[PATCH COMMITTED 5/6] linux ttyname{_r}: Don't bail prematurely [BZ #22145]

[PATCH COMMITTED 5/6] linux ttyname{_r}: Don't bail prematurely [BZ #22145]

[Christian Brauner]

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 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.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 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 00f388369f..566591047f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2017-11-15  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;
 }
 
-- 
2.14.1

[PATCH COMMITTED 4/6] linux ttyname{_r}: Make tty checks consistent

[Christian Brauner]

From: Luke Shumaker <lukeshu@parabola.nu>

In the ttyname and ttyname_r routines on Linux, at several points it needs to
check if a given TTY is the TTY we are looking for. It used to be that this
check was (to see if `maybe` is `mytty`):

       __xstat64(_STAT_VER, maybe_filename, &maybe) == 0
    #ifdef _STATBUF_ST_RDEV
       && S_ISCHR(maybe.st_mode) && maybe.st_rdev == mytty.st_rdev
    #else
       && maybe.st_ino == mytty.st_ino && maybe.st_dev == mytty.st_dev
    #endif

This check appears in several places.

Then, one of the changes made in commit 15e9a4f378c8607c2ae1aa465436af4321db0e23
was to change that check to:

       __xstat64(_STAT_VER, maybe_filename, &maybe) == 0
    #ifdef _STATBUF_ST_RDEV
       && S_ISCHR(maybe.st_mode) && maybe.st_rdev == mytty.st_rdev
    #endif
       && maybe.st_ino == mytty.st_ino && maybe.st_dev == mytty.st_dev

That is, it made the st_ino and st_dev parts of the check happen even if we have
the st_rdev member. This is an important change, because the kernel allows
multiple devpts filesystem instances to be created; a device file in one devpts
instance may share the same st_rdev with a file in another devpts instance, but
they aren't the same file.

This check appears twice in each file (ttyname.c and ttyname_r.c), once (in
ttyname and __ttyname_r) to check if a candidate file found by inspecting /proc
is the desired TTY, and once (in getttyname and getttyname_r) to check if a
candidate file found by searching /dev is the desired TTY. However, 15e9a4f
only updated the checks for files found via /proc; but the concern about
collisions between devpts instances is just as valid for files found via /dev.

So, update all 4 occurrences the check to be consistent with the version of the
check introduced in 15e9a4f. Make it easy to keep all 4 occurrences of the
check consistent by pulling it in to a static inline function, is_mytty.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog                           |  7 +++++++
 sysdeps/unix/sysv/linux/ttyname.c   | 40 ++++++++----------------------------
 sysdeps/unix/sysv/linux/ttyname.h   | 12 +++++++++++
 sysdeps/unix/sysv/linux/ttyname_r.c | 41 ++++++++-----------------------------
 4 files changed, 36 insertions(+), 64 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index bb960c04bd..00f388369f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,12 @@
 2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
 
+	[BZ #22145]
+	* sysdeps/unix/sysv/linux/ttyname.h (is_mytty): New function.
+	* sysdeps/unix/sysv/linux/ttyname.c (getttyname): Call is_mytty.
+	(ttyname): Likewise.
+	* sysdeps/unix/sysv/linux/ttyname_r.c (getttyname_r): Likewise.
+	(__ttyname_r): Likewise.
+
 	* sysdeps/unix/sysv/linux/ttyname.h (is_pty): Change return type from
 	int to bool.
 
diff --git a/sysdeps/unix/sysv/linux/ttyname.c b/sysdeps/unix/sysv/linux/ttyname.c
index 116cf350e5..6e97d2d455 100644
--- a/sysdeps/unix/sysv/linux/ttyname.c
+++ b/sysdeps/unix/sysv/linux/ttyname.c
@@ -35,14 +35,14 @@
 char *__ttyname;
 #endif
 
-static char *getttyname (const char *dev, dev_t mydev,
-			 ino64_t myino, int save, int *dostat);
+static char *getttyname (const char *dev, const struct stat64 *mytty,
+			 int save, int *dostat);
 
 libc_freeres_ptr (static char *getttyname_name);
 
 static char *
 attribute_compat_text_section
-getttyname (const char *dev, dev_t mydev, ino64_t myino, int save, int *dostat)
+getttyname (const char *dev, const struct stat64 *mytty, int save, int *dostat)
 {
   static size_t namelen;
   struct stat64 st;
@@ -63,7 +63,7 @@ getttyname (const char *dev, dev_t mydev, ino64_t myino, int save, int *dostat)
     *((char *) __mempcpy (getttyname_name, dev, devlen - 1)) = '/';
 
   while ((d = __readdir64 (dirstream)) != NULL)
-    if ((d->d_fileno == myino || *dostat)
+    if ((d->d_fileno == mytty->st_ino || *dostat)
 	&& strcmp (d->d_name, "stdin")
 	&& strcmp (d->d_name, "stdout")
 	&& strcmp (d->d_name, "stderr"))
@@ -85,12 +85,7 @@ getttyname (const char *dev, dev_t mydev, ino64_t myino, int save, int *dostat)
 	  }
 	memcpy (&getttyname_name[devlen], d->d_name, dlen);
 	if (__xstat64 (_STAT_VER, getttyname_name, &st) == 0
-#ifdef _STATBUF_ST_RDEV
-	    && S_ISCHR (st.st_mode) && st.st_rdev == mydev
-#else
-	    && d->d_fileno == myino && st.st_dev == mydev
-#endif
-	   )
+	    && is_mytty (mytty, &st))
 	  {
 	    (void) __closedir (dirstream);
 #if 0
@@ -167,12 +162,7 @@ ttyname (int fd)
       /* Verify readlink result, fall back on iterating through devices.  */
       if (ttyname_buf[0] == '/'
 	  && __xstat64 (_STAT_VER, ttyname_buf, &st1) == 0
-#ifdef _STATBUF_ST_RDEV
-	  && S_ISCHR (st1.st_mode)
-	  && st1.st_rdev == st.st_rdev
-#endif
-	  && st1.st_ino == st.st_ino
-	  && st1.st_dev == st.st_dev)
+	  && is_mytty (&st, &st1))
 	return ttyname_buf;
 
