/dev/stdin, /dev/fd/0 are not readable for the user (owned by root)
$ ls -lL /dev/fd/0 /dev/stdin
crw—w—— 1 root tty 136, 2 May 16 08:05 /dev/fd/0
crw—w—— 1 root tty 136, 2 May 16 08:05 /dev/stdin
$ : </dev/stdin
-ash: can’t open /dev/stdin: Permission denied
on Debian:
$ : </dev/stdin && echo OK
OK
(from redmine: issue id 8883, created on 2018-05-16)