From: Dipankar Sarma <dipankar@in.ibm.com>

There are places in the kernel where we look up files in fd tables and
access the file structure without holding refereces to the file.  So, we
need special care to avoid the race between looking up files in the fd
table and tearing down of the file in another CPU.  Otherwise, one might
see a NULL f_dentry or such torn down version of the file.  This patch
fixes those special places where such a race may happen.

Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 drivers/char/tty_io.c |    8 ++++++--
 fs/locks.c            |    9 +++++++--
 fs/proc/base.c        |   21 +++++++++++++++------
 3 files changed, 28 insertions(+), 10 deletions(-)

diff -puN drivers/char/tty_io.c~fix-file-lookup-without-ref drivers/char/tty_io.c
--- 25/drivers/char/tty_io.c~fix-file-lookup-without-ref	Wed Apr 12 16:10:09 2006
+++ 25-akpm/drivers/char/tty_io.c	Wed Apr 12 16:10:09 2006
@@ -2723,7 +2723,11 @@ static void __do_SAK(void *arg)
 		}
 		task_lock(p);
 		if (p->files) {
-			rcu_read_lock();
+			/*
+			 * We don't take a ref to the file, so we must
+			 * hold ->file_lock instead.
+			 */
+			spin_lock(&p->files->file_lock);
 			fdt = files_fdtable(p->files);
 			for (i=0; i < fdt->max_fds; i++) {
 				filp = fcheck_files(p->files, i);
@@ -2738,7 +2742,7 @@ static void __do_SAK(void *arg)
 					break;
 				}
 			}
-			rcu_read_unlock();
+			spin_unlock(&p->files->file_lock);
 		}
 		task_unlock(p);
 	} while_each_thread(g, p);
diff -puN fs/locks.c~fix-file-lookup-without-ref fs/locks.c
--- 25/fs/locks.c~fix-file-lookup-without-ref	Wed Apr 12 16:10:09 2006
+++ 25-akpm/fs/locks.c	Wed Apr 12 16:10:09 2006
@@ -2230,7 +2230,12 @@ void steal_locks(fl_owner_t from)
 
 	lock_kernel();
 	j = 0;
-	rcu_read_lock();
+
+	/*
+	 * We are not taking a ref to the file structures, so
+	 * we need to acquire ->file_lock.
+	 */
+	spin_lock(&files->file_lock);
 	fdt = files_fdtable(files);
 	for (;;) {
 		unsigned long set;
@@ -2248,7 +2253,7 @@ void steal_locks(fl_owner_t from)
 			set >>= 1;
 		}
 	}
-	rcu_read_unlock();
+	spin_unlock(&files->file_lock);
 	unlock_kernel();
 }
 EXPORT_SYMBOL(steal_locks);
diff -puN fs/proc/base.c~fix-file-lookup-without-ref fs/proc/base.c
--- 25/fs/proc/base.c~fix-file-lookup-without-ref	Wed Apr 12 16:10:09 2006
+++ 25-akpm/fs/proc/base.c	Wed Apr 12 16:10:09 2006
@@ -297,16 +297,20 @@ static int proc_fd_link(struct inode *in
 
 	files = get_files_struct(task);
 	if (files) {
-		rcu_read_lock();
+		/*
+		 * We are not taking a ref to the file structure, so we must
+		 * hold ->file_lock.
+		 */
+		spin_lock(&files->file_lock);
 		file = fcheck_files(files, fd);
 		if (file) {
 			*mnt = mntget(file->f_vfsmnt);
 			*dentry = dget(file->f_dentry);
-			rcu_read_unlock();
+			spin_unlock(&files->file_lock);
 			put_files_struct(files);
 			return 0;
 		}
-		rcu_read_unlock();
+		spin_unlock(&files->file_lock);
 		put_files_struct(files);
 	}
 	return -ENOENT;
@@ -1523,7 +1527,12 @@ static struct dentry *proc_lookupfd(stru
 	if (!files)
 		goto out_unlock;
 	inode->i_mode = S_IFLNK;
-	rcu_read_lock();
+
+	/*
+	 * We are not taking a ref to the file structure, so we must
+	 * hold ->file_lock.
+	 */
+	spin_lock(&files->file_lock);
 	file = fcheck_files(files, fd);
 	if (!file)
 		goto out_unlock2;
@@ -1531,7 +1540,7 @@ static struct dentry *proc_lookupfd(stru
 		inode->i_mode |= S_IRUSR | S_IXUSR;
 	if (file->f_mode & 2)
 		inode->i_mode |= S_IWUSR | S_IXUSR;
-	rcu_read_unlock();
+	spin_unlock(&files->file_lock);
 	put_files_struct(files);
 	inode->i_op = &proc_pid_link_inode_operations;
 	inode->i_size = 64;
@@ -1541,7 +1550,7 @@ static struct dentry *proc_lookupfd(stru
 	return NULL;
 
 out_unlock2:
-	rcu_read_unlock();
+	spin_unlock(&files->file_lock);
 	put_files_struct(files);
 out_unlock:
 	iput(inode);
_