Eric Sandeen [off-list ref] writes:

quoted

3) A lot of user even don't now mount ro can still modify device
   Yes, I didn't know this point until I checked the log replay code of
   btrfs.
   Adding such mount option alias may raise some attention of users.

Given that nothing in the documentation implies that the block device itself
must remain unchanged on a read-only mount, I don't see any problem which
needs fixing.  MS_RDONLY rejects user IO; that's all.

If you want to be sure your block device rejects all IO for forensics or
what have you, I'd suggest # blockdev --setro /dev/whatever prior to mount,
and take it out of the filesystem's control.  Or better yet, making an
image and not touching the original.

What we do for the petitboot bootloader in POWER and OpenPower firmware
(a linux+initramfs that does kexec to boot) is that we use device mapper
to make a snapshot in memory where we run recovery (for some
filesystems, notably XFS is different due to journal not being endian
safe). We also have to have an option *not* to do that, just in case
there's a bug in journal replay... and we're lucky in the fact that we
probably do have enough memory to complete replay, this solution could
be completely impossible on lower memory machines.

As such, I believe we're the only bit of firmware/bootloader ever that
has correctly parsed a journalling filesystem.

-- 
Stewart Smith