openpty: use TIOCGPTPEER to open slave side fd

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>

1 files changed, 3 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index e1e98ad..da396ff 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,9 @@
 
 	* login/openpty.c (openpty): Close slave pty file descriptor on error.
 
+	* login/openpty.c (openpty): If defined, use the TIOCGPTPEER ioctl()
+	call to allocate the slave pty file descriptor.
+
 2017-10-06  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/ieee754/ldbl-128/s_fma.c: Include <libm-alias-double.h>.