[PATCH 1/5] exec: binfmt_misc: don't nullify Node->dentry in kill_node()

[Oleg Nesterov]

kill_node() nullifies/checks Node->dentry to avoid double free. This
complicates the next changes and this is very confusing:

- we do not need to check dentry != NULL under entries_lock, kill_node()
  is always called under inode_lock(d_inode(root)) and we rely on this
  inode_lock() anyway, without this lock the MISC_FMT_OPEN_FILE cleanup
  could race with itself.

- if kill_inode() was already called and ->dentry == NULL we should not
  even try to close e->interp_file.

We can change bm_entry_write() to simply check !list_empty(list) before
kill_node. Again, we rely on inode_lock(), in particular it saves us from
the race with bm_status_write(), another caller of kill_node().

Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
 fs/binfmt_misc.c | 22 +++++++++-------------
 1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index f471809..f4de5ae 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -600,11 +600,7 @@ static void kill_node(Node *e)
        struct dentry *dentry;
 
        write_lock(&entries_lock);
-       dentry = e->dentry;
-       if (dentry) {
-               list_del_init(&e->list);
-               e->dentry = NULL;
-       }
+       list_del_init(&e->list);
        write_unlock(&entries_lock);
 
        if ((e->flags & MISC_FMT_OPEN_FILE) && e->interp_file) {
@@ -612,12 +608,11 @@ static void kill_node(Node *e)
                e->interp_file = NULL;
        }
 
-       if (dentry) {
-               drop_nlink(d_inode(dentry));
-               d_drop(dentry);
-               dput(dentry);
-               simple_release_fs(&bm_mnt, &entry_count);
-       }
+       dentry = e->dentry;
+       drop_nlink(d_inode(dentry));
+       d_drop(dentry);
+       dput(dentry);
+       simple_release_fs(&bm_mnt, &entry_count);
 }
 
 /* /<entry> */
@@ -662,7 +657,8 @@ static ssize_t bm_entry_write(struct file *file, const char 
__user *buffer,
                root = file_inode(file)->i_sb->s_root;
                inode_lock(d_inode(root));
 
-               kill_node(e);
+               if (!list_empty(&e->list))
+                       kill_node(e);
 
                inode_unlock(d_inode(root));
                break;
@@ -791,7 +787,7 @@ static ssize_t bm_status_write(struct file *file, const 
char __user *buffer,
                inode_lock(d_inode(root));
 
                while (!list_empty(&entries))
-                       kill_node(list_entry(entries.next, Node, list));
+                       kill_node(list_first_entry(&entries, Node, list));
 
                inode_unlock(d_inode(root));
                break;
-- 
2.5.0