[PATCH] Cygwin: pty: Additional race issue fix regarding pseudo console.

[PATCH] Cygwin: pty: Additional race issue fix regarding pseudo console.

[Takashi Yano]

- In commit bb93c6d7, the race issue was not completely fixed. In
  the pseudo console inheritance, if the destination process to
  which the ownership of pseudo console switches, is found but exits
  before switching, the inheritance fails. Currently, this extremely
  rarely happens. This patch fixes the issue.
---
 winsup/cygwin/fhandler_tty.cc | 47 +++++++++++------------------------
 1 file changed, 14 insertions(+), 33 deletions(-)

diff --git a/winsup/cygwin/fhandler_tty.cc b/winsup/cygwin/fhandler_tty.cc
index d44728795..e4480771b 100644
--- a/winsup/cygwin/fhandler_tty.cc
+++ b/winsup/cygwin/fhandler_tty.cc
@@ -64,6 +64,8 @@ struct pipe_reply {
 
 extern HANDLE attach_mutex; /* Defined in fhandler_console.cc */
 
+inline static bool pcon_pid_alive (DWORD pid);
+
 DWORD
 fhandler_pty_common::get_console_process_id (DWORD pid, bool match,
 					     bool cygwin, bool stub_only)
@@ -90,20 +92,18 @@ fhandler_pty_common::get_console_process_id (DWORD pid, bool match,
 	else
 	  {
 	    pinfo p (cygwin_pid (list[i]));
-	    if (!!p && p->dwProcessId && p->exec_dwProcessId
-		&& p->dwProcessId != p->exec_dwProcessId)
+	    if (!!p && p->exec_dwProcessId)
 	      {
-		res_pri = list[i];
+		res_pri = stub_only ? p->exec_dwProcessId : list[i];
 		break;
 	      }
-	    if (!!p && !res)
+	    if (!p && !res && pcon_pid_alive (list[i]) && stub_only)
+	      res = list[i];
+	    if (!!p && !res && !stub_only)
 	      res = list[i];
 	  }
       }
-  if (stub_only)
-    return res_pri;
-  else
-    return res_pri ?: res;
+  return res_pri ?: res;
 }
 
 static bool isHybrid;
@@ -3384,35 +3384,17 @@ fallback:
 void
 fhandler_pty_slave::close_pseudoconsole (tty *ttyp, DWORD force_switch_to)
 {
-  DWORD switch_to_stub = 0, switch_to = 0;
-  DWORD new_pcon_pid = 0;
+  DWORD switch_to = 0;
   if (force_switch_to)
     {
-      switch_to_stub = force_switch_to;
-      new_pcon_pid = force_switch_to;
+      switch_to = force_switch_to;
       ttyp->setpgid (force_switch_to + MAX_PID);
     }
   else if (pcon_pid_self (ttyp->pcon_pid))
     {
       /* Search another process which attaches to the pseudo console */
       DWORD current_pid = myself->exec_dwProcessId ?: myself->dwProcessId;
-      switch_to = get_console_process_id (current_pid, false, true);
-      if (switch_to)
-	{
-	  pinfo p (cygwin_pid (switch_to));
-	  if (p)
-	    {
-	      if (p->exec_dwProcessId)
-		switch_to_stub = p->exec_dwProcessId;
-	      new_pcon_pid = p->exec_dwProcessId;
-	    }
-	}
-      else
-	{
-	  switch_to = get_console_process_id (current_pid, false, false);
-	  if (switch_to)
-	    new_pcon_pid = switch_to;
-	}
+      switch_to = get_console_process_id (current_pid, false, true, true);
     }
   if (ttyp->pcon_activated)
     {
@@ -3420,7 +3402,6 @@ fhandler_pty_slave::close_pseudoconsole (tty *ttyp, DWORD force_switch_to)
       ttyp->previous_output_code_page = GetConsoleOutputCP ();
       if (pcon_pid_self (ttyp->pcon_pid))
 	{
-	  switch_to = switch_to_stub ?: switch_to;
 	  if (switch_to)
 	    {
 	      /* Change pseudo console owner to another process */
@@ -3454,7 +3435,7 @@ fhandler_pty_slave::close_pseudoconsole (tty *ttyp, DWORD force_switch_to)
 	      CloseHandle (ttyp->h_pcon_conhost_process);
 	      CloseHandle (ttyp->h_pcon_in);
 	      CloseHandle (ttyp->h_pcon_out);
-	      ttyp->pcon_pid = new_pcon_pid;
+	      ttyp->pcon_pid = switch_to;
 	      ttyp->h_pcon_write_pipe = new_write_pipe;
 	      ttyp->h_pcon_condrv_reference = new_condrv_reference;
 	      ttyp->h_pcon_conhost_process = new_conhost_process;
@@ -3513,8 +3494,8 @@ fhandler_pty_slave::close_pseudoconsole (tty *ttyp, DWORD force_switch_to)
     }
   else if (pcon_pid_self (ttyp->pcon_pid))
     {
-      if (switch_to_stub)
-	ttyp->pcon_pid = new_pcon_pid;
+      if (switch_to)
+	ttyp->pcon_pid = switch_to;
       else
 	{
 	  ttyp->pcon_pid = 0;
-- 
2.31.1

Re: [PATCH] Cygwin: pty: Additional race issue fix regarding pseudo console.

[Corinna Vinschen]

On Apr 21 12:06, Takashi Yano wrote:
> - In commit bb93c6d7, the race issue was not completely fixed. In
>   the pseudo console inheritance, if the destination process to
>   which the ownership of pseudo console switches, is found but exits
>   before switching, the inheritance fails. Currently, this extremely
>   rarely happens. This patch fixes the issue.
> ---
>  winsup/cygwin/fhandler_tty.cc | 47 +++++++++++------------------------
>  1 file changed, 14 insertions(+), 33 deletions(-)

This and the other two patches pushed.


Thanks,
Corinna