tracing @ mnt_id: 4294968373 > /sys/kernel/debug @ mnt_id: 4294968374 > /sys/fs/fuse/connections @ mnt_id: 4294968375 > /sys/kernel/config @ mnt_id: 4294968376 > > whereas with LISTMOUNT_RESERVE it gives: > > /sys/kernel/config @ mnt_id: 4294968376 > /sys/fs/fuse/connections @ mnt_id: 4294968375 > /sys/kernel/debug @ mnt_id: 4294968374 > /sys/kernel/tracing @ mnt_id: 4294968373 > /sys/fs/bpf @ mnt_id: 4294968372 > /sys/firmware/efi/efivars @ mnt_id: 4294968371 > /sys/fs/cgroup @ mnt_id: 4294968370 > /sys/kernel/security @ mnt_id: 4294968369 > > A few smaller cleanups included in this series. > I have some remarks about listmount, they are not patchset-specific though. If I'm reading things correctly put_user is performed with namespace_sem held? While it probably works, it is fundamentally unsound behavior -- said put_user introduces funky lock orderings which don't need to be there and can go off cpu indefinitely waiting for the fault to be serviced. Either the code needs to pre-wire user memory or it needs to export to userspace in chunks while dropping the lock in-between (say 16 or whatever IDs per batch, something stack-tolerable).
