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@xxxxxxxxxxxx (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@xxxxxxxxxx>
