[Bug 254005] Attempting to mount a slice containing a r/o mounted partition may make the mounted filesystem inaccessible

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254005

            Bug ID: 254005
           Summary: Attempting to mount a slice containing a r/o mounted
                    partition may make the mounted filesystem inaccessible
           Product: Base System
           Version: 11.4-STABLE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: ardovm at yahoo.it

When a BSD partition is mounted _read_only_ to / (suppose /dev/da0s2a), if I
try to mount its containing slice (/dev/da0s2) I receive a ``strange'' error
message, and from that moment the mounted filesystem becomes unreadable.

This bug is observed on memstick images from both:
 - 11.4-STABLE r369279 (built from source),
 - 12.2-RELEASE as downloaded from official mirror.

I tested the following scenarios, when attempting to mount /dev/da0s2:

 A- If the partition is mounted r/o from /dev/da0s2a, then mount(8) reports the
``strange'' message and the system becomes unstable (bug).

 B- If the partition is mounted from /dev/ufs/label, then mount(8) reports
"Operation not permitted" and the system remains stable. This is the expected
behavior IMHO.

 C- If the partition is mounted read_write, from any special device, then
mount(8) reports:
    - ``Operation not permitted'' if I try to mount the slice rw and the system
remains stable (expected behavior);
    - the ``strange'' error message, if I try to mount the slice ro, and the
system remains stable (tolerable behavior).

The "strange error message" is "invalid argument" on 11.4-STABLE.


Steps to reproduce:

 1- flash the memstick image into a USB pen drive

 2- edit /boot/loader.conf on the memstick adding the following lines (needed
to boot successfully on my test system, I am not sure they have any influence
on this problem):
    kern.vty=sc
    kern.cam.boot_delay=10000
    kern.cam.scsi_delay=10000 

 3- edit /etc/fstab on the memstick and change the root device from
    /dev/ufs/FreeBSD_Install 
    to 
    /dev/da0s2a [or whatever device the USB pen drive will be assigned to]

 4- boot the memstick and open a shell

 5- # mount /dev/da0s2 /mnt
    mount: /dev/da0s2: No such file or directory <--- strange message!


Additional information:

 - the partition /dev/da0s2a is offset 16 blocks from the beginning of its
containing slice /dev/da0s2, as per mkimg(1) default behavior.

 - the problem should also appear on i386 (I can check thoroughly if
necessary).


IMHO this is a form of foot-shooting and we can claim that root should be
allowed to ruin their own system. But it's worth mentioning that there _is_ a
safety check in place, that is only active when the partition is mounted from
/dev/ufs/label instead of /dev/da0s2. This is counter-intuitive.

I remain available to give further information and/or conducting further tests.

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