       /* If the link doesn't exist, then it points to a device in another
@@ -186,11 +176,7 @@ ttyname (int fd)
 
   if (__xstat64 (_STAT_VER, "/dev/pts", &st1) == 0 && S_ISDIR (st1.st_mode))
     {
-#ifdef _STATBUF_ST_RDEV
-      name = getttyname ("/dev/pts", st.st_rdev, st.st_ino, save, &dostat);
-#else
-      name = getttyname ("/dev/pts", st.st_dev, st.st_ino, save, &dostat);
-#endif
+      name = getttyname ("/dev/pts", &st, save, &dostat);
     }
   else
     {
@@ -200,21 +186,13 @@ ttyname (int fd)
 
   if (!name && dostat != -1)
     {
-#ifdef _STATBUF_ST_RDEV
-      name = getttyname ("/dev", st.st_rdev, st.st_ino, save, &dostat);
-#else
-      name = getttyname ("/dev", st.st_dev, st.st_ino, save, &dostat);
-#endif
+      name = getttyname ("/dev", &st, save, &dostat);
     }
 
   if (!name && dostat != -1)
     {
       dostat = 1;
-#ifdef _STATBUF_ST_RDEV
-      name = getttyname ("/dev", st.st_rdev, st.st_ino, save, &dostat);
-#else
-      name = getttyname ("/dev", st.st_dev, st.st_ino, save, &dostat);
-#endif
+      name = getttyname ("/dev", &st, save, &dostat);
     }
 
   return name;
diff --git a/sysdeps/unix/sysv/linux/ttyname.h b/sysdeps/unix/sysv/linux/ttyname.h
index cdaf60fb87..48181330a9 100644
--- a/sysdeps/unix/sysv/linux/ttyname.h
+++ b/sysdeps/unix/sysv/linux/ttyname.h
@@ -34,3 +34,15 @@ is_pty (struct stat64 *sb)
   return false;
 #endif
 }
+
+static inline bool
+is_mytty (const struct stat64 *mytty, const struct stat64 *maybe)
+{
+  return (maybe->st_ino == mytty->st_ino
+	  && maybe->st_dev == mytty->st_dev
+#ifdef _STATBUF_ST_RDEV
+	  && S_ISCHR (maybe->st_mode)
+	  && maybe->st_rdev == mytty->st_rdev
+#endif
+	  );
+}
diff --git a/sysdeps/unix/sysv/linux/ttyname_r.c b/sysdeps/unix/sysv/linux/ttyname_r.c
index 18f35ef2b7..58eb919c3f 100644
--- a/sysdeps/unix/sysv/linux/ttyname_r.c
+++ b/sysdeps/unix/sysv/linux/ttyname_r.c
@@ -31,12 +31,12 @@
 #include "ttyname.h"
 
 static int getttyname_r (char *buf, size_t buflen,
-			 dev_t mydev, ino64_t myino, int save,
+			 const struct stat64 *mytty, int save,
 			 int *dostat);
 
 static int
 attribute_compat_text_section
-getttyname_r (char *buf, size_t buflen, dev_t mydev, ino64_t myino,
+getttyname_r (char *buf, size_t buflen, const struct stat64 *mytty,
 	      int save, int *dostat)
 {
   struct stat64 st;
@@ -52,7 +52,7 @@ getttyname_r (char *buf, size_t buflen, dev_t mydev, ino64_t myino,
     }
 
   while ((d = __readdir64 (dirstream)) != NULL)
-    if ((d->d_fileno == myino || *dostat)
+    if ((d->d_fileno == mytty->st_ino || *dostat)
 	&& strcmp (d->d_name, "stdin")
 	&& strcmp (d->d_name, "stdout")
 	&& strcmp (d->d_name, "stderr"))
@@ -72,12 +72,7 @@ getttyname_r (char *buf, size_t buflen, dev_t mydev, ino64_t myino,
 	cp[0] = '\0';
 
 	if (__xstat64 (_STAT_VER, buf, &st) == 0
-#ifdef _STATBUF_ST_RDEV
-	    && S_ISCHR (st.st_mode) && st.st_rdev == mydev
-#else
-	    && d->d_fileno == myino && st.st_dev == mydev
-#endif
-	   )
+	    && is_mytty (mytty, &st))
 	  {
 	    (void) __closedir (dirstream);
 	    __set_errno (save);
@@ -151,12 +146,7 @@ __ttyname_r (int fd, char *buf, size_t buflen)
       /* Verify readlink result, fall back on iterating through devices.  */
       if (buf[0] == '/'
 	  && __xstat64 (_STAT_VER, buf, &st1) == 0
-#ifdef _STATBUF_ST_RDEV
-	  && S_ISCHR (st1.st_mode)
-	  && st1.st_rdev == st.st_rdev
-#endif
-	  && st1.st_ino == st.st_ino
-	  && st1.st_dev == st.st_dev)
+	  && is_mytty (&st, &st1))
 	return 0;
 
