9 Feb
2018
9 Feb
'18
12:59 a.m.
On Wed, Feb 7, 2018, at 17:48, Ron Natalie wrote:
Amusingly, we still found bugs. I was griping that
people would mount
on just any directory on the system, the mtab wouldn’t show the absolute
path. I suggested we chdir to / early on or better yet, cd to /dev.
That way I wouldn’t have to type /dev/ on to the device name. I say
the user would have to give an absolute path anyhow since there’s
nothing on /dev to mount on. Well my roommate immediately tries to
prove me wrong. He tries passing various devnodes and regular files
that happen to sit in /dev. Then he gets the great idea of mounting
on /dev/. It works, but now we have no way to unmount it. We had
to reboot the machine. I quickly modified mount to require an EMPTY
directory owned by the user to be required for mounting.
Would a mknod (in the new /dev or anywhere else) to "recreate" the device file
have worked? Requiring an empty directory seems a bit overkill vs simply requiring an
absolute path (perhaps one that does not begin with /dev) or having the mount command
calculate the absolute path.
(I'm mildly surprised that if this was in the era where . and .. were simply
hardlinks created by mkdir(1), mounting on /dev/. didn't simply literally mount on
/dev/. leaving /dev alone, which would have been an interesting state but wouldn't
have prevented unmounting)
Why is it that umount(2) took the device special file name rather than the mount point
directory name, anyway? This seems to have been fixed in NFSv2, 4.3BSD-Uwisc, and Linux,
but why was it like that in the first place? It *seems* like in the V6 era code the system
could have just as well checked m_inodp for a match to the directory as m_dev for the
device.