Bug 21375 - XF86_SVGA, lxc/lib/font/fc/fserve.c can dereference a NULL pointer.
Summary: XF86_SVGA, lxc/lib/font/fc/fserve.c can dereference a NULL pointer.
Alias: None
Product: Red Hat Linux
Classification: Retired
Component: XFree86
Version: 7.0
Hardware: i386
OS: Linux
Target Milestone: ---
Assignee: Mike A. Harris
QA Contact: David Lawrence
Depends On:
TreeView+ depends on / blocked
Reported: 2000-11-27 15:27 UTC by Need Real Name
Modified: 2007-04-18 16:30 UTC (History)
0 users

Clone Of:
Last Closed: 2001-03-01 21:28:13 UTC

Attachments (Terms of Use)

Description Need Real Name 2000-11-27 15:27:10 UTC
NOTE:           Since I'm not sure if this will interest Redhat or XFree
                I've sent it to both. Thus I've included references 
                to the redhat packages involved.
VERSION:        Redhat package: XFree86-SVGA-3.3.6-33
                R6.3, public-patch-3
                The problem was initially discovered in the XF86_SVGA
                contained in the above Redhat package. It is also present
                in XF86_SVGA compiled from sources X336-src-x.tgz with and
                without the following fixes...
                fix-01-r128,            fix-02-svr4,            
                fix-03-mmap,            fix-04-s3trio3d2x,
                fix-05-s3trio3d,        fix-06-s3trio3d2x,
CLIENT MACHINE and OPERATING SYSTEM:   i386/Redhat Linux 7.0
                Dell Cpt Laptop, Intel Celeron 333Mhz Processer,
                Kernel 2.2.17.
                Kernel is unpatched and compiled from source, not a redhat
                Also PCMCIA version 3.1.21 compiled from source.
DISPLAY TYPE:   Neomagic NM2360 driving internal LCD panel
                (Chipset forced to NM2200 in XF86Config)
WINDOW MANAGER: None -- see later.
COMPILER:       gcc version 2.96 20000731 (Red Hat Linux 7.0)
AREA:           xc/lib/font/fc
                fs_handle_unexpected() [in xc/lib/font/fc/fserve.c] can
                _fs_eat_rest_of_error() [in .../fsio.c] with an FSFpeRec
                structure in which the field trans_conn is NULL.
                This ultimately leads to TRANS(Read)() [_FontTransRead()]
                xc/lib/xtrans/Xtrans.c] attempting to dereference a NULL
                pointer followed by "Caught signal 11" and a server crash.
                A brief description of my setup...
                I'm running XFree86 4.0.1 as shipped in Redhat 7.0. Redhat
                supplies both the new 4.0.1 XFree86 server and the older 
                individual servers from XFree86 3.3.6.
                Due to some (very minor) glitches with the newer server on
                system I'm still running the XF86_SVGA server from 3.3.6.
                I'm also running vnc (version: 3.3.3r2, with some
                modifications, installed from source) and kdm (from kde
                1.1.2, redhat package kdebase-1.1.2-48).
                I log in using kdm and my .xsession script runs the
                vncviewer in fullscreen mode in order to simulate an X
                session which I can share to other displays as I move
                Fonts are provided by xfs (from 4.0.1, redhat package
                This setup worked correctly for several weeks until I
                the microsoft web truetype fonts
                (http://www.microsoft.com/typography/fontpack/) to be
                by xfs.
                Now if the laptop is suspended for more than about 5-10
                minutes the X server will probably crash shortly after the
                system is resumed.
                The specifics of the problem...
                I've traced the execution path by using a combination of
                call trace and judicious insertion of printf(). I can get
                as far as fs_wakeup() [in xc/lib/font/fc/fserve.c] although
                I'm unsure where this is getting called from (as a callback
                At somepoint after the resume, fs_wakeup() is called, and
                finding a matching block record for the data it reads from
                connection, it calls fs_handle_unexpected() [in the same
----->8 Snip 8<-----
static void
fs_handle_unexpected(conn, rep)
    FSFpePtr    conn;
    fsGenericReply *rep;
    if (rep->type == FS_Event && rep->data1 == KeepAlive) {
        fsNoopReq   req;
        /* ping it back */
         req.reqType = FS_Noop;
         req.length = SIZEOF(fsNoopReq) >> 2;
         _fs_add_req_log(conn, FS_Noop);
         _fs_write(conn, (char *) &req, SIZEOF(fsNoopReq));
                                                 <-- NO ERROR CHECK HERE.
     /* this should suck up unexpected replies and events */