       /* If the link doesn't exist, then it points to a device in another
@@ -175,13 +165,8 @@ __ttyname_r (int fd, char *buf, size_t buflen)
 
   if (__xstat64 (_STAT_VER, buf, &st1) == 0 && S_ISDIR (st1.st_mode))
     {
-#ifdef _STATBUF_ST_RDEV
-      ret = getttyname_r (buf, buflen, st.st_rdev, st.st_ino, save,
+      ret = getttyname_r (buf, buflen, &st, save,
 			  &dostat);
-#else
-      ret = getttyname_r (buf, buflen, st.st_dev, st.st_ino, save,
-			  &dostat);
-#endif
     }
   else
     {
@@ -193,26 +178,16 @@ __ttyname_r (int fd, char *buf, size_t buflen)
     {
       buf[sizeof ("/dev/") - 1] = '\0';
       buflen += sizeof ("pts/") - 1;
-#ifdef _STATBUF_ST_RDEV
-      ret = getttyname_r (buf, buflen, st.st_rdev, st.st_ino, save,
-			  &dostat);
-#else
-      ret = getttyname_r (buf, buflen, st.st_dev, st.st_ino, save,
+      ret = getttyname_r (buf, buflen, &st, save,
 			  &dostat);
-#endif
     }
 
   if (ret && dostat != -1)
     {
       buf[sizeof ("/dev/") - 1] = '\0';
       dostat = 1;
-#ifdef _STATBUF_ST_RDEV
-      ret = getttyname_r (buf, buflen, st.st_rdev, st.st_ino,
-			  save, &dostat);
-#else
-      ret = getttyname_r (buf, buflen, st.st_dev, st.st_ino,
+      ret = getttyname_r (buf, buflen, &st,
 			  save, &dostat);
-#endif
     }
 
   return ret;
-- 
2.14.1

[PATCH COMMITTED 3/6] linux ttyname: Change return type of is_pty from int to bool

[Christian Brauner]

From: Luke Shumaker <lukeshu@parabola.nu>

is_pty returning a bool is fine since there's no possible outcome other than
true or false, and bool is used throughout the codebase.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog                         | 3 +++
 sysdeps/unix/sysv/linux/ttyname.h | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 6194237230..bb960c04bd 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,8 @@
 2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
 
+	* sysdeps/unix/sysv/linux/ttyname.h (is_pty): Change return type from
+	int to bool.
+
 	* sysdeps/unix/sysv/linux/ttyname.h (is_pty): Update doc reference.
 
 	* manual/terminal.texi (Is It a Terminal):
diff --git a/sysdeps/unix/sysv/linux/ttyname.h b/sysdeps/unix/sysv/linux/ttyname.h
index cbcdbab607..cdaf60fb87 100644
--- a/sysdeps/unix/sysv/linux/ttyname.h
+++ b/sysdeps/unix/sysv/linux/ttyname.h
@@ -16,6 +16,7 @@
    not, see <http://www.gnu.org/licenses/>.  */
 
 #include <unistd.h>
+#include <stdbool.h>
 #include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -23,7 +24,7 @@
 /* Return true if this is a UNIX98 pty device, as defined in
    linux/Documentation/devices.txt (on linux < 4.10) or
    linux/Documentation/admin-guide/devices.txt (on linux >= 4.10).  */
-static inline int
+static inline bool
 is_pty (struct stat64 *sb)
 {
 #ifdef _STATBUF_ST_RDEV
-- 
2.14.1

[PATCH COMMITTED 1/6] manual: Update to mention ENODEV for ttyname and ttyname_r

[Christian Brauner]

From: Luke Shumaker <lukeshu@parabola.nu>

Commit 15e9a4f378c8607c2ae1aa465436af4321db0e23 introduced ENODEV as a possible
error condition for ttyname and ttyname_r. Update the manual to mention this GNU
extension.

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog            | 5 +++++
 manual/terminal.texi | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index f8f46ae45d..7916ad639f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
+
+	* manual/terminal.texi (Is It a Terminal):
+	Mention ENODEV for ttyname and ttyname_r.
+
 2017-11-15  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/unix/sysv/linux/bits/socket.h (MSG_ZEROCOPY): New enum
diff --git a/manual/terminal.texi b/manual/terminal.texi
index 4fef5045b8..4aace48b14 100644
--- a/manual/terminal.texi
+++ b/manual/terminal.texi
@@ -109,6 +109,11 @@ The @var{filedes} is not associated with a terminal.
 @item ERANGE
 The buffer length @var{len} is too small to store the string to be
 returned.
+
+@item ENODEV
+The @var{filedes} is associated with a terminal device that is a slave
+pseudo-terminal, but the file name associated with that device could
+not be determined.  This is a GNU extension.
 @end table
 @end deftypefun
 
-- 
2.14.1

[PATCH COMMITTED 6/6] linux ttyname{_r}: Add tests

[Christian Brauner]

From: Luke Shumaker <lukeshu@parabola.nu>

Add a new tst-ttyname test that includes several named sub-testcases.

