linux - Open file by inode

Question

Is it possible to open a file knowing its inode?

ls -i /tmp/test/test.txt
529965 /tmp/test/test.txt

I can provide path, inode (above 529965) and I am looking to get in return a file descriptor.

Answer 1

This is not possible because it would open a loophole in the access control rules. Whether you can open a file depends not only on its own access permission bits, but on the permission bits of every containing directory. (For instance, in your example, if test.txt were mode 644 but the containing directory test were mode 700, then only root and the owner of test could open test.txt.) Inode numbers only identify the file, not the containing directories (it's possible for a file to be in more than one directory; read up on "hard links") so the kernel cannot perform a complete set of access control checks with only an inode number.

(Some Unix implementations have offered nonstandard root-only APIs to open a file by inode number, bypassing some of the access-control rules, but if current Linux has such an API, I don't know about it.)