To reproduce this crash:
Start a failsafe X.org session using the vesa driver
execute '/usr/X11/bin/xrandr -s 800x600' in a terminal
The X server crashes with the following backtrace (note that in the X.org sources xf86SetRootClip is a static function only called from within EnableDisableFBAccess)
0: /usr/X11/bin/i386/Xorg'xf86SigHandler+0x84 [0x80f89d4]
1: /lib/libc.so.1'__sighndlr+0xf [0xd2b00d5f]
2: /lib/libc.so.1'call_user_handler+0x2bf [0xd2af5f82]
3: /usr/X11/lib/modules/libfb.so'fbSolid+0x24d [0xcc42b00d]
4: /usr/X11/lib/modules/libfb.so'fbFillRegionSolid+0x13d [0xcc42cd4d]
5: /usr/X11/lib/modules/libfb.so'fbPaintWindow+0xfb [0xcc42d07b]
6: /usr/X11/bin/i386/Xorg'cwPaintWindowBackground+0x71 [0x81b87f1]
7: /usr/X11/bin/i386/Xorg'damagePaintWindow+0x91 [0x81b6ec1]
8: /usr/X11/bin/i386/Xorg'compPaintWindowBackground+0x71 [0x813dff1]
9: /usr/X11/bin/i386/Xorg'miWindowExposures+0xeb [0x815b09b]
10: /usr/X11/bin/i386/Xorg'miHandleValidateExposures+0x86 [0x8173e06]
11: /usr/X11/bin/i386/Xorg'xf86SetRootClip+0x3e5 [0x81029a5]
12: /usr/X11/lib/modules/drivers/vesa_drv.so'VESASwitchMode+0xf8
[0xd28b3848]
13: /usr/X11/bin/i386/Xorg'CMapSwitchMode+0x4a [0x810056a]
14: /usr/X11/bin/i386/Xorg'xf86SwitchMode+0xc9 [0x80f3299]
15: /usr/X11/bin/i386/Xorg'xf86RandRSetMode+0xe8 [0x8119bb8]
16: /usr/X11/bin/i386/Xorg'xf86RandRSetConfig+0x1b0 [0x8119e30]
17: /usr/X11/bin/i386/Xorg'RRCrtcSet+0xec [0x81a87ec]
18: /usr/X11/bin/i386/Xorg'ProcRRSetScreenConfig+0x41b [0x81aefbb]
19: /usr/X11/bin/i386/Xorg'ProcRRDispatch+0x45 [0x81a8165]
20: /usr/X11/bin/i386/Xorg'XaceCatchExtProc+0x9d [0x818e36d]
21: /usr/X11/bin/i386/Xorg'Dispatch+0x4f9 [0x809c3b9]
22: /usr/X11/bin/i386/Xorg'main+0x491 [0x80b0611]
23: /usr/X11/bin/i386/Xorg'_start+0x80 [0x808a380]
This appears to be already reported in X.Org community bugzilla:
http://bugs.freedesktop.org/show_bug.cgi?id=10348
and fixed in the community git master, but not yet in a released tarball:
http://cgit.freedesktop.org/xorg/driver/xf86-video-vesa/commit/?id=f5315667c0d827cca56a0a855b33e0efe83e3fa0