Invalid cross-device link while Hardlinking in the same file system

.netrchard linkhomerhelroot

I have /home/myuser/Desktop/rc/.netrc file that i want hardlink to /root, ie home directory of root user.
When i do:

ln /home/user/Destkop/rc/.netrc /root

it gives the following error:

ln: creating hard link /root/.netrc' =>.netrc': Invalid
cross-device link

but it works when i hardlink the file to myuser's home, ie to /home/myuser.

So, what's the problem, why it says invalid cross-devices when there is only one file system here?

PS. I am using RHEL6

Best Answer

it's most likely that your /home directory isn't on the same partition as the /root directory.

You can easily check this with cat /etc/fstab hardlinks cannot be created between different partitions, only symlinks can.

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Different Hard Link Permissions Behavior in CentOS 6 vs CentOS 7

I spun up some fresh CentOS 6 and 7 vm's and was able to recreate the exact behavior you showed. After doing some digging, it turns out that this is actually a change in the kernel regarding default behavior with respect to hard and soft links for the sake of security. The following pages pointed me in the right direction:

http://kernel.opensuse.org/cgit/kernel/commit/?id=561ec64ae67ef25cac8d72bb9c4bfc955edfd415

http://kernel.opensuse.org/cgit/kernel/commit/?id=800179c9b8a1

If you make the file world writable, your admin user will be able to create the hard link.

To revert to the behavior of CentOS 6 system wide, new kernel parameters were added. Set the following in /etc/sysctl.conf:

fs.protected_hardlinks = 0
fs.protected_symlinks = 0

then run

sysctl -p

As for why your program opts to use links instead of copying files, why create an exact copy of a file you need to use when you can just create an entry that points to the original blocks? This saves disk space and the operation is less costly in terms of CPU and I/O. The new hard link is the same file, just with different metadata/inode. If you were to delete the original file after creating a hard link, it won't affect the link. A file is only 'deleted' once all links have been removed.

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The useradd code calls a mkdir library function to (attempt to) create the specified directory. useradd checks the return code, but only for being non-zero; in this case, I suspect that mkdir is returning ENOENT -- A directory component in pathname does not exist or is a dangling symbolic link because the parent directory (/opt/atlassian) didn't exist, or had been removed during your attempts to add the user.

As Kusalananda / roaima point out, the simplest solution here is to create the parent directory structure before calling useradd:

sudo mkdir -p /opt/atlassian
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Best Answer

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