[v7,5/6] linux ttyname and ttyname_r: Don't bail prematurely [BZ #22145]

Message ID 20171113212841.28518-6-lukeshu@lukeshu.com
State New
Headers show
Series
  • Fixup linux ttyname and ttyname_r [BZ #22145]
Related show

Commit Message

Luke Shumaker Nov. 13, 2017, 9:28 p.m.
From: Luke Shumaker <lukeshu@parabola.nu>

Commit 15e9a4f378c8607c2ae1aa465436af4321db0e23 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 try to find it by allowing the normal fall back on
iterating through devices.

An example scenario where this happens is with "/dev/console" in
containers.  It's a common practice among container managers (including
systemd-nspawn) to allocate a PTY master/slave pair in the host's mount
namespace (the slave having a path like "/dev/pty/$X"), bind mount the
slave to "/dev/console" in the container's mount namespace, and send the
slave FD to a process in the container.  Inside of the container, the
slave-end isn't available at its original path ("/dev/pts/$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 file
at the original "/dev/pts/$X" path doesn't match the FD passed to it, and
fails early and gives up, even though if it kept searching it would find
the TTY at "/dev/console".  Fix that; don't have the ENODEV path force an
early return inhibiting the fall-back search.

This change is based on the previous patch that adds use of is_mytty in
getttyname and getttyname_r.  Without that change, this effectively reverts
15e9a4f, which made us disregard the false similarity of file pointed to by
"/proc/self/fd/$Y", because if it doesn't bail prematurely then that file
("/dev/pts/$X") will just come up again anyway in the fall-back search.

v2:
 - Rearrange commit order
 - Revise commit message
v3:
 - Revise commit message
 - Revise ChangeLog message
 - Fix style of block comments
---
 ChangeLog                           |  5 +++++
 sysdeps/unix/sysv/linux/ttyname.c   | 19 ++++++++++++-------
 sysdeps/unix/sysv/linux/ttyname_r.c | 20 ++++++++++++--------
 3 files changed, 29 insertions(+), 15 deletions(-)

Patch

diff --git a/ChangeLog b/ChangeLog
index 7b1e6462f6..ddf41444f0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@ 
 2017-11-07  Luke Shumaker  <lukeshu@parabola.nu>
 
+	[BZ #22145]
+	* sysdeps/unix/sysv/linux/ttyname.c (ttyname):
+	Defer is_pty check until end of the function.
+	* 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 (getttyname): Call is_mytty.
diff --git a/sysdeps/unix/sysv/linux/ttyname.c b/sysdeps/unix/sysv/linux/ttyname.c
index 6e97d2d455..f4c955f25b 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 58eb919c3f..00eefc2c5c 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;
 }