[PATCH 1/2] Cygwin: stat(): Fix "Bad address" error on stat() for /dev/tty.

[Takashi Yano]

As reported in
https://cygwin.com/pipermail/cygwin/2023-June/253888.html,
"Bad address" error occurs when stat() is called after the commit
3721a756b0d8 ("Cygwin: console: Make the console accessible from
other terminals.").

There are two problems in the current code. One is fhandler_console::
fstat() calls get_ttyp()->getsid(). However, fh_alloc() in dtable.cc
omits to initialize the fhandler_console instance when stat() is
called. Due to this, get_ttyp() returns NULL and access violation
occurs. The other problem is fh_alloc() assigns fhandler_console
even if the CTTY is not a console. So the first problem above occurs
even if the CTTY is a pty.

This patch fixes the issue by:
1) Call set_unit() to initialize _tc if the get_ttyp() returns NULL.
2) Assign fhandler_pty_slave for /dev/tty if CTTY is a pty in fh_alloc().

Fixes: 3721a756b0d8 ("Cygwin: console: Make the console accessible
  from other terminals.").
Fixes: 23771fa1f7028 ("dtable.cc (fh_alloc): Make different decisions
  when generating fhandler for not-opened devices. Add kludge to deal
  with opening /dev/tty.")
Reported-by: Bruce Jerrick <bmj...@gmail.com>
Signed-off-by: Takashi Yano <takashi.y...@nifty.ne.jp>
---
 winsup/cygwin/dtable.cc           | 8 +++++++-
 winsup/cygwin/fhandler/console.cc | 6 ++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/dtable.cc b/winsup/cygwin/dtable.cc
index 18e0f3097..2aae2fd65 100644
--- a/winsup/cygwin/dtable.cc
+++ b/winsup/cygwin/dtable.cc
@@ -600,7 +600,13 @@ fh_alloc (path_conv& pc)
        case FH_TTY:
          if (!pc.isopen ())
            {
-             fhraw = cnew_no_ctor (fhandler_console, -1);
+             if (CTTY_IS_VALID (myself->ctty))
+               {
+                 if (iscons_dev (myself->ctty))
+                   fhraw = cnew_no_ctor (fhandler_console, -1);
+                 else
+                   fhraw = cnew_no_ctor (fhandler_pty_slave, -1);
+               }
              debug_printf ("not called from open for /dev/tty");
            }
          else if (!CTTY_IS_VALID (myself->ctty) && last_tty_dev
diff --git a/winsup/cygwin/fhandler/console.cc 
b/winsup/cygwin/fhandler/console.cc
index 7768a9941..6aa3b50bf 100644
--- a/winsup/cygwin/fhandler/console.cc
+++ b/winsup/cygwin/fhandler/console.cc
@@ -4554,6 +4554,12 @@ fhandler_console::set_disable_master_thread (bool x, 
fhandler_console *cons)
 int
 fhandler_console::fstat (struct stat *st)
 {
+  /* When stat() is called, fh_alloc() in dtable.cc omits to initialize
+     the console instance. Due to this, get_ttyp() returns NULL here.
+     So, calling set_unit() is necessary to access getsid(). */
+  if (!get_ttyp ())
+    set_unit ();
+
   fhandler_base::fstat (st);
   st->st_mode = S_IFCHR | S_IRUSR | S_IWUSR;
   pinfo p (get_ttyp ()->getsid ());
-- 
2.39.0