Re: [PATCH v1 0/7] Remove in-tree usage of MAP_DENYWRITE

J. Bruce Fields
2021-08-19T14:33:50Z

On Thu, Aug 19, 2021 at 08:56:52AM -0500, Eric W. Biederman wrote:
> bfields@fieldses.org (J. Bruce Fields) writes:
>
> > On Fri, Aug 13, 2021 at 05:49:19PM -0700, Andy Lutomirski wrote:
> >> I’ll bite. How about we attack this in the opposite direction: remove
> >> the deny write mechanism entirely.
> >
> > For what it's worth, Windows has open flags that allow denying read or
> > write opens. They also made their way into the NFSv4 protocol, but
> > knfsd enforces them only against other NFSv4 clients. Last I checked,
> > Samba attempted to emulate them using flock (and there's a comment to
> > that effect on the flock syscall in fs/locks.c). I don't know what Wine
> > does.
> >
> > Pavel Shilovsky posted flags adding O_DENY* flags years ago:
> >
> > https://lwn.net/Articles/581005/
> >
> > I keep thinking I should look back at those some day but will probably
> > never get to it.
> >
> > I've no idea how Windows applications use them, though I'm told it's
> > common.
>
> I don't know in any detail. I just have this memory of not being able
> to open or do anything with a file on windows while any application has
> it open.
>
> We limit mandatory locks to filesystems that have the proper mount flag
> and files that are sgid but are not executable. Reusing that limit we
> could probably allow such a behavior in Linux without causing chaos.

I'm pretty confused about how we're using the term "mandatory locking".

The locks you're thinking of are basically ordinary posix byte-range
locks which we attempt to enforce as mandatory under certain conditions
(e.g. in fs/read_write.c:rw_verify_area). That means we have to check
them on ordinary reads and writes, which is a pain in the butt. (And we
don't manage to do it correctly--the code just checks for the existence
of a conflicting lock before performing IO, ignoring the obvious
time-of-check/time-of-use race.)

This has nothing to do with Windows share locks which from what I
understand are whole-file locks that are only enforced against opens.

--b.

> Without being very strict about which files can participate I can just
> imagine someone hiding their presence by not allowing other applications
> the ability to write to utmp or a log file.
>
> In the windows world where everything evolved with those kinds of
> restrictions it is probably fine (although super annoying).
>
> Eric