     _fs_eat_rest_of_error(conn, (fsError *) rep);
----->8 Snip 8<-----
                fs_handle_unexpected() finds that this is a "KeepAlive" and
                attempts to send a Noop back to the font server.
                This involves calling _fs_write() in [.../fsio.c].
                When _fs_write() calls _FontTransWrite() [in
                xc/lib/xtrans/Xtrans.c] it fails setting errno to EPIPE.
                _fs_write() then calls _fs_connection_died() [in
                which (amongst other things) sets conn->trans_conn to NULL.
                _fs_write() then sets errno to EPIPE and returns -1 to
                the error.
                Crucially fs_handle_unexpected() doesn't check the return
                value of _fs_write() and goes on to call
                _fs_eat_rest_of_error() [in .../fsio.c] with "conn"
                the NULL pointer in the field trans_conn.
----->8 Snip 8<-----
    FSFpePtr    conn;
   if (!conn->attemptReconnect)
   conn->attemptReconnect = FALSE;
   conn->time_to_try = time((Time_t *) 0) + FS_RECONNECT_WAIT;
   conn->reconnect_delay = FS_RECONNECT_WAIT;
   conn->fs_fd = -1;
   conn->trans_conn = NULL;                                      <--- HERE.
   conn->next_reconnect = awaiting_reconnect;
   awaiting_reconnect = conn;
----->8 Snip 8<-----
                _fs_eat_rest_of_error() just does a call to
                [in the same file] passing on "conn" containing the NULL
                _fs_drain_bytes() calls _fs_read() [in the same file] to
                read the data from the connection, once again passing on 
                _fs_read() calls TRANS(Read) [_FontTransRead()] in
                xc/lib/xtrans/Xtrans.c passing conn->trans_conn as the
                first paramater (Ie. NULL).
----->8 Snip 8<----- [From _fs_read()]
    while ((bytes_read = _FontTransRead(conn->trans_conn,
            data, (int) size)) != size) {
----->8 Snip 8<-----
                _FontTransRead() tries to dereference this NULL pointer and
                we catch sig11.
----->8 Snip 8<-----
TRANS(Read) (ciptr, buf, size)      <-- ciptr is NULL.
    return ciptr->transptr->Read (ciptr, buf, size);
----->8 Snip 8<-----
                This sequence seems to cause the crash everytime on my
                system (but it can also happen even if the exact sequence
                is not followed).
                Initially: We've logged in using kdm and are running
                a normal X session under Xvnc with vncviewer running 
                full screen so we can see it.
                [Windowmaker 0.62.1, redhat package WindowMaker-0.62.1-14,
                running under Xvnc; no window manager is running under
                XF86_SVGA (just vncviewer), nothing else.]
                1, Run xscreensaver to lock the X display with the
                   "xscreensaver-command -lock" command
                   (XScreeSaver 3.25, redhat package xscreensaver-3.25-4).
                   Note xscreensaver is locking Xvnc NOT XF86_SVGA.
                2, Press a key so that xscreensaver prompts for a password.
                3, While the password dialog is displayed hit Fn+Suspend to
                   place the laptop into suspend mode.
                4, Go and have a cup of coffee.
                   Note this step is important!
                   You must wait at least 10 minutes (say 15 for good
                   If you try to resume immediately it won't crash.
                5, After a 15 minute delay hit the power button. The
                   dialog will pop back up (unless the panel is set to
                   after 10 minutes in which case press Ctrl or something
                   wake it up).
                6, Type your password, hit Enter and XF86_SVGA will crash
                   with the "Caught signal 11." message.
                   Xvnc will be fine, and you can restart the X server and
                   reconnect to it with vncviewer and your Xvnc session
                   be uneffected.
                This fix adds an error check such that
                checks the return value of _fs_write() and exits
                if it is -1 (thus skipping the call to
                It also adds a few warning messages so you know that the
                problem is still there, although now better handled.
----->8 Snip 8<-- (diff -c xc/lib/font/fc/fserve.c
*** xc/lib/font/fc/fserve.c     Wed Jun 11 13:08:41 1997
--- xc.new/lib/font/fc/fserve.c Sun Nov 26 14:49:27 2000
*** 1,4 ****
--- 1,5 ----
  /* $TOG: fserve.c /main/49 1997/06/10 11:23:56 barstow $ */
+ /* Modified to prevent a seg.fault crash -- R.Kay (26-11-00) */
  Copyright (c) 1990  X Consortium
*** 92,97 ****
--- 93,99 ----
