kill: add '--' option to separate options from arguments

Using a negative pid to send TERM to a process group results in an
obscure error:

   $ ./busybox kill -12345
   kill: bad signal name '12345'

This is intended. Manpage says:
ARGUMENTS
       pid    Each pid can be one of four things:
...
              -n     where n is larger than 1.  All processes in process group
                     n are signaled.  When an argument of  the  form  '-n'  is
                     given,  and it is meant to denote a process group, either
                     a signal must be specified first, or the argument must be
                     preceded  by a '--' option, otherwise it will be taken as
                     the signal to send.

However, we did not support "--". Add this capability to BusyBox.

function                                             old     new   delta
kill_main                                            993     999      +6

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/procps/kill.c b/procps/kill.c
index 0ddae2f70..24cc903fc 100644
--- a/procps/kill.c
+++ b/procps/kill.c
@@ -184,6 +184,10 @@ int kill_main(int argc UNUSED_PARAM, char **argv)
 	if (is_killall5 && arg[0] == 'o')
 		goto do_it_now;
 
+	/* "--" separates options from args. Testcase: "kill -- -123" */
+	if (!is_killall5 && arg[0] == '-' && arg[1] == '\0')
+		goto do_it_sooner;
+
 	if (argv[1] && arg[0] == 's' && arg[1] == '\0') { /* -s SIG? */
 		arg = *++argv;
 	} /* else it must be -SIG */
@@ -192,6 +196,7 @@ int kill_main(int argc UNUSED_PARAM, char **argv)
 		bb_error_msg("bad signal name '%s'", arg);
 		return EXIT_FAILURE;
 	}
+ do_it_sooner:
 	arg = *++argv;
 
  do_it_now: