use SIOCGIFCONF much more carefully

1 files changed, 34 insertions, 14 deletions

diff --git a/src/lib/libc/net/if_nameindex.c b/src/lib/libc/net/if_nameindex.c
index 8714c1f95a..8643397db2 100644
--- a/src/lib/libc/net/if_nameindex.c
+++ b/src/lib/libc/net/if_nameindex.c
@@ -43,29 +43,48 @@
 struct if_nameindex *
 if_nameindex(void)
 {
-        int     i, j, fd, len;
+        int     i, j, fd = -1, extra, len = 0;
         struct if_nameindex *nameindex = NULL;
         struct ifconf ifconf;
-        char    lastname[IFNAMSIZ], *c;
+        char    lastname[IFNAMSIZ], *c, *inbuf;
         struct if_nameindex *n;
         struct sockaddr_dl *sd;
         struct sockaddr *sa;
         void   *p;
 
-        if ((fd = socket(PF_INET, SOCK_DGRAM, 0)) < 0)
-                return NULL;
+        ifconf.ifc_buf = NULL;
 
-        ifconf.ifc_len = 0;
-        ifconf.ifc_buf = 0;
-        if (ioctl(fd, SIOCGIFCONF, (void *) &ifconf))
-                goto ret;
-        if (ifconf.ifc_len < IFNAMSIZ)
-                goto ret;
-        if (!(ifconf.ifc_buf = malloc(ifconf.ifc_len)))
-                goto ret;
-        if (ioctl(fd, SIOCGIFCONF, (void *) &ifconf))
+        if ((fd = socket(PF_INET, SOCK_DGRAM, 0)) < 0)
                 goto ret;
 
+        /*
+         * Try ifc_len == 0 hack first, to get the actual length.
+         * If that fails, revert to a loop which grows the ifc_buf
+         * until it is sufficiently large.
+         */
+        extra = sizeof(struct ifreq);
+        while (1) {
+                ifconf.ifc_len = len;
+                if (ioctl(fd, SIOCGIFCONF, (void *) &ifconf) == -1 &&
+                    ifconf.ifc_buf)
+                        goto ret;
+                if (ifconf.ifc_buf &&
+                    ifconf.ifc_len + extra < len)
+                        break;
+                if (ifconf.ifc_buf) {
+                        if (len == 0)
+                                len = 4096;
+                        ifconf.ifc_len = len *= 2;
+                } else {
+                        len = ifconf.ifc_len;
+                        extra = 0;
+                }
+                inbuf = realloc(ifconf.ifc_buf, ifconf.ifc_len);
+                if (inbuf == NULL)
+                        goto ret;
+                ifconf.ifc_buf = inbuf;
+        }
+
         i = sizeof(struct if_nameindex);
         j = 0;
         p = ifconf.ifc_buf;
@@ -138,7 +157,8 @@ if_nameindex(void)
                 n->if_index = i;
 
 ret:
-        close(fd);
+        if (fd != -1)
+                close(fd);
         if (ifconf.ifc_buf)
                 free(ifconf.ifc_buf);
         return (nameindex);