diff mbox

[2/2,v4] openpty: use TIOCGPTPEER to open slave side fd

Message ID 20170829134515.9345-2-christian.brauner@ubuntu.com
State New, archived
Headers show

Commit Message

Christian Brauner Aug. 29, 2017, 1:45 p.m. UTC
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(-)
diff mbox

Patch

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 a7b1ab5dde..766aa958ff 100644
--- a/login/openpty.c
+++ b/login/openpty.c
@@ -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;