getgrgid_r fails with ERANGE if buffer is too small. Retry with a larger buffer.

Signed-off-by: Alexander Kriventsov <akriventsov@nic.ru>

src/lxc/cmd/lxc_user_nic.c

@@ -206,7 +206,28 @@ static char **get_groupnames(void)
 	}
 
 	for (i = 0; i < ngroups; i++) {
-		ret = getgrgid_r(group_ids[i], &grent, buf, bufsize, &grentp);
+		while ((ret = getgrgid_r(group_ids[i], &grent, buf, bufsize, &grentp)) == ERANGE) {
+			bufsize <<= 1;
+			if (bufsize > MAX_GRBUF_SIZE) {
+				usernic_error("Failed to get group members: %u\n",
+				      group_ids[i]);
+				free(buf);
+				free(group_ids);
+				free_groupnames(groupnames);
+				return NULL;
+			}
+			char *new_buf = realloc(buf, bufsize);
+			if (!new_buf) {
+				usernic_error("Failed to allocate memory while getting group "
+					      "names: %s\n",
+					      strerror(errno));
+				free(buf);
+				free(group_ids);
+				free_groupnames(groupnames);
+				return NULL;
+			}
+			buf = new_buf;
+		}
 		if (!grentp) {
 			if (ret == 0)
 				usernic_error("%s", "Could not find matched group record\n");

src/lxc/utils.h

@@ -26,6 +26,10 @@
 /* Properly support loop devices on 32bit systems. */
 #define _FILE_OFFSET_BITS 64
 
+#ifndef MAX_GRBUF_SIZE
+#define MAX_GRBUF_SIZE 65536
+#endif
+
 #include <errno.h>
 #include <linux/loop.h>
 #include <linux/types.h>