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.) 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
