commit 1b93f4032a246e0aa43c2f424921e0aace57b994
Author: Eugene Syromyatnikov <evgsyr@gmail.com>
Date:   Fri Aug 4 11:33:04 2017 +0200

    Improve handling of unexpected tracees
    
    When receiving a ptrace stop of an unexpected child, handle it
    in the most transparent way possible:
    - detach it instead of PTRACE_CONT'ing;
    - send it the signal with which it has been stopped.
    This should hopefully help to deal with processes that have been created
    with misused CLONE_PTRACE flag set.
    
    * strace.c (maybe_allocate_tcb) <WIFSTOPPED(status) && !followfork>:
    Calculate the signal similarly to the way next_event does,
    forward it to the unexpected tracee, and detach the tracee.

commit 7a35b711df127664e7430b2644ae92c75f3d5f67
Author: Dmitry V. Levin <ldv@altlinux.org>
Date:   Sun Aug 6 13:27:07 2017 +0000

    Fix handling of unexpected tracees when PTRACE_SEIZE is not in use
    
    * strace.c (maybe_allocate_tcb) <WIFSTOPPED(status) && !followfork>:
    The expected ptrace stop signal in case of !use seize is not
    syscall_trap_sig but SIGSTOP.  An idea of using PTRACE_GETSIGINFO to
    distinguish signal stops that should be re-injected from other kinds
    of stops didn't work out due to kernel implementation peculiarities
    of initial ptrace-stop.

commit 330f4633d5103938982602b6f21f761570e3482c
Author: Dmitry V. Levin <ldv@altlinux.org>
Date:   Sun Aug 6 15:10:56 2017 +0000

    Simplify handling of unexpected tracees
    
    * strace.c (maybe_allocate_tcb) <WIFSTOPPED(status) && !followfork>:
    Remove the dance around possible re-injection of WSTOPSIG(status)
    as the only observable stop here is the initial ptrace-stop.

diff -rup a/strace.c b/strace.c
--- a/strace.c	2017-08-31 14:04:21.000000000 -0400
+++ b/strace.c	2017-08-31 14:51:32.523134570 -0400
@@ -1958,11 +1958,15 @@ maybe_allocate_tcb(const int pid, int st
 			error_msg("Process %d attached", pid);
 		return tcp;
 	} else {
-		/* This can happen if a clone call used
-		 * CLONE_PTRACE itself.
+		/*
+		 * This can happen if a clone call misused CLONE_PTRACE itself.
+		 *
+		 * There used to be a dance around possible re-injection of
+		 * WSTOPSIG(status), but it was later removed as the only
+		 * observable stop here is the initial ptrace-stop.
 		 */
-		ptrace(PTRACE_CONT, pid, (char *) 0, 0);
-		error_msg("Stop of unknown pid %u seen, PTRACE_CONTed it", pid);
+		ptrace(PTRACE_DETACH, pid, NULL, 0L);
+		error_msg("Detached unknown pid %d", pid);
 		return NULL;
 	}
 }