[zbentley]

This isn't recursion. "strace strace foo" first runs the "strace" binary (PID X), which attaches one ptrace to "foo" (PID Y) and runs an I/O loop printing retrieved trace data. The outer "strace" call launches as PID Z, and attaches a ptrace to PID Y: the inner strace. The inner strace is not itself being ptraced until that happens, so there's no special case needed to allow "stracing strace".

However, you may be referring to a different scenario: multiple ptrace calls do hit the same process if you (for example) run "strace -p PID" for the same PID more than once simultaneously. In that case, strace invocations after the first one will fail with an error like 'strace: attach: ptrace(PTRACE_SEIZE, PID): Operation not permitted'.