In case the kernel was compiled with the yama security module (CONFIG_SECURITY_YAMA), tracing processes with ptrace(2) can be restricted.

    0 => No restrictions.
    1 => Restricted attach, only the following can attach
            - A process in the parent hierarchy.
            - A process with CAP_SYS_PTRACE.
            - A process with the PID that the tracee allowed by via
    2 => Only processes with CAP_SYS_PTRACE in the user namespace of the tracee
         can attach.
    3 => No tracing allowed.

Further details in ptrace(2).