[PATCH] Fix segfault in pptpctrl argument parser

Hello,

it's easily possible to trigger a segfault in pptpctrl:

# /usr/sbin/pptpctrl 0 0 1 /etc/ppp/pptpd-options 0 1

This happened when triggering a bug in pptpmanager I am currently
working on. The check for (argc < 7) isn't sufficient, my suggested
fix adds a check to any GETARG_* invocation.

    Christoph

--- a/pptpctrl.c
+++ b/pptpctrl.c
@@ -92,19 +92,29 @@ static void launch_pppd(char **pppaddrs, struct in_addr *inetaddrs);
 #define OUR_NB_MODE O_NDELAY
 #endif
 
+void usage()
+{
+        fprintf(stderr, "pptpctrl: insufficient arguments, see man pptpctrl\n");
+        exit(2);
+}
+
 /* read a command line argument, a flag alone */
 #define GETARG_INT(X) \
+        if (arg >= argc) usage() ; \
         X = atoi(argv[arg++])
 
 /* read a command line argument, a string alone */
 #define GETARG_STRING(X) \
+        if (arg >= argc) usage() ; \
         X = strdup(argv[arg++])
 
 /* read a command line argument, a presence flag followed by string */
 #define GETARG_VALUE(X) \
-        if(atoi(argv[arg++]) != 0) \
+        if (arg >= argc) usage() ; \
+        if(atoi(argv[arg++]) != 0) { \
+                if (arg >= argc) usage() ; \
                 strlcpy(X, argv[arg++], sizeof(X)); \
-        else \
+        } else \
                 *X = '\0'
 
 int main(int argc, char **argv)
@@ -122,10 +132,8 @@ int main(int argc, char **argv)
         gargv = argv;
 
         /* fail if argument count invalid */
-        if (argc < 7) {
-                fprintf(stderr, "pptpctrl: insufficient arguments, see man pptpctrl\n");
-                exit(2);
-        }
+        if (argc < 7)
+                usage();
 
         /* open a connection to the syslog daemon */
         openlog("pptpd", LOG_PID, PPTP_FACILITY);

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