This patch is ordered after the patches with the fixes that it tests for (to
avoid breaking `git bisect`), but for reference, here's how each relevant change
so far affected the testcases in this commit, starting with
15e9a4f378c8607c2ae1aa465436af4321db0e23:

  |                                 | before  |         | make checks | don't |
  |                                 | 15e9a4f | 15e9a4f | consistent  | bail  |
  |---------------------------------+---------+---------+-------------+-------|
  | basic smoketest                 | PASS    | PASS    | PASS        | PASS  |
  | no conflict, no match           | PASS[1] | PASS    | PASS        | PASS  |
  | no conflict, console            | PASS    | FAIL!   | FAIL        | PASS! |
  | conflict, no match              | FAIL    | PASS!   | PASS        | PASS  |
  | conflict, console               | FAIL    | FAIL    | FAIL        | PASS! |
  | with readlink target            | PASS    | PASS    | PASS        | PASS  |
  | with readlink trap; fallback    | FAIL    | FAIL    | FAIL        | PASS! |
  | with readlink trap; no fallback | FAIL    | PASS!   | PASS        | PASS  |
  | with search-path trap           | FAIL    | FAIL    | PASS!       | PASS  |
  |---------------------------------+---------+---------+-------------+-------|
  |                                 | 4/9     | 5/9     | 6/9         | 9/9   |

  [1]: 15e9a4f introduced a semantic that, under certain failure
       conditions, ttyname sets errno=ENODEV, where previously it didn't
       set errno; it's not quite fair to hold "before 15e9a4f" ttyname to
       those new semantics.  This testcase actually fails, but would have
       passed if we tested for the old the semantics.

Each of the failing tests before 15e9a4f are all essentially the same bug: that
it returns a PTY slave with the correct minor device number, but from the wrong
devpts filesystem instance.

15e9a4f sought to fix this, but missed several of the cases that can cause this
to happen, and also broke the case where both the erroneous PTY and the correct
PTY exist.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog                             |   4 +
 sysdeps/unix/sysv/linux/Makefile      |   3 +-
 sysdeps/unix/sysv/linux/tst-ttyname.c | 625 ++++++++++++++++++++++++++++++++++
 3 files changed, 631 insertions(+), 1 deletion(-)
 create mode 100644 sysdeps/unix/sysv/linux/tst-ttyname.c

diff --git a/ChangeLog b/ChangeLog
index 566591047f..167ff200e8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
 
+	[BZ #22145]
+	* sysdeps/unix/sysv/linux/tst-ttyname.c: New file.
+	* sysdeps/unix/sysv/linux/Makefile: Add tst-ttyname to tests.
+
 	[BZ #22145]
 	* sysdeps/unix/sysv/linux/ttyname.c (ttyname):
 	Defer is_pty check until end of the function.
diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index bf76b8773d..c6675b3aa5 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -43,7 +43,8 @@ sysdep_headers += sys/mount.h sys/acct.h sys/sysctl.h \
 		  bits/siginfo-arch.h bits/siginfo-consts-arch.h
 
 tests += tst-clone tst-clone2 tst-clone3 tst-fanotify tst-personality \
-	 tst-quota tst-sync_file_range test-errno-linux tst-sysconf-iov_max
+	 tst-quota tst-sync_file_range tst-sysconf-iov_max tst-ttyname \
+	 test-errno-linux
 
