Tested with truss -f on native binaries, and Linux. Both work.
% truss -f /compat/linux/bin/bash -c "id & id &"
P78
% truss -f bash -c "id & id &"
...
% truss -f sh -c "id & id &"
The Kyua tests as of r288949 work as well.
/usr/tests/sys/kern # kyua test ptrace_test
ptrace_test:ptrace__follow_fork_both_attached -> passed [0.010s]
ptrace_test:ptrace__follow_fork_both_attached_unrelated_debugger -> passed [0.010s]
ptrace_test:ptrace__follow_fork_child_detached -> passed [0.009s]
ptrace_test:ptrace__follow_fork_child_detached_unrelated_debugger -> passed [0.010s]
ptrace_test:ptrace__follow_fork_parent_detached -> passed [0.010s]
ptrace_test:ptrace__follow_fork_parent_detached_unrelated_debugger -> passed [0.010s]
ptrace_test:ptrace__getppid -> passed [0.009s]
ptrace_test:ptrace__new_child_pl_syscall_code_fork -> passed [0.009s]
ptrace_test:ptrace__new_child_pl_syscall_code_thread -> passed [0.009s]
ptrace_test:ptrace__new_child_pl_syscall_code_vfork -> passed [0.009s]
ptrace_test:ptrace__parent_sees_exit_after_child_debugger -> passed [0.015s]
ptrace_test:ptrace__parent_sees_exit_after_unrelated_debugger -> passed [0.015s]
ptrace_test:ptrace__parent_wait_after_attach -> passed [0.009s]
ptrace_test:ptrace__parent_wait_after_trace_me -> passed [0.009s]
Results file id is usr_tests_sys_kern.20151006-222504-676851
Results saved to /root/.kyua/store/results.usr_tests_sys_kern.20151006-222504-676851.db
14/14 passed (0 failed)