[PATCH 5/5] linux ttyname and ttyname_r: Fix namespace check [BZ #22145]

[Luke Shumaker <lukeshu at parabola dot nu>]

In commit 15e9a4f Christian Brauner introduced logic for ttyname() sending
back ENODEV to signal that we can't get a name for the TTY because we
inherited it from a different mount namespace.

However, just because we inherited it from a different mount namespace, and
it isn't available at its original path, doesn't mean that its name is
unknowable; we can still find it by allowing the normal fall back on
iterating through devices.

A common scenario where this happens is with "/dev/console" in
containers.  Common container managers (including systemd-nspawn) will
call openpty() on a ptmx device in the host's mount namespace to
allocate a pty master/slave pair, then send the slave FD to the
container, and bind-mounted at "/dev/console" in the container's mount
namespace.  Inside of the container, the slave-end isn't available at
its original path ("/dev/pts/$X"), since the container mount namespace
has a separate devpts instance from the host (that path may or may not
exist in the container; if it does exist, it's not the same PTY slave
device).  Currently ttyname{_r}() sees that the original path isn't a
match, and fails early and gives up, even though if it kept searching
it would find the TTY at "/dev/console".  This fixes that so that the
ENODEV path does not force an early return inhibiting the fall-back
search.
---
 ChangeLog                           |  5 +++++
 sysdeps/unix/sysv/linux/ttyname.c   | 19 ++++++++++++-------
 sysdeps/unix/sysv/linux/ttyname_r.c | 20 ++++++++++++--------
 3 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 5973b9d50b..28f31d345b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2017-10-11  Luke Shumaker  <lukeshu@parabola.nu>
 
+	[BZ #22145]
+	* sysdeps/unix/sysv/linux/ttyname.c (ttyname):
+	Defer is_pty check until end.
+	* sysdeps/unix/sysv/linux/ttyname_r.c (ttyname_r): Likewise.
+
 	[BZ #22145]
 	* sysdeps/unix/sysv/linux/ttyname.h (is_mytty): New function.
 	* sysdeps/unix/sysv/linux/ttyname.c: Call it.
diff --git a/sysdeps/unix/sysv/linux/ttyname.c b/sysdeps/unix/sysv/linux/ttyname.c
index 138a8a57f8..ebd916f68e 100644
--- a/sysdeps/unix/sysv/linux/ttyname.c
+++ b/sysdeps/unix/sysv/linux/ttyname.c
@@ -115,6 +115,7 @@ ttyname (int fd)
   char procname[30];
   struct stat64 st, st1;
   int dostat = 0;
+  int doispty = 0;
   char *name;
   int save = errno;
   struct termios term;
@@ -165,13 +166,7 @@ ttyname (int fd)
 	  && is_mytty (&st, &st1))
 	return ttyname_buf;
 
-      /* If the link doesn't exist, then it points to a device in another
-	 namespace. */
-      if (is_pty (&st))
-	{
-	  __set_errno (ENODEV);
-	  return NULL;
-	}
+      doispty = 1;
     }
 
   if (__xstat64 (_STAT_VER, "/dev/pts", &st1) == 0 && S_ISDIR (st1.st_mode))
@@ -195,5 +190,15 @@ ttyname (int fd)
       name = getttyname ("/dev", &st, save, &dostat);
     }
 
+  if (!name && doispty && is_pty (&st))
+    {
+      /* We failed to figure out the TTY's name, but we can at least
+       * signal that we did verify that it really is a PTY slave.
+       * This happens when we have inherited the file descriptor from
+       * a different mount namespace. */
+      __set_errno (ENODEV);
+      return NULL;
+    }
+
   return name;
 }
diff --git a/sysdeps/unix/sysv/linux/ttyname_r.c b/sysdeps/unix/sysv/linux/ttyname_r.c
index d975d95d0d..adcffacb2c 100644
--- a/sysdeps/unix/sysv/linux/ttyname_r.c
+++ b/sysdeps/unix/sysv/linux/ttyname_r.c
@@ -95,6 +95,7 @@ __ttyname_r (int fd, char *buf, size_t buflen)
   char procname[30];
   struct stat64 st, st1;
   int dostat = 0;
+  int doispty = 0;
   int save = errno;
 
   /* Test for the absolute minimal size.  This makes life easier inside
@@ -149,14 +150,7 @@ __ttyname_r (int fd, char *buf, size_t buflen)
 	  && is_mytty (&st, &st1))
 	return 0;
 
-      /* If the link doesn't exist, then it points to a device in another
-       * namespace.
-       */
-      if (is_pty (&st))
-	{
-	  __set_errno (ENODEV);
-	  return ENODEV;
-	}
+      doispty = 1;
     }
 
   /* Prepare the result buffer.  */
@@ -190,6 +184,16 @@ __ttyname_r (int fd, char *buf, size_t buflen)
 			  save, &dostat);
     }
 
+  if (ret && doispty && is_pty (&st))
+    {
+      /* We failed to figure out the TTY's name, but we can at least
+       * signal that we did verify that it really is a PTY slave.
+       * This happens when we have inherited the file descriptor from
+       * a different mount namespace. */
+      __set_errno (ENODEV);
+      return ENODEV;
+    }
+
   return ret;
 }
 
-- 
2.14.2