fcntl_setlease uses a struct file_lock on the stack to find the
file_lock it's actually looking for. The problem with this approach is
that lease_init will attempt to free the file_lock if the arg argument
is invalid, causing kmem_cache_free to be called with the address of the
on-stack file_lock.
After running the following test-case, it doesn't take long for an i686
machine running 2.6.16.13 to stop responding completely. 
2.6.17-rc3-git12 shows similar results, although it takes a bit more 
activity before crashing.

Björn Steinbrink also identified a slab leak in the same piece of code, 
caused by the fasync_helper call which will allocate a new slab every 
time because it uses the wrong structure (the one on the stack) when the 
a lease on that file already exists.

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

void die(const char *msg)
{
	perror(msg);
	exit(1);
}
int main(int argc, char *argv[])
{
	int fd;
	if (argc == 1)
	{
		fprintf(stderr, "Usage: %s file\n", argv[0]);
		exit(1);
	}
	fd = open(argv[1], O_RDONLY);
	if (fd == -1)
		die("open()");
	if (fcntl(fd, F_SETLEASE, F_RDLCK) == -1)
		die("setlease(RDLCK)");
	if (fcntl(fd, F_SETLEASE, F_RDLCK) == -1)
		die("setlease(RDLCK2)");
	if (fcntl(fd, F_SETLEASE, 5) == -1)
		die("setlease(invalid)");
	close(fd);
	return 0;
}

Signed-off-by: Daniel Hokka Zakrisson <daniel <at> hozac.com>

---
--- a/fs/locks.c	2006-05-07 00:29:26.000000000 +0200
+++ b/fs/locks.c	2006-05-07 23:29:12.000000000 +0200
@@ -1363,6 +1363,7 @@ static int __setlease(struct file *filp,
  		goto out;

  	if (my_before != NULL) {
+		*flp = *my_before;
  		error = lease->fl_lmops->fl_change(my_before, arg);
  		goto out;
  	}
@@ -1433,7 +1434,7 @@ EXPORT_SYMBOL(setlease);
   */
  int fcntl_setlease(unsigned int fd, struct file *filp, long arg)
  {
-	struct file_lock fl, *flp = &fl;
+	struct file_lock *fl, *flp;
  	struct dentry *dentry = filp->f_dentry;
  	struct inode *inode = dentry->d_inode;
  	int error;
@@ -1446,14 +1447,15 @@ int fcntl_setlease(unsigned int fd, stru
  	if (error)
  		return error;

-	locks_init_lock(&fl);
-	error = lease_init(filp, arg, &fl);
+	error = lease_alloc(filp, arg, &fl);
  	if (error)
  		return error;
+	flp = fl;

  	lock_kernel();

  	error = __setlease(filp, arg, &flp);
+	locks_free_lock(fl);
  	if (error || arg == F_UNLCK)
  		goto out_unlock;