[PATCH v2] xl: close nullfd after dup2'ing it to stdin

[Ian Campbell <ian.campbell@citrix.com>]

We assert that nullfd if not std{in,out,err} since that would result
in closing one of the just dup2'd fds. For this to happen
std{in,out,err} would have needed to be closed, at which point all
sorts of other things could go wrong.

CID: 1130519

It was previously hypothesised[0] that fixing 1130516 would solve this
too, but that appears to not have been the case.

Compile tested only.

[0] http://lists.xenproject.org/archives/html/xen-devel/2013-11/msg02931.html

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: andrew.cooper3@citrix.com
---
v2: Assert logfile and nullfd are not stdio fds
---
 tools/libxl/xl_cmdimpl.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
index 9958d8a..a377de1 100644
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -499,12 +499,17 @@ static int do_daemonize(char *name, const char *pidfile)
 
     CHK_SYSCALL(logfile = open(fullname, O_WRONLY|O_CREAT|O_APPEND, 0644));
     free(fullname);
+    assert(logfile >= 3);
 
     CHK_SYSCALL(nullfd = open("/dev/null", O_RDONLY));
+    assert(nullfd >= 3);
+
     dup2(nullfd, 0);
     dup2(logfile, 1);
     dup2(logfile, 2);
 
+    close(nullfd);
+
     CHK_SYSCALL(daemon(0, 1));
 
     if (pidfile) {
-- 
2.1.4