 # Generate the list of SYS_* macros for the system calls (__NR_*
 # macros).  The file syscall-names.list contains all possible system
diff --git a/sysdeps/unix/sysv/linux/tst-ttyname.c b/sysdeps/unix/sysv/linux/tst-ttyname.c
new file mode 100644
index 0000000000..32d7a65938
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-ttyname.c
@@ -0,0 +1,625 @@
+/* Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If
+   not, see <http://www.gnu.org/licenses/>.  */
+
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mount.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include <support/check.h>
+#include <support/namespace.h>
+#include <support/support.h>
+#include <support/temp_file.h>
+#include <support/test-driver.h>
+#include <support/xunistd.h>
+
+/* generic utilities */
+
+#define VERIFY(expr)                                                    \
+  do {                                                                  \
+    if (!(expr))                                                        \
+      {                                                                 \
+        printf ("error: %s:%d: %s: %m\n",                               \
+                __FILE__, __LINE__, #expr);                             \
+        exit (1);                                                       \
+      }                                                                 \
+  } while (0)
+
+static void
+touch (const char *path, mode_t mode)
+{
+  xclose (xopen (path, O_WRONLY|O_CREAT|O_NOCTTY, mode));
+}
+
+static size_t
+trim_prefix (char *str, size_t str_len, const char *prefix)
+{
+  size_t prefix_len = strlen (prefix);
+  if (str_len > prefix_len && memcmp (str, prefix, prefix_len) == 0)
+    {
+      memmove (str, str + prefix_len, str_len - prefix_len);
+      return str_len - prefix_len;
+    }
+  return str_len;
+}
+
+/* returns a pointer to static storage */
+static char *
+proc_fd_readlink (const char *linkname)
+{
+  static char target[PATH_MAX+1];
+  ssize_t target_len = readlink (linkname, target, PATH_MAX);
+  VERIFY (target_len > 0);
+  target_len = trim_prefix (target, target_len, "(unreachable)");
+  target[target_len] = '\0';
+  return target;
+}
+
+static void
+become_root_in_mount_ns (void)
+{
+  uid_t orig_uid = getuid ();
+  gid_t orig_gid = getgid ();
+
+  support_become_root ();
+
+  if (unshare (CLONE_NEWNS) < 0)
+    FAIL_UNSUPPORTED ("could not enter new mount namespace");
+
+  /* support_become_root might have put us in a new user namespace;
+     most filesystems (including tmpfs) don't allow file or directory
+     creation from a user namespace unless uid and gid maps are set,
+     even if we have root privileges in the namespace (failing with
+     EOVERFLOW, since the uid overflows the empty (0-length) uid map).
+
+     Also, stat always reports that uid and gid maps are empty, so we
+     have to try actually reading from them to check if they are
+     empty.  */
+  int fd;
+
+  if ((fd = open ("/proc/self/uid_map", O_RDWR, 0)) >= 0)
+    {
+      char buf;
+      if (read (fd, &buf, 1) == 0)
+	{
+	  char *str = xasprintf ("0 %ld 1\n", (long)orig_uid);
+	  if (write (fd, str, strlen (str)) < 0)
+	    FAIL_EXIT1 ("write (uid_map, \"%s\"): %m", str);
+	  free (str);
+	}
+      xclose (fd);
+    }
+
+  /* Setting the gid map has the additional complexity that we have to
+     first turn off setgroups.  */
+  if ((fd = open ("/proc/self/setgroups", O_WRONLY, 0)) >= 0)
+    {
+      const char *str = "deny";
+      if (write (fd, str, strlen (str)) < 0)
+	FAIL_EXIT1 ("write (setroups, \"%s\"): %m", str);
+      xclose (fd);
+    }
+
+  if ((fd = open ("/proc/self/gid_map", O_RDWR, 0)) >= 0)
+    {
+      char buf;
+      if (read (fd, &buf, 1) == 0)
+	{
+	  char *str = xasprintf ("0 %ld 1\n", (long)orig_gid);
+	  if (write (fd, str, strlen (str)) < 0)
+	    FAIL_EXIT1 ("write (gid_map, \"%s\"): %m", str);
+	  free (str);
+	}
+      xclose (fd);
+    }
+}
+
+/* plain ttyname runner */
+
+struct result
+{
+  const char *name;
+  int err;
+};
+
+/* strings in result structure are in static storage */
+static struct result
+run_ttyname (int fd)
+{
+  struct result ret;
+  errno = 0;
+  ret.name = ttyname (fd);
+  ret.err = errno;
+  return ret;
+}
+
+static bool
+eq_ttyname (struct result actual, struct result expected)
+{
+  char *actual_name, *expected_name;
+
+  if ((actual.err == expected.err) &&
+      (!actual.name == !expected.name) &&
+      (actual.name ? strcmp (actual.name, expected.name) == 0 : true))
+    {
+      if (expected.name)
+        expected_name = xasprintf ("\"%s\"", expected.name);
+      else
+	expected_name = xstrdup ("NULL");
+
+      printf ("info:      ttyname: PASS {name=%s, errno=%d}\n",
+	      expected_name, expected.err);
+
+      free (expected_name);
+      return true;
+    }
+
+  if (actual.name)
+    actual_name = xasprintf ("\"%s\"", actual.name);
+  else
+    actual_name = xstrdup ("NULL");
+
+  if (expected.name)
+    expected_name = xasprintf ("\"%s\"", expected.name);
+  else
+    expected_name = xstrdup ("NULL");
+
+  printf ("error:     ttyname: actual {name=%s, errno=%d} != expected {name=%s, errno=%d}\n",
+	  actual_name, actual.err,
+	  expected_name, expected.err);
+
+  free (actual_name);
+  free (expected_name);
+  return false;
+}
+
+/* ttyname_r runner */
+
+struct result_r
+{
+  const char *name;
+  int ret;
+  int err;
+};
+
+/* strings in result structure are in static storage */
+static struct result_r
+run_ttyname_r (int fd)
+{
+  static char buf[TTY_NAME_MAX];
+
+  struct result_r ret;
+  errno = 0;
+  ret.ret = ttyname_r (fd, buf, TTY_NAME_MAX);
+  ret.err = errno;
+  if (ret.ret == 0)
+    ret.name = buf;
+  else
+    ret.name = NULL;
+  return ret;
+}
+
+static bool
+eq_ttyname_r (struct result_r actual, struct result_r expected)
+{
+  char *actual_name, *expected_name;
+
+  if ((actual.err == expected.err) &&
+      (actual.ret == expected.ret) &&
+      (!actual.name == !expected.name) &&
+      (actual.name ? strcmp (actual.name, expected.name) == 0 : true))
+    {
+      if (expected.name)
+        expected_name = xasprintf ("\"%s\"", expected.name);
+      else
+        expected_name = xstrdup ("NULL");
+
+      printf ("info:      ttyname_r: PASS {name=%s, ret=%d, errno=%d}\n",
