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

[Giorgos Keramidas]

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

From: Giorgos Keramidas <keramida at FreeBSD.org>
To: Jukka Ukkonen <jau at iki.fi>
Cc: freebsd-gnats-submit at FreeBSD.org, jilles 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 06:25:25 +0100

 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;