Question

I have a file from which I have suppressed every permission. No one should be able to read the file, right? In fact, if I run

File f = new File("not_readable.pdf");
System.out.println(f.canRead())

I get

false

However, if I call

File f = new File("not_readable.pdf");
System.out.println(f.length())

I get

455074

It is my understanding that in order to get the size of a file one must open and read the file first, but this result strongly suggests that I’m wrong. Does anyone know why this happens? Also, is there a way to prevent Java’s file.length() method from accessing the size of a file?

I am using Ubuntu 12.10

Answer 1

You are mistaken: The length of a file is file system metadata (at least for file systems running under the Linux VFS). Anyone who has read permissions to a directory can see all the contained files and their sizes. To prevent users from seeing a file’s size, you have to prevent them from seeing it altogether, i.e. place the file in a directory that has permissions like drwxr-x--- if the user is not in the group associated to the directory, or drwx------ if the user is in that group.