+              expected_name, expected.ret, expected.err);
+
+      free (expected_name);
+      return true;
+    }
+
+  if (actual.name)
+    actual_name = xasprintf ("\"%s\"", actual.name);
+  else
+    actual_name = xstrdup ("NULL");
+
+  if (expected.name)
+    expected_name = xasprintf ("\"%s\"", expected.name);
+  else
+    expected_name = xstrdup ("NULL");
+
+  printf ("error:     ttyname_r: actual {name=%s, ret=%d, errno=%d} != expected {name=%s, ret=%d, errno=%d}\n",
+	  actual_name, actual.ret, actual.err,
+	  expected_name, expected.ret, expected.err);
+
+  free (actual_name);
+  free (expected_name);
+  return false;
+}
+
+/* combined runner */
+
+static bool
+doit (int fd, const char *testname, struct result_r expected_r)
+{
+  struct result expected = {.name=expected_r.name, .err=expected_r.ret};
+  bool ret = true;
+
+  printf ("info:    testcase: %s\n", testname);
+
+  if (!eq_ttyname (run_ttyname (fd), expected))
+    ret = false;
+  if (!eq_ttyname_r (run_ttyname_r (fd), expected_r))
+    ret = false;
+
+  if (!ret)
+    support_record_failure ();
+
+  return ret;
+}
+
+/* chroot setup */
+
+static char *chrootdir;
+
+static void
+prepare (int argc, char **argv)
+{
+  chrootdir = xasprintf ("%s/tst-ttyname-XXXXXX", test_dir);
+  if (mkdtemp (chrootdir) == NULL)
+    FAIL_EXIT1 ("mkdtemp (\"%s\"): %m", chrootdir);
+  add_temp_file (chrootdir);
+}
+#define PREPARE prepare
+
+/* These chroot setup functions put the TTY at at "/console" (where it
+   won't be found by ttyname), and create "/dev/console" as an
+   ordinary file.  This way, it's easier to write test-cases that
+   expect ttyname to fail; test-cases that expect it to succeed need
+   to explicitly remount it at "/dev/console".  */
+
+static int
+do_in_chroot_1 (int (*cb)(const char *, int))
+{
+  printf ("info:  entering chroot 1\n");
+
+  /* Open the PTS that we'll be testing on.  */
+  int master;
+  char *slavename;
+  VERIFY ((master = posix_openpt (O_RDWR|O_NOCTTY|O_NONBLOCK)) >= 0);
+  VERIFY ((slavename = ptsname (master)));
+  VERIFY (unlockpt (master) == 0);
+  if (strncmp (slavename, "/dev/pts/", 9) != 0)
+    FAIL_UNSUPPORTED ("slave pseudo-terminal is not under /dev/pts/: %s",
+                      slavename);
+  int slave = xopen (slavename, O_RDWR, 0);
+  if (!doit (slave, "basic smoketest",
+             (struct result_r){.name=slavename, .ret=0, .err=0}))
+    return 1;
+
+  pid_t pid = xfork ();
+  if (pid == 0)
+    {
+      xclose (master);
+
+      become_root_in_mount_ns ();
+
+      VERIFY (mount ("tmpfs", chrootdir, "tmpfs", 0, "mode=755") == 0);
+      VERIFY (chdir (chrootdir) == 0);
+
+      xmkdir ("proc", 0755);
+      xmkdir ("dev", 0755);
+      xmkdir ("dev/pts", 0755);
+
+      VERIFY (mount ("/proc", "proc", NULL, MS_BIND|MS_REC, NULL) == 0);
+      VERIFY (mount ("devpts", "dev/pts", "devpts",
+                     MS_NOSUID|MS_NOEXEC,
+                     "newinstance,ptmxmode=0666,mode=620") == 0);
+      VERIFY (symlink ("pts/ptmx", "dev/ptmx") == 0);
+
+      touch ("console", 0);
+      touch ("dev/console", 0);
+      VERIFY (mount (slavename, "console", NULL, MS_BIND, NULL) == 0);
+
+      xchroot (".");
+
+      char *linkname = xasprintf ("/proc/self/fd/%d", slave);
+      char *target = proc_fd_readlink (linkname);
+      VERIFY (strcmp (target, slavename) == 0);
+      free (linkname);
+
+      _exit (cb (slavename, slave));
+    }
+  int status;
+  xwaitpid (pid, &status, 0);
+  VERIFY (WIFEXITED (status));
+  xclose (master);
+  xclose (slave);
+  return WEXITSTATUS (status);
+}
+
+static int
+do_in_chroot_2 (int (*cb)(const char *, int))
+{
+  printf ("info:  entering chroot 2\n");
+
+  int pid_pipe[2];
+  xpipe (pid_pipe);
+  int exit_pipe[2];
+  xpipe (exit_pipe);
+
+  /* Open the PTS that we'll be testing on.  */
+  int master;
+  char *slavename;
+  VERIFY ((master = posix_openpt (O_RDWR|O_NOCTTY|O_NONBLOCK)) >= 0);
+  VERIFY ((slavename = ptsname (master)));
+  VERIFY (unlockpt (master) == 0);
+  if (strncmp (slavename, "/dev/pts/", 9) != 0)
+    FAIL_UNSUPPORTED ("slave pseudo-terminal is not under /dev/pts/: %s",
+                      slavename);
+  /* wait until in a new mount ns to open the slave */
+
+  /* enable `wait`ing on grandchildren */
+  VERIFY (prctl (PR_SET_CHILD_SUBREAPER, 1) == 0);
+
+  pid_t pid = xfork (); /* outer child */
+  if (pid == 0)
+    {
+      xclose (master);
+      xclose (pid_pipe[0]);
+      xclose (exit_pipe[1]);
+
+      become_root_in_mount_ns ();
+
+      int slave = xopen (slavename, O_RDWR, 0);
+      if (!doit (slave, "basic smoketest",
+                 (struct result_r){.name=slavename, .ret=0, .err=0}))
+        _exit (1);
+
+      VERIFY (mount ("tmpfs", chrootdir, "tmpfs", 0, "mode=755") == 0);
+      VERIFY (chdir (chrootdir) == 0);
+
+      xmkdir ("proc", 0755);
+      xmkdir ("dev", 0755);
+      xmkdir ("dev/pts", 0755);
+
+      VERIFY (mount ("devpts", "dev/pts", "devpts",
+                     MS_NOSUID|MS_NOEXEC,
+                     "newinstance,ptmxmode=0666,mode=620") == 0);
+      VERIFY (symlink ("pts/ptmx", "dev/ptmx") == 0);
+
+      touch ("console", 0);
+      touch ("dev/console", 0);
+      VERIFY (mount (slavename, "console", NULL, MS_BIND, NULL) == 0);
+
+      xchroot (".");
+
+      if (unshare (CLONE_NEWNS | CLONE_NEWPID) < 0)
+        FAIL_UNSUPPORTED ("could not enter new PID namespace");
+      pid = xfork (); /* inner child */
+      if (pid == 0)
+        {
+          xclose (pid_pipe[1]);
+
+          /* wait until the outer child has exited */
+          char c;
+          VERIFY (read (exit_pipe[0], &c, 1) == 0);
+          xclose (exit_pipe[0]);
+
+          VERIFY (mount ("proc", "/proc", "proc",
+                         MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL) == 0);
+
+          char *linkname = xasprintf ("/proc/self/fd/%d", slave);
+          char *target = proc_fd_readlink (linkname);
+          VERIFY (strcmp (target, strrchr (slavename, '/')) == 0);
+          free (linkname);
+
+          _exit (cb (slavename, slave));
+        }
+      xwrite (pid_pipe[1], &pid, sizeof pid);
+      _exit (0);
+    }
+  xclose (pid_pipe[1]);
+  xclose (exit_pipe[0]);
+  xclose (exit_pipe[1]);
+
+  /* wait for the outer child */
+  int status;
+  xwaitpid (pid, &status, 0);
