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Hello Jerome.

In my case the output is not empty, but it is missing accessible processes where the parents are inaccessible.

I cannot reproduce the issue either. Output not empty on RHEL6.6, 2.6.32-642.el6.x86_64.

(In reply to Jan Rybar from comment #6)
> I cannot reproduce the issue either. Output not empty on RHEL6.6,
> 2.6.32-642.el6.x86_64.

The output is not empty, but does it contains all accessible processes?

[jmarchan@virt_rhel6 ~]$ pstree
init─┬─NetworkManager─┬─dhclient
     │                └─{NetworkManager}
[...]
     ├─sshd─┬─sshd───sshd───bash─┬─pstree
     │      │                    └─vi
     │      └─sshd───bash
     ├─udevd───2*[udevd]
     └─wpa_supplicant
[jmarchan@virt_rhel6 ~]$ ps -u jmarchan
  PID TTY          TIME CMD
 2698 ?        00:00:00 sshd
 2699 pts/0    00:00:01 bash
 9262 pts/0    00:00:00 vi
23192 pts/0    00:00:00 ps
[jmarchan@virt_rhel6 ~]$ sudo mount -o remount,hidepid=1 /proc/
[jmarchan@virt_rhel6 ~]$ pstree
[jmarchan@virt_rhel6 ~]$ ps -u jmarchan
  PID TTY          TIME CMD
 2699 pts/0    00:00:01 bash
 9262 pts/0    00:00:00 vi
23196 pts/0    00:00:00 ps

Note that sshd disappeared from ps output too. This is because it is suid root and thus it directory in /proc/ is also inaccessible to unprivileged processes.

If I run a program that forks a lot, the output is not empty anymore, yet there are still a number of process missing (see Jaromir's comment about the parent being inaccessible).

[jmarchan@virt_rhel6 ~]$ ./bz1151823/fork_snake
[jmarchan@virt_rhel6 ~]$ pstree
?───11*[fork_snake───fork_snake───fork_snake───fork_snake───fork_snake───fork_sn+
[jmarchan@virt_rhel6 ~]$

The changelog of upstream psmisc mention a fix regarding hidepid that landed in 22.17:
"Changes in 22.17
[...]
Fake pstree root for kernels with hidepid turned on"

Created attachment 1200836 [details]
Proposed patch - show accessible processes if hidepid on

Created attachment 1227351 [details]
Upstream backport patch, fix orphan processes

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2017-0771.html