start: only use host's /dev/null when absolutely necessary

See comments for details, but basically, only use the host's /dev/null when
absolutely necessary (i.e. there is no reasonable /dev/null in the
container).

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>

diff --git a/src/lxc/start.c b/src/lxc/start.c
index fc98eb396493744c38c9e8bc0667c737fe8f9df1..6d2c7b5f5494171ccdc5e6fac4efb88f5487512e 100644 (file)
--- a/src/lxc/start.c
+++ b/src/lxc/start.c
@@ -710,7 +710,8 @@ static int do_start(void *data)
 {
        struct lxc_list *iterator;
        struct lxc_handler *handler = data;
-       int devnull_fd = -1;
+       int devnull_fd = -1, ret;
+       char path[PATH_MAX];
 
        if (sigprocmask(SIG_SETMASK, &handler->oldmask, NULL)) {
                SYSERROR("failed to set sigprocmask");
@@ -789,11 +790,28 @@ static int do_start(void *data)
        }
        #endif
 
-       if (handler->backgrounded) {
+       ret = sprintf(path, "%s/dev/null", handler->conf->rootfs.mount);
+       if (ret < 0 || ret >= sizeof(path)) {
+               SYSERROR("sprintf'd too many chars");
+               goto out_warn_father;
+       }
+
+       /* In order to checkpoint restore, we need to have everything in the
+        * same mount namespace. However, some containers may not have a
+        * reasonable /dev (in particular, they may not have /dev/null), so we
+        * can't set init's std fds to /dev/null by opening it from inside the
+        * container.
+        *
+        * If that's the case, fall back to using the host's /dev/null. This
+        * means that migration won't work, but at least we won't spew output
+        * where it isn't wanted.
+        */
+       if (handler->backgrounded && !handler->conf->autodev && access(path, F_OK) < 0) {
                devnull_fd = open_devnull();
 
                if (devnull_fd < 0)
                        goto out_warn_father;
+               WARN("using host's /dev/null for container init's std fds, migraiton won't work");
        }
 
        /* Setup the container, ip, names, utsname, ... */
@@ -861,6 +879,13 @@ static int do_start(void *data)
 
        close(handler->sigfd);
 
+       if (devnull_fd < 0) {
+               devnull_fd = open_devnull();
+
+               if (devnull_fd < 0)
+                       goto out_warn_father;
+       }
+
        if (handler->backgrounded && set_stdfds(devnull_fd))
                goto out_warn_father;