[2/2,v4] openpty: use TIOCGPTPEER to open slave side fd
Commit Message
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(-)
@@ -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;