+  VERIFY (WIFEXITED (status));
+  int ret = WEXITSTATUS (status);
+  if (ret != 0)
+    return ret;
+
+  /* set 'pid' to the inner child */
+  VERIFY (read (pid_pipe[0], &pid, sizeof pid) == sizeof pid);
+  xclose (pid_pipe[0]);
+
+  /* wait for the inner child */
+  xwaitpid (pid, &status, 0);
+  VERIFY (WIFEXITED (status));
+  xclose (master);
+  return WEXITSTATUS (status);
+}
+
+/* main test */
+
+static int
+run_chroot_tests (const char *slavename, int slave)
+{
+  struct stat st;
+  bool ok = true;
+
+  /* There are 3 groups of tests here.  The first group fairly
+     generically does things known to mess up ttyname, and verifies
+     that ttyname copes correctly.  The remaining groups are
+     increasingly convoluted, as we target specific parts of ttyname
+     to try to confuse.  */
+
+  /* Basic tests that it doesn't get confused by multiple devpts
+     instances.  */
+  {
+    VERIFY (stat (slavename, &st) < 0); /* sanity check */
+    if (!doit (slave, "no conflict, no match",
+               (struct result_r){.name=NULL, .ret=ENODEV, .err=ENODEV}))
+      ok = false;
+    VERIFY (mount ("/console", "/dev/console", NULL, MS_BIND, NULL) == 0);
+    if (!doit (slave, "no conflict, console",
+               (struct result_r){.name="/dev/console", .ret=0, .err=0}))
+      ok = false;
+    VERIFY (umount ("/dev/console") == 0);
+
+    /* keep creating PTYs until we we get a name collision */
+    while (stat (slavename, &st) < 0)
+      posix_openpt (O_RDWR|O_NOCTTY|O_NONBLOCK);
+    VERIFY (stat (slavename, &st) == 0);
+
+    if (!doit (slave, "conflict, no match",
+               (struct result_r){.name=NULL, .ret=ENODEV, .err=ENODEV}))
+      ok = false;
+    VERIFY (mount ("/console", "/dev/console", NULL, MS_BIND, NULL) == 0);
+    if (!doit (slave, "conflict, console",
+               (struct result_r){.name="/dev/console", .ret=0, .err=0}))
+      ok = false;
+    VERIFY (umount ("/dev/console") == 0);
+  }
+
+  /* The first tests kinda assumed that they hit certain code-paths
+     based on assuming that the readlink target is 'slavename', but
+     that's not quite always true.  They're still a good preliminary
+     sanity check, so keep them, but let's add tests that make sure
+     that those code-paths are hit by doing a readlink ourself.  */
+  {
+    char *linkname = xasprintf ("/proc/self/fd/%d", slave);
+    char *target = proc_fd_readlink (linkname);
+    free (linkname);
+    /* Depeding on how we set up the chroot, the kernel may or may not
+       trim the leading path to the target (it may give us "/6",
+       instead of "/dev/pts/6").  We test it both ways (do_in_chroot_1
+       and do_in_chroot_2).  This test group relies on the target
+       existing, so guarantee that it does exist by creating it if
+       necessary.  */
+    if (stat (target, &st) < 0)
+      {
+        VERIFY (errno == ENOENT);
+        touch (target, 0);
+      }
+
+    VERIFY (mount ("/console", "/dev/console", NULL, MS_BIND, NULL) == 0);
+    VERIFY (mount ("/console", target, NULL, MS_BIND, NULL) == 0);
+    if (!doit (slave, "with readlink target",
+               (struct result_r){.name=target, .ret=0, .err=0}))
+      ok = false;
+    VERIFY (umount (target) == 0);
+    VERIFY (umount ("/dev/console") == 0);
+
+    VERIFY (mount ("/console", "/dev/console", NULL, MS_BIND, NULL) == 0);
+    VERIFY (mount (slavename, target, NULL, MS_BIND, NULL) == 0);
+    if (!doit (slave, "with readlink trap; fallback",
+               (struct result_r){.name="/dev/console", .ret=0, .err=0}))
+      ok = false;
+    VERIFY (umount (target) == 0);
+    VERIFY (umount ("/dev/console") == 0);
+
+    VERIFY (mount (slavename, target, NULL, MS_BIND, NULL) == 0);
+    if (!doit (slave, "with readlink trap; no fallback",
+               (struct result_r){.name=NULL, .ret=ENODEV, .err=ENODEV}))
+      ok = false;
+    VERIFY (umount (target) == 0);
+  }
+
+  /* This test makes sure that everything still works OK if readdir
+     finds a pseudo-match before and/or after the actual match.  Now,
+     to do that, we need to control that readdir finds the
+     pseudo-matches before and after the actual match; and there's no
+     good way to control that order in absence of whitebox testing.
+     So, just create 3 files, then use opendir/readdir to see what
+     order they are in, and assign meaning based on that order, not by
+     name; assigning the first to be a pseudo-match, the second to be
+     the actual match, and the third to be a pseudo-match.  This
+     assumes that (on tmpfs) ordering within the directory is stable
+     in the absence of modification, which seems reasonably safe.  */
+  {
+    /* since we're testing the fallback search, disable the readlink
+       happy-path */
+    VERIFY (umount2 ("/proc", MNT_DETACH) == 0);
+
+    touch ("/dev/console1", 0);
+    touch ("/dev/console2", 0);
+    touch ("/dev/console3", 0);
+
+    char *c[3];
+    int ci = 0;
+    DIR *dirstream = opendir ("/dev");
+    VERIFY (dirstream != NULL);
+    struct dirent *d;
+    while ((d = readdir (dirstream)) != NULL && ci < 3)
+      {
+        if (strcmp (d->d_name, "console1") &&
+            strcmp (d->d_name, "console2") &&
+            strcmp (d->d_name, "console3") )
+          continue;
+        c[ci++] = xasprintf ("/dev/%s", d->d_name);
+      }
+    VERIFY (ci == 3);
+    VERIFY (closedir (dirstream) == 0);
+
+    VERIFY (mount (slavename, c[0], NULL, MS_BIND, NULL) == 0);
+    VERIFY (mount ("/console", c[1], NULL, MS_BIND, NULL) == 0);
+    VERIFY (mount (slavename, c[2], NULL, MS_BIND, NULL) == 0);
+    VERIFY (umount2 ("/dev/pts", MNT_DETACH) == 0);
+    if (!doit (slave, "with search-path trap",
+               (struct result_r){.name=c[1], .ret=0, .err=0}))
+      ok = false;
+    for (int i = 0; i < 3; i++)
+      {
+        VERIFY (umount (c[i]) == 0);
+        VERIFY (unlink (c[i]) == 0);
+        free (c[i]);
+      }
+  }
+
+  return ok ? 0 : 1;
+}
+
+static int
+do_test (void)
+{
+  int ret1 = do_in_chroot_1 (run_chroot_tests);
+  if (ret1 == EXIT_UNSUPPORTED)
+    return ret1;
+
+  int ret2 = do_in_chroot_2 (run_chroot_tests);
+  if (ret2 == EXIT_UNSUPPORTED)
+    return ret2;
+
+  return  ret1 | ret2;
+}
+
+#include <support/test-driver.c>
-- 
2.14.1

