httpd(8): dup2() fix for proc.c

[Rafael Zalamena]

This diff fixes the same problem ntpd(8) had with the dup2() when oldd == newd.

Quick background:
when you dup2(oldd, newd) and oldd == newd the CLOEXEC flag won't be removed
by the descriptor. We could use dup3() to detect this, but it is easier/faster
just to compare the fds and do the fcntl() ourselves.

ok?


Index: proc.c
===================================================================
RCS file: /home/obsdcvs/src/usr.sbin/httpd/proc.c,v
retrieving revision 1.27
diff -u -p -r1.27 proc.c
--- proc.c      28 Sep 2016 12:01:04 -0000      1.27
+++ proc.c      5 Oct 2016 16:50:40 -0000
@@ -22,6 +22,7 @@
 #include <sys/socket.h>
 #include <sys/wait.h>
 
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -131,7 +132,12 @@ proc_exec(struct privsep *ps, struct pri
                                break;
                        case 0:
                                /* Prepare parent socket. */
-                               dup2(fd, PROC_PARENT_SOCK_FILENO);
+                               if (fd != PROC_PARENT_SOCK_FILENO) {
+                                       if (dup2(fd, PROC_PARENT_SOCK_FILENO)
+                                           == -1)
+                                               fatal("dup2");
+                               } else if (fcntl(fd, F_SETFD, 0) == -1)
+                                       fatal("fcntl");
 
                                execvp(argv[0], nargv);
                                fatal("%s: execvp", __func__);