Re: [PATCH] locks: Set fl_nspid at file_lock allocation

[Jeff Layton <jlayton@poochiereds.net>]

On Thu, 2017-05-18 at 13:36 -0400, Benjamin Coddington wrote:
> > On Thu, 2017-05-18 at 12:02 -0400, Benjamin Coddington wrote:
> > > diff --git a/fs/locks.c b/fs/locks.c
> > > index af2031a1fcff..959b3f93f4bd 100644
> > > --- a/fs/locks.c
> > > +++ b/fs/locks.c
> > > @@ -249,7 +249,7 @@ locks_dump_ctx_list(struct list_head *list, char 
> > > *list_type)
> > >  	struct file_lock *fl;
> > > 
> > >  	list_for_each_entry(fl, list, fl_list) {
> > > -		pr_warn("%s: fl_owner=%p fl_flags=0x%x fl_type=0x%x fl_pid=%u\n", 
> > > list_type, fl->fl_owner, fl->fl_flags, fl->fl_type, fl->fl_pid);
> > > +		pr_warn("%s: fl_owner=%p fl_flags=0x%x fl_type=0x%x fl_pid=%u\n", 
> > > list_type, fl->fl_owner, fl->fl_flags, fl->fl_type, 
> > > pid_vnr(fl->fl_nspid));
> > >  	}
> > >  }
> > > 
> > 
> > Probably should change the format to say fl_nspid=%u here, just to be
> > clear. Might also want to keep fl_pid in there since the lock manager
> > could set it.
> 
> Yes, I thought about just spitting out both.  Let's do that.
> 
> > > @@ -2074,7 +2075,7 @@ static int posix_lock_to_flock(struct flock 
> > > *flock, struct file_lock *fl)
> > >  #if BITS_PER_LONG == 32
> > >  static void posix_lock_to_flock64(struct flock64 *flock, struct 
> > > file_lock *fl)
> > >  {
> > > -	flock->l_pid = IS_OFDLCK(fl) ? -1 : fl->fl_pid;
> > > +	flock->l_pid = IS_OFDLCK(fl) ? -1 : pid_vnr(fl->fl_nspid);
> > 
> > What about the lock managers that _do_ set fl->fl_pid. With nfsv2/3,
> > this is always going to give you back the pid of lockd, AFAICT.
> 
> But isn't this really what you want?  If a local process wants to know 
> who
> holds a conflicting lock, the fl_pid of a remote system is really pretty
> useless.  Not only that, but there's no way for the local process to 
> know
> when the pid is local or remote.  Better to be consistent and always 
> return
> something that's useful.
> 

The l_pid field in struct flock (and by extension fl_pid) is pretty
poorly defined in the spec(s), especially when there is a remote host
involved. Programs that rely on it are insane, of course...but Linux has
always behaved this way.

In the absence of a compelling reason to change it, I think we should
keep the behavior in this respect as close as possible to what we have
now.

> > Do we want to present the pid value that the client sent here instead 
> > in
> > that case? Maybe the lm could set a fl_flag to indicate that the pid
> > should be taken directly from fl_pid here? Then you could move the 
> > above
> > logic to a static inline or something.
> > 
> > Alternately, you could add a new lm_present_pid operation to lock
> > managers to format the pid as they see fit.
> 
> Either works to solve the problem, but I still think that F_GETLK and
> /proc/locks should only return local pids.
> 
> > > @@ -2322,6 +2325,7 @@ int fcntl_getlk64(struct file *filp, unsigned 
> > > int cmd, struct flock64 __user *l)
> > >  	int error;
> > > 
> > >  	error = -EFAULT;
> > > +	file_lock.fl_nspid = get_pid(task_tgid(current));
> > 
> > Might it be cleaner to create a FILE_LOCK(name) macro that does this 
> > on
> > the stack (like LIST_HEAD()) ?
> 
> Yes, it would.  I'll do it.
> 

In some of those places it might not hurt to consider allocating and
freeing a file_lock instead. file_lock is not exactly small (208 bytes
on my latest build)...

> > > @@ -2520,6 +2527,7 @@ locks_remove_flock(struct file *filp, struct 
> > > file_lock_context *flctx)
> > > 
> > >  	if (fl.fl_ops && fl.fl_ops->fl_release_private)
> > >  		fl.fl_ops->fl_release_private(&fl);
> > > +	put_pid(fl.fl_nspid);
> > 
> > I think we only need to take a reference for when the file_lock can
> > outlive the current task, so the get/put may not be necessary in these
> > functions.
> 
> Right.. of course.  I can clean those up.
> 
-- 
Jeff Layton <jlayton@poochiereds.net>