In Ubuntu 18.04, I can create a file as one user in /tmp, and write to it as user root (despite the sticky bit being set on /tmp).

In Ubuntu 20.04, I get a Permission Denied error as user root.

Ubuntu 18.04:

# As user "ubuntu":

touch /tmp/tmplock



# As user "root":

echo "foo" > /tmp/tmplock



cat /tmp/tmplock

# Outputs "foo"



ls -lah /tmp

drwxrwxrwt 32 root   root    20K Jun 16 18:36 .

-rw-rw-r--  1 ubuntu ubuntu    4 Jun 16 18:36 tmplock

Ubuntu 20.04:

# As user "ubuntu":

touch /tmp/tmplock



# As user "root":

echo "foo" > /tmp/tmplock

# Outputs: bash: /tmp/tmplock: Permission denied



ls -lah /tmp

drwxrwxrwt 12 root   root   4.0K Jun 16 18:15 .

-rw-rw-r--  1 ubuntu ubuntu    0 Jun 16 18:14 tmplock

I can't find any information on if this is related to the sticky bit, or an underlying Linux kernel change, or anything.

Does anyone know what can explain the change in behavior?