[Bug 210316] panic after trying to r/w mount msdosfs on write protected media

[Bruce Evans]

On Thu, 17 May 2018, Bruce Evans wrote:

> On Thu, 17 May 2018 a bug that doesn't want replies at freebsd.org wrote:
>
>> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210316
>> 
>> --- Comment #14 from Andriy Gapon <avg at FreeBSD.org> ---
>> (In reply to Conrad Meyer from comment #13)
>> 
>> Indeed, if we talk about the general behaviour.
>> 
>> I see that I utterly failed to explain that I was thinking purely in a 
>> context
>> of what msdos does in markvoldirty.
> ...
> Markvoldirty() was obtained from apple and fixed a bit by me, but is still
> very bad, without even this write protection bug.
> ...

I forgot to mention its main bug.  msdosfs doesn't check the dirty flag, so
it allows even rw mounts of dirty file systems (and I think it is too
dangerous to allow even ro mounts for fs's with FAT).  So maintaining the
dirty flag is useless for FreeBSD.  It is just a courtesy for OS'es that do
check the flag.

Bruce