[PATCH COMMITTED 2/6] linux ttyname: Update a reference to kernel docs for kernel 4.10

[Christian Brauner]

From: Luke Shumaker <lukeshu@parabola.nu>

Linux 4.10 moved many of the documentation files around.

4.10 came out between the time the patch adding the comment (commit
15e9a4f378c8607c2ae1aa465436af4321db0e23) was submitted and the time
it was applied (in February, January, and March 2017; respectively).

Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog                         | 2 ++
 sysdeps/unix/sysv/linux/ttyname.h | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index 7916ad639f..6194237230 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2017-11-15  Luke Shumaker  <lukeshu@parabola.nu>
 
+	* sysdeps/unix/sysv/linux/ttyname.h (is_pty): Update doc reference.
+
 	* manual/terminal.texi (Is It a Terminal):
 	Mention ENODEV for ttyname and ttyname_r.
 
diff --git a/sysdeps/unix/sysv/linux/ttyname.h b/sysdeps/unix/sysv/linux/ttyname.h
index 2e415e4e9c..cbcdbab607 100644
--- a/sysdeps/unix/sysv/linux/ttyname.h
+++ b/sysdeps/unix/sysv/linux/ttyname.h
@@ -21,7 +21,8 @@
 #include <sys/stat.h>
 
 /* Return true if this is a UNIX98 pty device, as defined in
-   linux/Documentation/devices.txt.  */
+   linux/Documentation/devices.txt (on linux < 4.10) or
+   linux/Documentation/admin-guide/devices.txt (on linux >= 4.10).  */
 static inline int
 is_pty (struct stat64 *sb)
 {
-- 
2.14.1