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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2021-08-19 at 10:33 -0400, J. Bruce Fields wrote:
> 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.)
> 

Yeah, the locks we're talking about are the locks described in:

    Documentation/filesystems/mandatory-locking.rst

They've always been racy. You have to mount the fs with '-o mand' and
set a special mode on the file (setgid bit set, with group execute bit
cleared). It's a crazypants interface.

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

Yep. Those are different.

Confusingly, there is also LOCK_MAND|LOCK_READ|LOCK_WRITE for flock(),
which are purported to be for emulating Windows share modes. They aren't
really mandatory though.

> --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

-- 
Jeff Layton <jlayton@kernel.org>