[Bug 235322] it should be possible to remount nullfs mount from readonly to writable and back

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235322

            Bug ID: 235322
           Summary: it should be possible to remount nullfs mount from
                    readonly to writable and back
           Product: Base System
           Version: 12.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: eto.freebsd at ethome.sk

This has been bugging me for quite some time, but it seems, that it is
impossible to remount nullfs mount from "ro" to "rw" and back, without actually
unmounting it first and then remounting it back the other way. Since nobody
seems to know or cares about it, I am creating this bug.

Case in point:

01 # mkdir ~/test
02 # mount -o ro -t nullfs /etc /usr/home/eto/test

So far all is good, but now:

01 # mount -o rw /usr/home/eto/test
02 > mount: /usr/home/eto/test: unknown special file or file system
03 # mount -o rw /etc /usr/home/eto/test
04 > mount: /etc: Block device required

It's been some time when I played with nmount() from C, but if I remember
correctly, one gets "not supported" error when trying to update mount flags.

I tried to look at nullfs code, but it's hard to follow for non-kernel
experienced programmers.

Anyway I believe sysadmins should be able to change writability flags of nuffs
mounts (like when updating jails). "ro" mounted jails are very sturdy and
protected against unexpected modifications, but are quite the hassle during
base updates.

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