View source for How to open a file without open system call

== Opening a file without open() ==

Linux provides a way to do this. The way is called <code>open_by_handle_at</code> system call. Introduced to make the user-space NFS server work, this call allows to open an inode using a blob called ''handle''. The handle is (almost) meaning-less sequence of bytes by which filesystem promises to find the inode and open one. And the handle itself can be generated by the kernel using the inode object. Since fsnotify object references inode we can try to ask the kernel to generate the respective inode's handle. And we did that and patched the kernel to show this handle in the /proc/$pid/fdindo/$fd file for the fsnotify.

So when dumping the fsnotify we read the handle out of proc and save one in the images, and on restore time we call the <code>open_by_handle_at</code> with the handle value and get the inode back. Then we need to ask the kernel to put the fsnotify on this inode. To do this CRIU calls fsnotify init call on the /proc/self/fd/$fd path. While resolving the path kernel finds the inode opened previously and restores the handle in the proper place. Thus we fool the kernel and put fsnotify on an inode without even knowing its path.