                             (pci)->descent || \
+ #include <stdio.h>     /* So we can print some warnings -- RKAY */
  extern FontPtr find_old_font();
*** 1214,1220 ****
        req.reqType = FS_Noop;
        req.length = SIZEOF(fsNoopReq) >> 2;
        _fs_add_req_log(conn, FS_Noop);
!       _fs_write(conn, (char *) &req, SIZEOF(fsNoopReq));
      /* this should suck up unexpected replies and events */
      _fs_eat_rest_of_error(conn, (fsError *) rep);
--- 1216,1229 ----
        req.reqType = FS_Noop;
        req.length = SIZEOF(fsNoopReq) >> 2;
        _fs_add_req_log(conn, FS_Noop);
!       /* If _fs_write fails, conn->tran_conn will be NULL and calling
!        * _fs_eat_rest_of_error will eventually cause a segfault in
!        * _FontTransRead()                           -- RKAY           */
!       if (_fs_write(conn, (char *) &req, SIZEOF(fsNoopReq)) == -1) {
!           fprintf(stderr, "Warning: _fs_write failed in " 
!                           "fs_handle_unexpected.\n");
!           return;
!       }
      /* this should suck up unexpected replies and events */
      _fs_eat_rest_of_error(conn, (fsError *) rep);
----->8 Snip 8<------------------------
----->8 Snip 8<-- (diff -c xc/lib/font/fc/fsio.c xc.new/lib/font/fc/fsio.c)
*** xc/lib/font/fc/fsio.c       Fri Jul 23 14:42:00 1999
--- xc.new/lib/font/fc/fsio.c   Sun Nov 26 14:49:38 2000
*** 1,5 ****
--- 1,6 ----
  /* $XConsortium: fsio.c,v 1.37 95/04/05 19:58:13 kaleb Exp $ */
  /* $XFree86: xc/lib/font/fc/fsio.c,v 1999/07/23 13:22:20 hohndel
Exp $
+ /* Modified to prevent a seg.fault crash -- R.Kay (26-11-00) */
   * Copyright 1990 Network Computing Devices
*** 457,462 ****
--- 458,467 ----
        } else if (ECHECK(EINTR)) {
        } else {                /* something bad happened */
+           /* RKAY */
+           if (ECHECK(EPIPE))
+               fprintf(stderr, "Warning: EPIPE while writing to font "
+                               "server.\n");
            return -1;
----->8 Snip 8<------------------
                Of course there remains the question of why writing to the
                font server fails after a resume. I've not had chance to
                investigate that aspect of the problem.
                However, whatever the answer may be, a problem with the
                server shouldn't result in the X server seg faulting.
                One side effect of the above patch is that subsequent to
                averted crash XF86_SVGA start consuming ~90% of the CPU.
                This appears to be because the mechanism for calculating
                timeouts for the Select() calls in WaitForSomething()
                [xc/programs/Xserver/os/WaitFor.c] now decides on a timeout
                of 0. And so XF86_SVGA runs in a busy loop. Aside from that
                it works fine (if it wasn't for the fan switching in I
                wouldn't have noticed).
                There appears to be a function _fs_try_reconnect() 
                [xc/lib/font/fc/fserve.c] that looks
                like it should re-establish the font server connection.
                The only place I can find where this is called is
                fs_wakeup(). However, if the connection has died and
                fs_connection_died() has been called, as in this case, then
                conn->fs_fd == -1 and fs_wakeup() exits immediately and the
                call to fs_try_reconnect() is never reached.
----->8 Snip 8<-----
    if (conn->fs_fd == -1)
            return FALSE;
----->8 Snip 8<-----
                I tried sleeping for 10 seconds and then calling
                fs_try_reconnect in fs_handle_unexpected() but
                conn->trans_conn was still NULL. I guess an examination of 
                xfs is would be a good idea.

                Hopefully this is of some use,

Comment 1 Mike A. Harris 2001-02-28 06:50:48 UTC
Thanks very much for doing the debugging session and analasis, and also for
the patch as well.  I believe we have a fix for 4.x based servers now and
will be testing it out soon.  i will also try out your patch soon too, if
by chance you've come across any more info or patches please feel free to
submit them, and I will try to get a fix out ASAP.  Sorry for the delay
of response.  I'm playing catchup with an inherited bug report pile from
XFree86, and hope to get caught up sometime this year..  ;o)

This bug is a duplicate of Bug #17991 and countless others I wont mention,
however I'm not marking it duplicate, as it is the most detailed report
of the bunch.  Thanks again.

Comment 2 Mike A. Harris 2001-07-16 08:01:26 UTC
Fixed in latest errata

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