Why can't files be manipulated by inode?

问题:

Why is it that you cannot access a file when you only know its inode, without searching for a file that links to that inode? A hard link to the file contains nothing but a name and a number telling you where to find the inode with all the real information about the file. I was surprised when I was told that there was no usermode way to use the inode number directly to open a file.

This seems like such a harmless and useful capability for the system to provide. Why is it not provided?

回答1:

Some Operating Systems do have that facility. For example, OS X needs it to support the Carbon File Manager, and on Linux you can use debugfs. Of course, you can do it on any UNIX from the command-line via find -inum, but the real reason you can't access files by inode is that it isn't particularly useful. It does kindof circumvent file permissions, because if there's a file you can read in a folder you can't read or execute, then opening the inode lets you discover it.

The reason it isn't very useful is that you need to find an inode number via a *stat() call, at which point you already have the filename (or an open fd)...or you need to guess the inum.

回答2:

Security reasons -- to access a file you need permission on the file AS WELL AS permission to search all the directories from the root needed to get at the file. If you could access a file by inode, you could bypass the checks on the containing directories.

This allows you to create a file that can be accessed by a set of users (or a set of groups) and not anyone else -- create directories that are only accessable by the the users (one dir per user), and then hard-link the file into all of those directories -- the file itself is accessable by anyone, but can only actually be accessed by someone who has search permissions on one of the directories it is linked into.

回答3:

In response to your comment: To "pass a file", you can use fd passing over AF_LOCAL sockets by means of SCM_RIGHTS (see man 7 unix).

回答4:

Btrfs does have an ioctl for that (BTRFS_IOC_INO_PATHS added in this patch), however it does no attempt to check permissions along the path, and is simply reserved to root.

回答5:

Surely if you've already looked up a file via a path, you shouldn't have to do it again and again?

stat(f,&s); i=open(f,O_MODE);

involves two trawls through a directory structure. This wastes CPU cycles with unnecessary string operations. Yes, the well designed fs cache will hind most of this inefficiency from a casual end-user, but repeating work for no reason is ugly if not plain silly.