kern/175674: sem_open() should use O_EXLOCK with open() instead of a separate flock() call

[Jilles Tjoelker]

The following reply was made to PR kern/175674; it has been noted by GNATS.

From: Jilles Tjoelker <jilles at stack.nl>
To: Giorgos Keramidas <keramida at FreeBSD.org>
Cc: Jukka Ukkonen <jau at iki.fi>, freebsd-gnats-submit at FreeBSD.org,
	davidxu at FreeBSD.org
Subject: Re: kern/175674: sem_open() should use O_EXLOCK with open() instead
 of a separate flock() call
Date: Sun, 3 Feb 2013 21:20:31 +0100

 On Sun, Feb 03, 2013 at 06:25:25AM +0100, Giorgos Keramidas wrote:
 > On 2013-01-29 18:03, Jukka Ukkonen <jau at iki.fi> wrote:
 > > >Number:         175674
 > > >Category:       kern
 > > >Synopsis:       sem_open() should use O_EXLOCK with open() instead of a separate flock() call
 > 
 > > >Environment:
 > > FreeBSD sleipnir 9.1-STABLE FreeBSD 9.1-STABLE #2 r246056M: Tue Jan 29 07:33:01 EET 2013     root at sleipnir:/usr/obj/usr/src/sys/Sleipnir  amd64
 > > >Description:
 > > sem_open() is calling flock() to set a lock on a newly created file descriptor.
 > > That is pointless. The open() call a few lines before the flock() could, and
 > > in my opinion should, be done with the O_EXLOCK flag set.
 
 > It's also a bit safer to obtain the exclusive lock atomically before
 > open() returns. Waiting for open() to complete and then calling flock()
 > has a race condition.
 
 > Jilles and David, do you think this patch looks ok for libc?
 
 > > Patch attached with submission follows:
 > > 
 > > --- lib/libc/gen/sem_new.c.flock	2012-11-09 18:50:05.000000000 +0200
 > > +++ lib/libc/gen/sem_new.c	2012-11-09 18:44:59.000000000 +0200
 > > @@ -198,11 +198,13 @@
 > >  		goto error;
 > >  	}
 > >  
 > > -	fd = _open(path, flags|O_RDWR|O_CLOEXEC, mode);
 > > +	fd = _open(path, flags|O_RDWR|O_CLOEXEC|O_EXLOCK, mode);
 > >  	if (fd == -1)
 > >  		goto error;
 > > +#if 0
 > >  	if (flock(fd, LOCK_EX) == -1)
 > >  		goto error;
 > > +#endif
 > >  	if (_fstat(fd, &sb)) {
 > >  		flock(fd, LOCK_UN);
 > >  		goto error;
 
 For a reason unknown to me, open(2) does not restart but always returns
 [EINTR] when a signal is caught. This is not POSIX-compliant. On the
 other hand, flock(2) is not broken in this way. So this change breaks
 sem_open(3) in the unlikely case that a signal with SA_RESTART arrives
 while it is waiting for the lock.
 
 The best way to fix this is in kern_openat() in the kernel but this
 might cause compatibility issues.
 
 The #if 0 is silly; we have version control to restore old code if need
 be.
 
 -- 
 Jilles Tjoelker