researched my recent error and found that it's a security feature:
"This is not a bug, it is a design issue. It is also possible to prevent your program being traced by using Linux specific prctl call and clear PR_SET_DUMPABLE flag, which OpenSSH agent in your system already does.
But, not many programmers aware of this behaviour. So, when looking at the Linux distributions perspective, it is good to activate additional protection mechanism on ptrace system call to make more secure system without need to change of any software code.
Newer Linux kernels has the Yama security module. When the Yama is built into the kernel, its behaviour can be changed through /proc/sys/kernel/yama/ptrace_scope file.
If you're trying to use strace to attach another process which is also belongs to you but you're not a parent of it and the value in ptrace_scope file greater than 0, it is normal to get "Operation not permitted" error.
Some recent Linux distributions sets ptrace_scope value to 1 in default. You can still use strace in this scenario with starting the programs with strace like below:
$ strace ./my_prog
but you can not attach a process which is already started and has different parent of your newly executed strace.