@@ -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.
@@ -94,6 +94,8 @@ openpty (int *amaster, int *aslave, char *name,
char *buf = _buf;
int master, ret = -1, slave = -1;
+ *buf = '\0';
+
master = getpt ();
if (master == -1)
return -1;
@@ -104,12 +106,22 @@ openpty (int *amaster, int *aslave, char *name,
if (unlockpt (master))
goto on_error;
- if (pts_name (master, &buf, sizeof (_buf)))
- goto on_error;
-
- slave = open (buf, O_RDWR | O_NOCTTY);
+#ifdef TIOCGPTPEER
+ /* Try to allocate slave fd solely based on master fd first. */
+ slave = ioctl (master, TIOCGPTPEER, O_RDWR | O_NOCTTY);
+#endif
if (slave == -1)
- goto on_error;
+ {
+ /* Fallback to path-based slave fd allocation in case kernel doesn't
+ * support TIOCGPTPEER.
+ */
+ if (pts_name (master, &buf, sizeof (_buf)))
+ goto on_error;
+
+ slave = open (buf, O_RDWR | O_NOCTTY);
+ if (slave == -1)
+ goto on_error;
+ }
/* XXX Should we ignore errors here? */
if (termp)
@@ -122,7 +134,13 @@ openpty (int *amaster, int *aslave, char *name,
*amaster = master;
*aslave = slave;
if (name != NULL)
- strcpy (name, buf);
+ {
+ if (*buf == '\0')
+ if (pts_name (master, &buf, sizeof (_buf)))
+ goto on_error;
+
+ strcpy (name, buf);
+ }
ret = 0;
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 2017-08-28: * Instead of #ifdefing the TIOCGPTPEER ioctl flag we now try the ioctl() first and if it fails we fallback to path-based allocation of the slave fd. This allows us retain backward compatibility with kernels that do not support this ioctl call. * A note on the following codepath if (name != NULL) { if (*buf == '\0') if (pts_name (master, &buf, sizeof (_buf))) goto fail; strcpy (name, buf); } "buf" is guaranteed to be allocated in this case. If the pts_name() call above failed we would have never reached this code path. If it has been called succesfully it will either have handed us a valid buffer or "buf" will still point to the static char array "_buf" which is initialized to 0. Changelog 2017-08-28: * Preserve #ifdef for TIOCGPTPEER since it needs to work on non-Linux distros too. * Only intialize first byte of "_buf". Changelog 2017-08-29: * Adapt to unified error handling as suggested by Florian. --- ChangeLog | 5 +++++ login/openpty.c | 30 ++++++++++++++++++++++++------ 2 files changed, 29 insertions(+), 6 deletions(-)