New locking/refcounting for fs_struct

* all changes of current->fs are done under task_lock and write_lock of
  old fs->lock
* refcount is not atomic anymore (same protection)
* its decrements are done when removing reference from current; at the
  same time we decide whether to free it.
* put_fs_struct() is gone
* new field - ->in_exec.  Set by check_unsafe_exec() if we are trying to do
  execve() and only subthreads share fs_struct.  Cleared when finishing exec
  (success and failure alike).  Makes CLONE_FS fail with -EAGAIN if set.
* check_unsafe_exec() may fail with -EAGAIN if another execve() from subthread
  is in progress.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 49 insertions, 20 deletions

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 36e0a12..41cff72 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -72,25 +72,27 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
 		path_put(old_root);
 }
 
-void put_fs_struct(struct fs_struct *fs)
+void free_fs_struct(struct fs_struct *fs)
 {
-	/* No need to hold fs->lock if we are killing it */
-	if (atomic_dec_and_test(&fs->count)) {
-		path_put(&fs->root);
-		path_put(&fs->pwd);
-		kmem_cache_free(fs_cachep, fs);
-	}
+	path_put(&fs->root);
+	path_put(&fs->pwd);
+	kmem_cache_free(fs_cachep, fs);
 }
 
 void exit_fs(struct task_struct *tsk)
 {
-	struct fs_struct * fs = tsk->fs;
+	struct fs_struct *fs = tsk->fs;
 
 	if (fs) {
+		int kill;
 		task_lock(tsk);
+		write_lock(&fs->lock);
 		tsk->fs = NULL;
+		kill = !--fs->users;
+		write_unlock(&fs->lock);
 		task_unlock(tsk);
-		put_fs_struct(fs);
+		if (kill)
+			free_fs_struct(fs);
 	}
 }
 
@@ -99,7 +101,8 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 	struct fs_struct *fs = kmem_cache_alloc(fs_cachep, GFP_KERNEL);
 	/* We don't need to lock fs - think why ;-) */
 	if (fs) {
-		atomic_set(&fs->count, 1);
+		fs->users = 1;
+		fs->in_exec = 0;
 		rwlock_init(&fs->lock);
 		fs->umask = old->umask;
 		read_lock(&old->lock);
@@ -114,28 +117,54 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 
 int unshare_fs_struct(void)
 {
-	struct fs_struct *fsp = copy_fs_struct(current->fs);
-	if (!fsp)
+	struct fs_struct *fs = current->fs;
+	struct fs_struct *new_fs = copy_fs_struct(fs);
+	int kill;
+
+	if (!new_fs)
 		return -ENOMEM;
-	exit_fs(current);
-	current->fs = fsp;
+
+	task_lock(current);
+	write_lock(&fs->lock);
+	kill = !--fs->users;
+	current->fs = new_fs;
+	write_unlock(&fs->lock);
+	task_unlock(current);
+
+	if (kill)
+		free_fs_struct(fs);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(unshare_fs_struct);
 
 /* to be mentioned only in INIT_TASK */
 struct fs_struct init_fs = {
-	.count		= ATOMIC_INIT(1),
+	.users		= 1,
 	.lock		= __RW_LOCK_UNLOCKED(init_fs.lock),
 	.umask		= 0022,
 };
 
 void daemonize_fs_struct(void)
 {
-	struct fs_struct *fs;
+	struct fs_struct *fs = current->fs;
+
+	if (fs) {
+		int kill;
+
+		task_lock(current);
 
-	exit_fs(current);	/* current->fs->count--; */
-	fs = &init_fs;
-	current->fs = fs;
-	atomic_inc(&fs->count);
+		write_lock(&init_fs.lock);
+		init_fs.users++;
+		write_unlock(&init_fs.lock);
+
+		write_lock(&fs->lock);
+		current->fs = &init_fs;
+		kill = !--fs->users;
+		write_unlock(&fs->lock);
+
+		task_unlock(current);
+		if (kill)
+			free_fs_struct(fs);
+	}
 }