[PATCH] fanotify: handle permission events without holding fsnotify_mark_srcu

[Amir Goldstein]

Handling fanotify events does not entail dereferencing fsnotify_mask
beyond the point of fanotify_should_send_event().

For the case of permission events, which may block indefinetely,
return -EAGAIN and then fsnotify() will call handle_event() again
without a reference to the mark.

Without a reference to the mark, there is no need to call
handle_event() under fsnotify_mark_srcu read side lock
which is protecting the inode/vfsmount mark lists.
We just need to hold a reference to the group

Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
---
 fs/notify/fanotify/fanotify.c | 14 +++++++++++---
 fs/notify/fsnotify.c          | 26 ++++++++++++++++++++++----
 2 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 604e307..0ffa9ed 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -298,9 +298,17 @@ static int fanotify_handle_event(struct
fsnotify_group *group,
        BUILD_BUG_ON(FAN_ACCESS_PERM != FS_ACCESS_PERM);
        BUILD_BUG_ON(FAN_ONDIR != FS_ISDIR);

-       if (!fanotify_should_send_event(inode_mark, vfsmnt_mark, mask, data
-                                       data_type))
-               return 0;
+       if (inode_mark || vfsmnt_mark) {
+               if (!fanotify_should_send_event(inode_mark, vfsmnt_mark, mask,
+                                               data, data_type))
+                       return 0;
+
+#ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
+               /* Ask to be called again without a reference to mark */
+               if (mask & FAN_ALL_PERM_EVENTS)
+                       return -EAGAIN;
+#endif
+       }

        pr_debug("%s: group=%p inode=%p mask=%x\n", __func__, group, inode,
                 mask);
diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 34bccbe..dc0c86e 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -199,7 +199,7 @@ int fsnotify(struct inode *to_tell, __u32 mask,
void *data, int data_is,
 {
        struct hlist_node *inode_node = NULL, *vfsmount_node = NULL;
        struct fsnotify_mark *inode_mark = NULL, *vfsmount_mark = NULL;
-       struct fsnotify_group *inode_group, *vfsmount_group;
+       struct fsnotify_group *inode_group, *vfsmount_group, *group;
        struct mount *mnt;
        int idx, ret = 0;
        /* global tests shouldn't care about events on child only the
specific event */
@@ -282,15 +282,33 @@ int fsnotify(struct inode *to_tell, __u32 mask,
void *data, int data_is,
                ret = send_to_group(to_tell, inode_mark, vfsmount_mark, mask,
                                    data, data_is, cookie, file_name);

-               if (ret && (mask & ALL_FSNOTIFY_PERM_EVENTS))
-                       goto out;
-
                if (inode_group)
                        inode_node = srcu_dereference(inode_node->next,
                                                      &fsnotify_mark_srcu);
                if (vfsmount_group)
                        vfsmount_node = srcu_dereference(vfsmount_node->next,
                                                         &fsnotify_mark_srcu);
+
+               if (ret != -EAGAIN)
+                       continue;
+
+               group = inode_group ? : vfsmount_group;
+               fsnotify_get_group(group);
+               srcu_read_unlock(&fsnotify_mark_srcu, idx);
+               /*
+                * Calling handle_event() again without reference to mark,
+                * so we do not need to call it under fsnotify_mark_srcu
+                * which is protecting the inode/vfsmount mark lists.
+                * We just need to hold a reference to the group
+                */
+               return group->ops->handle_event(group, to_tell, NULL, NULL,
+                                               mask, data, data_is,
+                                               file_name, cookie);
+               idx = srcu_read_lock(&fsnotify_mark_srcu);
+               fsnotify_put_group(group);
+
+               if (ret && (mask & ALL_FSNOTIFY_PERM_EVENTS))
+                       goto out;
        }
        ret = 0;
 out:
-- 
2.7.4