Bug #844 tap: ignore TUNSETIFF EBUSY errors

This ioctl error suggests that the tap interface has already been set.

Backed out #411 and #651 as they only partially address the issue and introduced new bugs.

docs/CHANGELOG

@@ -10,10 +10,13 @@
     - add check for IPv6 extension header length (#827 #842)
     - GitHub template for pull requests (#839)
     - handle IPv6 fragment extension header (#832 #837)
+    - Linux tap interfaces fail intermittently (#828)
     - Infinite loop in tcprewrite at get.c (#827 #842)
+    - SLL incorrect size and protocal when converted to Ethernet (#826)
     - CVE-2023-43279 add check for empty CIDR (#824 #843)
     - AF_XDF socket extension (#822 #823)
     - configure.ac: unify search dirs for pcap and add lib32 (#819)
+    - tcpreplay-edit recomputes IPv4 checksums unnecessarily (#815 #846)
     - CVE-2023-4256 double free in tcprewrite DLT_JUNIPER_ETHER (#813 #851)
     - dlt_jnpr_ether_cleanup: check config before cleanup (#812 #851)
     - SEGV on invalid Juniper Ethernet header length (#811)
@@ -25,7 +28,8 @@
 06/04/2023 Version 4.4.4
     - overflow check fix for parse_mpls (#795)
     - tcpreplay-edit: prevent L2 flooding of ipv6 unicast packets (#793)
-    - CVE-2023-27784 CVE-2023-27785 CVE-2023-27786 CVE-2023-27787 CVE-2023-27788 CVE-2023-27789 bugs caused by strtok_r (#782 #784 #785 #786 #787 #788)
+    - CVE-2023-27784 CVE-2023-27785 CVE-2023-27786 CVE-2023-27787 CVE-2023-27788 CVE-2023-27789
+      bugs caused by strtok_r (#782 #784 #785 #786 #787 #788)
     - CVE-2023-27783 reachable assert in tcpedit_dlt_cleanup (#780)
     - add CI and C/C++ Linter and CodeQL (#773)
     - reachable assert in fast_edit_packet (#772)

src/common/sendpacket.c

@@ -524,10 +524,6 @@ sendpacket_open(const char *device,
                 sendpacket_type_t sendpacket_type _U_,
                 void *arg _U_)
 {
-#ifdef HAVE_TUNTAP
-    char sys_dev_dir[128];
-    bool device_exists;
-#endif
     sendpacket_t *sp;
     struct stat sdata;
 
@@ -536,11 +532,6 @@ sendpacket_open(const char *device,
 
     errbuf[0] = '\0';
 
-#ifdef HAVE_TUNTAP
-    snprintf(sys_dev_dir, sizeof(sys_dev_dir), "/sys/class/net/%s/", device);
-    device_exists = access(sys_dev_dir, R_OK) == 0;
-#endif
-
     /* khial is universal */
     if (stat(device, &sdata) == 0) {
         if (((sdata.st_mode & S_IFMT) == S_IFCHR)) {
@@ -563,7 +554,7 @@ sendpacket_open(const char *device,
             }
         }
 #ifdef HAVE_TUNTAP
-    } else if (strncmp(device, "tap", 3) == 0 && !device_exists) {
+    } else if (strncmp(device, "tap", 3) == 0) {
         sp = sendpacket_open_tuntap(device, errbuf);
 #endif
     } else {
@@ -895,9 +886,12 @@ sendpacket_open_tuntap(const char *device, char *errbuf)
     strncpy(ifr.ifr_name, device, sizeof(ifr.ifr_name) - 1);
 
     if (ioctl(tapfd, TUNSETIFF, (void *)&ifr) < 0) {
-        snprintf(errbuf, SENDPACKET_ERRBUF_SIZE, "Unable to create tuntap interface: %s", device);
-        close(tapfd);
-        return NULL;
+        // ignore EBUSY - it just means that the tunnel has already been opened
+        if (errno != EBUSY) {
+            snprintf(errbuf, SENDPACKET_ERRBUF_SIZE, "Unable to create tuntap interface: %s errno=%d", device, errno);
+            close(tapfd);
+            return NULL;
+        }
     }
 #elif defined(HAVE_FREEBSD)
     if (*device == '/') {