Убедитесь, что группа процессов была установлена ​​правильно при запуске

What's unusual here is that the open of /proc/pid/stack succeeds, but an attempt to read from it returns EPERM.

The kernel source code for showing the contents of /proc/pid/stack, fs/proc/base.c:proc_pid_stack(), calls lock_trace(), which will return a permission error if the caller doesn't have access to ptrace the target process.

On many Linux distributions, there are Linux Security Modules that may restrict the use of ptrace. One of these is Yama, which you'll encounter if you use Ubuntu. Permission to use ptrace is restricted to root or to an ancestor (typically the parent) process of the target. You can check this with

root@ubuntu:~# cat /proc/sys/kernel/yama/ptrace_scope 
1

And can disable it by doing

root@ubuntu:~# echo 0 > /proc/sys/kernel/yama/ptrace_scope 

Thereafter, you'll be able to cat /proc/pid/stack.