[gentoo-user] Root can't write to files owned by others?

[Dr Rainer Woitok]

Greetings,

until recently my system behaves sort of strangely:

   $ touch /tmp/file
   $ ls -l /tmp/file
   -rw------- 1 rainer rainer 0 2022-03-09 19:06 /tmp/file
   $ echo x | sudo tee /tmp/file
   Password: 
   tee: /tmp/file: Permission denied
   x
   $ chmod a+w /tmp/file
   $ ls -l /tmp/file
   -rw--w--w- 1 rainer rainer 0 2022-03-09 19:06 /tmp/file
   $ echo x | sudo tee /tmp/file
   tee: /tmp/file: Permission denied
   x
   $

Since when can't root write to files  it doesn't own?   And not even, if
the file has write permission for everybody?

This worked as long as I can think of.  My last routine upgrade install-
ed new  kernel package  "sys-kernel/gentoo-sources-5.15.26",  so I built
the new kernel and booted from it.  May this be the reason?

When I'll have time to reboot,  I'll test the  above commands  on my old
kernel, 5.15.19.  But perhaps there's another reason?

Slightly puzzled
     Rainer