[Toybox] [landley/toybox] the mount command behaves incorrectly (#49)

[Rob Landley]

On 07/10/2017 02:56 PM, Elliott Hughes wrote:
> another report of this bug today: android-ndk/ndk#380 (comment)
> <https://github.com/android-ndk/ndk/issues/380#issuecomment-306905516>

Blah. Sorry about the delay. (The underfunded startup I work at can't
afford to replace people it lost over the past year, so the rest of us
gained more plates to spin. Very distracting. Yes, I am catching up on
this at 4am.)

The simple fix is:

-  remount = (void *)(long)comma_scan(opts, "remount", 1);
+  remount = (void *)(long)comma_scan(opts, "remount", 0);

The problem is if comma_scan removes "remount" from the option string
during early parsing (setting the "remount" flag that changes the
behavior to do /proc/mounts lookup to find existing flags to delta
against), then we never set the remount flag when collecting the set of
flags to apply as a delta, meaning it tries to do a _new_ mount over the
existing mount, which either gives you -EBUSY or stacks a new mount on
top of the old mount (for tmpfs).

The deeper question is "why the heck does rw,remount work then"? And the
answer is that comma_scan() is failing to remove entries that match at
the end of the list. (Ok, fixed. And that _failure_ is why it passed my
initial testing way back when. Sigh...)

Hmmm... if /proc/filesystems says that a filesystem type is nodev, I
don't _have_ to require two arguments to mount it. I.E. there's no reason:

  mount -t tmpfs sub

Can't work? Try the fstab lookup first, but if it's not there just use
basename of sub as the device?

Hmmm... that works for tmpfs, ramfs, and devtmpfs, and ntfs claims to
need a device, but cifs and fuse are nodev and I'm pretty sure they
care. (Use the "device" field as other info).

Meanwhile the insane v9fs expects the address to live in the options as
special arguments in the csv, because IBM. Sigh... (I need to write
nfsmount, cifsmount, and v9fsmount commands. For two of those I could
even do a reasonable server. It's on the todo list. But not at this time
of morning. :)

Rob