[PATCH 2/2] openpty: use TIOCGPTPEER to open slave side fd

[Christian Brauner <christian dot brauner at ubuntu dot com>]

Newer kernels expose the ioctl TIOCGPTPEER [1] call to userspace which allows to
safely allocate a file descriptor for a pty slave based solely on the master
file descriptor. This allows us to avoid path-based operations and makes this
function a lot safer in the face of devpts mounts in different mount namespaces.

[1]: https://patchwork.kernel.org/patch/9760743/

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog       |  5 +++++
 login/openpty.c | 12 +++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index bc5fb8e27f..30829e4c16 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-08-26  Christian Brauner  <christian.brauner@ubuntu.com>
+
+	* login/openpty.c (openpty): If defined, use the TIOCGPTPEER ioctl call
+	to allocate the slave pty file descriptor.
+
 2017-08-26  Christian Brauner  <christian.brauner@ubuntu.com>
 
 	* login/openpty.c (openpty): Close slave pty file descriptor on error.
diff --git a/login/openpty.c b/login/openpty.c
index 8fbc66a3ef..293cc0a0db 100644
--- a/login/openpty.c
+++ b/login/openpty.c
@@ -104,10 +104,14 @@ openpty (int *amaster, int *aslave, char *name,
   if (unlockpt (master))
     goto fail;
 
+#ifdef TIOCGPTPEER
+  slave = ioctl (master, TIOCGPTPEER, O_RDWR | O_NOCTTY);
+#else
   if (pts_name (master, &buf, sizeof (_buf)))
     goto fail;
 
   slave = open (buf, O_RDWR | O_NOCTTY);
+#endif
   if (slave == -1)
     {
       if (buf != _buf)
@@ -127,7 +131,13 @@ openpty (int *amaster, int *aslave, char *name,
   *amaster = master;
   *aslave = slave;
   if (name != NULL)
-    strcpy (name, buf);
+    {
+#ifdef TIOCGPTPEER
+      if (pts_name (master, &buf, sizeof (_buf)))
+        goto fail;
+#endif
+      strcpy (name, buf);
+    }
 
   if (buf != _buf)
     free (buf);
-- 
2.14.1