How to open a file without open system call

Sometimes CRIU meets an inode object (check this article for details of what inode is) without a name. This article describes when this happens and what CRIU does in this case.

When this happens

There are two nasty API calls in the Linux kernel -- the inotify_init and the fanotify_init. Both take a file path as an argument and screw one up. What they do is find an inode object using this path, then put an events generator on it and then forget the path completely. The result of both calls is a file-descriptor pointing to the created event generator object.

When CRIU meets the inotify/fanotify (called fsnotify later for convenience) FD it has to find out the file on which the generator sits. But, since the inode's path is lost, this cannot be done in general case.

Chances to get the name back

Chances to get the name back exist. To understand when let's dive a little bit more in how Linux manages dentries and inodes.

Inodes and dentries

So, every file on disk is represented by an inode object. Inode has an ID (inode number), access rights, owner, link count and some more data. Names are only stored in special files called directories -- in directories there's a set of name-to-inode mappings. When accessing a file by its name Linux kernel sequentially reads from disk these mapping tables and for every name found in it creates a dentry object in memory. It's important to know, that dentry is created not only for existing files, but also for non-existing to speed up the ENOENT report for second file lookup. IOW dentries form a cache, which contains records for both present and absent objects on disk.

Since the tree of dentries can gorw infinitely Linux sometimes shrinks one, by freeing the unused dentries. The dentry is unused if no other object references one, and a dentry can be referenced by child dentries and by files (as described in another article).

Having said that, at the time the fsnotify creating happens we have a full dentry chain and the inode sitting in memory. Then the events generator is put on the inode and that's it. Neither inode nor fsnotify object references the dentry, so eventually the whole dentry chain can be shrunk from memory.

So, returning to the "can I get the name back" question. The answer is -- if the dentry cache is still alive -- yes, you can. But CRIU cannot rely on this, since it should also support situations when the dentry cache is not there.

Tmpfs

One filesystem, however, behaves friendly to this problem. The tempfs one pins the dentries in memory, since it has no other media on which to store the information about files on it. So for tmpfs the name is always at hands.