dns: Fix bug in error handling when talking to script

Comparing the result of read/write to a size_t value
is dangerous C. Since ssize_t and size_t have the same
size ssize_t is promoted to size_t, so -1 becomes
size_t max value and is not smaller than the expected
length.

Make sure to compare ssize_t to ssize_t to avoid any
suprises.

Change-Id: Ic395b6d1dce510bb4b499c5beba61f033a2a860b
Signed-off-by: Frank Lichtenheld <[email protected]>
Acked-by: Heiko Hund <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1208
Message-Id: <[email protected]>
URL: https://sourceforge.net/p/openvpn/mailman/message/59238099/
Signed-off-by: Gert Doering <[email protected]>

Author: flichtenheld

src/openvpn/dns.c

@@ -642,11 +642,10 @@ run_updown_runner(bool up, struct options *o, const struct tuntap *tt,
 
         while (1)
         {
-            ssize_t rlen, wlen;
             char path[PATH_MAX];
 
             /* Block here until parent sends a path */
-            rlen = read(dns_pipe_fd[0], &path, sizeof(path));
+            ssize_t rlen = read(dns_pipe_fd[0], &path, sizeof(path));
             if (rlen < 1)
             {
                 if (rlen == -1 && errno == EINTR)
@@ -665,8 +664,8 @@ run_updown_runner(bool up, struct options *o, const struct tuntap *tt,
             /* Unblock parent process */
             while (1)
             {
-                wlen = write(ack_pipe_fd[1], &res, sizeof(res));
-                if ((wlen == -1 && errno != EINTR) || wlen < sizeof(res))
+                ssize_t wlen = write(ack_pipe_fd[1], &res, sizeof(res));
+                if ((wlen == -1 && errno != EINTR) || wlen < (ssize_t)sizeof(res))
                 {
                     /* Not much we can do about errors but exit */
                     close(dns_pipe_fd[0]);
@@ -727,7 +726,7 @@ run_up_down_command(bool up, struct options *o, const struct tuntap *tt,
         env_set_write_file(dvf, es);
 
         int wfd = updown_runner->fds[1];
-        size_t dvf_size = strlen(dvf) + 1;
+        ssize_t dvf_size = strlen(dvf) + 1;
         while (1)
         {
             ssize_t len = write(wfd, dvf, dvf_size);
@@ -746,7 +745,7 @@ run_up_down_command(bool up, struct options *o, const struct tuntap *tt,
         while (1)
         {
             ssize_t len = read(rfd, &status, sizeof(status));
-            if (len < sizeof(status))
+            if (len < (ssize_t)sizeof(status))
             {
                 if (len == -1 && errno == EINTR)
                 {