Re: [PATCH 1/1] linux ttyname: return link if appropriate

[Serge Hallyn <serge dot hallyn at ubuntu dot com>]

Quoting Serge Hallyn (serge.hallyn@ubuntu.com):
> Quoting Florian Weimer (fweimer@redhat.com):
> > On 04/15/2016 06:46 PM, Serge Hallyn wrote:
> > >Quoting Florian Weimer (fweimer@redhat.com):
> > >>On 04/15/2016 05:29 PM, Serge Hallyn wrote:
> > >>>The current ttyname does the wrong thing in two cases:
> > >>>
> > >>>1. If the passed-in link (say /proc/self/fd/0) points to a
> > >>>device, say /dev/pts/2, in a parent mount namespace, and a
> > >>>/dev/pts/2 exists (in a different devpts) in the current
> > >>>namespace, then it returns /dev/pts/2.  But /dev/pts/2 is
> > >>>NOT the current tty, it is a different file and device.
> > >>
> > >>Is this the first change?
> > >
> > >Right, it ensures that the filesystem of the two files is
> > >the same.
> > >
> > >>>2. If the passed-in link (say /proc/self/fd/0) points to
> > >>>a device, say /dev/pts/2, in a parent mount namespace, and
> > >>>/dev/pts/2 does not exist in the current namespace, it
> > >>>returns success but an empty name.  As far as I can tell,
> > >>>there is no reason for it to not return /proc/self/fd/0.
> > >>>http://pubs.opengroup.org/onlinepubs/009695399/functions/ttyname.html
> > >>>does not say anything about not returning a link.
> > >>
> > >>Is it safe to drop the verification that ttyname ordinarily would do?
> > >
> > >Which verification do you mean exactly?
> > 
> > That the file descriptor actually belongs to a PTY device listed
> > under /dev/pts.
> 
> Oh, yeah.  I think that adding a chck that this is a pts (using st_rdev)
> before returning "/proc/self/fd/N" (in my newly added block) would be good.

Something like:

>From ba0dc51e90d884b107145924821a1e8caf43a468 Mon Sep 17 00:00:00 2001
From: Serge Hallyn <serge.hallyn@ubuntu.com>
Date: Fri, 15 Apr 2016 10:21:07 -0500
Subject: [PATCH 1/1] linux ttyname: return link if appropriate

The current ttyname does the wrong thing in two cases:

1. If the passed-in link (say /proc/self/fd/0) points to a
device, say /dev/pts/2, in a parent mount namespace, and a
/dev/pts/2 exists (in a different devpts) in the current
namespace, then it returns /dev/pts/2.  But /dev/pts/2 is
NOT the current tty, it is a different file and device.

2. If the passed-in link (say /proc/self/fd/0) points to
a device, say /dev/pts/2, in a parent mount namespace, and
/dev/pts/2 does not exist in the current namespace, it
returns success but an empty name.  As far as I can tell,
there is no reason for it to not return /proc/self/fd/0.
http://pubs.opengroup.org/onlinepubs/009695399/functions/ttyname.html
does not say anything about not returning a link.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
---
 sysdeps/unix/sysv/linux/ttyname.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/sysdeps/unix/sysv/linux/ttyname.c b/sysdeps/unix/sysv/linux/ttyname.c
index 7a001b4..54d0e6b 100644
--- a/sysdeps/unix/sysv/linux/ttyname.c
+++ b/sysdeps/unix/sysv/linux/ttyname.c
@@ -33,6 +33,21 @@
 char *__ttyname;
 #endif
 
+/*
+ * Return true if this is a UNIX98 pty device, as defined in
+ * linux/Documentation/devices.txt
+ */
+static int
+is_pty (struct stat *sb)
+{
+#ifdef _STATBUF_ST_RDEV
+  int m = major (sb.st_rdev);
+  return (136 <= m && m <= 143);
+#else
+  return false;
+#endif
+}
+
 static char *getttyname (const char *dev, dev_t mydev,
 			 ino64_t myino, int save, int *dostat)
      internal_function;
@@ -170,12 +185,22 @@ ttyname (int fd)
 #ifdef _STATBUF_ST_RDEV
 	  && S_ISCHR (st1.st_mode)
 	  && st1.st_rdev == st.st_rdev
+	  && st1.st_dev == st.st_dev
 #else
 	  && st1.st_ino == st.st_ino
 	  && st1.st_dev == st.st_dev
 #endif
 	  )
 	return ttyname_buf;
+      /* If the link doesn't exist, then it points to a dvice in another
+       * namespace.  If it is a UNIX98 pty, then return the /proc/self
+       * fd, as it points to a name unreachable in our namespace */
+      if (is_pty (st) && strlen (procname) < buflen - 1)
+        {
+          memcpy (ttyname_buf, procname, strlen (procname));
+          ttyname_buf[strlen (procname)] = '\0';
+          return ttyname_buf;
+        }
     }
 
   if (__xstat64 (_STAT_VER, "/dev/pts", &st1) == 0 && S_ISDIR (st1.st_mode))
-- 
2.7.4