Re: [PATCH 0/2] mount: add OPEN_TREE_NAMESPACE

[Rob Landley <rob@landley.net>]

>>>> You want rootfs to be a NULLFS instead of ramfs. You don't seem to want it to
>>>> actually _be_ a filesystem. Even with your "fix", containers could communicate
>>>> with each _other_ through it if it becomes accessible. If a container can get
>>>> access to an empty initramfs and write into it, it can ask/answer the question
>>>> "Are there any other containers on this machine running stux24" and then coordinate.

Or you could just make the ROOT= codepath remount the empty initramfs -o 
ro like some switch_root implementations do. If the PID 1 you launch 
isn't in initramfs, don't leave initramfs writeable. That seems unlikely 
to break userspace.

(Having permissions to remount initramfs but _not_ having already 
"cracked root" seems... a bit funky? You have waaaaay more faith in 
security modules than I do...)

>> I think this new OPEN_TREE_NAMESPACE is nifty, but I don't think the
>> path that gives it sensible behavior should be conditional like this.
>> Either make it *always* mount on top of nullfs (regardless of boot
>> options) or find some way to have it actually be the root.  I assume
>> the latter is challenging for some reason.
> 
> I think that's the plan. I suggested the same to Christian last week,
> and he was amenable to removing the option and just always doing a
> nullfs_rootfs mount.

Since 2013, initramfs might be ramfs or tmpfs depending on 
circumstances. Adding a third option for it be nullfs when there's no 
cpio.gz extracted into it seems reasonable. (You can always mount a 
tmpfs _over_ it if you need that later, it's writeable so a PID 1 
launched in it has workspace.)

That said, if you are changing the semantics, right now we switch_root 
from initramfs instead of pivot_root because initramfs couldn't be 
unmounted. With this change would pivot_root become the mechanism for 
initramfs too? (If the cpio.gz recipient wasn't actually rootfs but was 
an overmount the way ROOT= does it.)

Aside: it would be nice if inaccessible mount points could automatically 
be garbage collected. There's already some "lazy umount" plumbing that 
does that when explicitly requested to, but last I checked there were 
cases that didn't get caught. It's been a while though, might already 
have been fixed. Presumably initramfs would always get pinned because 
it's PID 0's / reference...

Also, could you guys make CONFIG_DEVTMPFS_MOUNT work with initramfs? 
I've posted patches for that on and off since 2017, most recent one's 
probably 
https://landley.net/bin/mkroot/0.8.13/linux-patches/0003-Wire-up-CONFIG_DEVTMPFS_MOUNT-to-initramfs.patch 
(tested on a 6.17 kernel).

> We think that older runtimes should still "just work" with this scheme.
> Out of an abundance of caution, we _might_ want a command-line option
> to make it go back to old way, in case we find some userland stuff that
> doesn't like this for some reason, but hopefully we won't even need
> that.

I assume it will break stuff, but I also assume the systems it breaks 
will never upgrade to a 7.x kernel because the kernel itself would 
consume all available memory before launching PID 1.

Rob