On 24/11/14 12:23 PM, DRC wrote:
> I think it might be prudent to enable it only with a 'vglrun +xcb'
> switch or something like that. I just predict that this is not the
> last we've heard of issues like this.
Oooh, here's a nice one:
xcb_get_extension_data doesn't just say "Yes I have GLX". It also
returns the extension major opcode that xcb_send_request uses for the io
vector index. So when the VGL_PROBEGLX code does its thing we get:
#0 XGetXCBConnection (dpy=0x608620) at x11_xcb.c:9
#1 0x00007ffff78d23b6 in _XGetXCBConnection (dpy=0x608620) at
/home/nathan/src/virtualgl/server/faker-sym.h:529
#2 0x00007ffff78e97e9 in xcb_get_extension_data (conn=0x61a3a0,
ext=0x7ffff5b90050 <xcb_glx_id>)
at /home/nathan/src/virtualgl/server/faker-xcb.cpp:149
#3 0x00007ffff55607d8 in xcb_send_request (c=c@entry=0x61a3a0,
flags=flags@entry=1,
vector=vector@entry=0x7fffffffda50, req=req@entry=0x7ffff5b8f950
<xcb_req.12371>) at xcb_out.c:177
#4 0x00007ffff59850b2 in xcb_glx_query_server_string
(c=c@entry=0x61a3a0, screen=screen@entry=0,
name=name@entry=2) at glx.c:2375
#5 0x00007ffff76a057e in __glXQueryServerString
(dpy=dpy@entry=0x619150, opcode=<optimized out>,
screen=screen@entry=0, name=name@entry=2) at glx_query.c:47
#6 0x00007ffff7682dd6 in AllocAndFetchScreenConfigs (priv=0x679dc0,
dpy=0x619150) at glxext.c:764
#7 __glXInitialize (dpy=dpy@entry=0x619150) at glxext.c:879
#8 0x00007ffff767f71b in GetGLXPrivScreenConfig (dpy=0x619150, scrn=0,
ppriv=ppriv@entry=0x7fffffffdb20,
ppsc=ppsc@entry=0x7fffffffdb28) at glxcmds.c:174
#9 0x00007ffff767f7ab in GetGLXPrivScreenConfig (ppsc=0x7fffffffdb28,
ppriv=0x7fffffffdb20,
scrn=<optimized out>, dpy=<optimized out>) at glxcmds.c:170
#10 glXGetConfig (dpy=<optimized out>, vis=0x631630, attribute=5,
value_return=0x63a1b8) at glxcmds.c:880
#11 0x00007ffff78fd498 in _glXGetConfig (dpy=0x619150, vis=0x631630,
attrib=5, value=0x63a1b8)
at /home/nathan/src/virtualgl/server/faker-sym.h:178
#12 0x00007ffff78fddf8 in buildVisAttribTable (dpy=0x619150, screen=0)
at /home/nathan/src/virtualgl/server/glxvisual.cpp:131
#13 0x00007ffff78fee96 in __vglMatchVisual (dpy=0x619150, screen=0,
depth=24, c_class=4, level=0, stereo=0,
trans=0) at /home/nathan/src/virtualgl/server/glxvisual.cpp:354
#14 0x00007ffff78dc484 in glXChooseVisual (dpy=0x619150, screen=0,
attrib_list=0x7fffffffdf00)
at /home/nathan/src/virtualgl/server/faker-glx.cpp:424
#15 0x0000000000405909 in main (argc=2, argv=0x7fffffffe078)
at /home/nathan/src/virtualgl/glxdemos/glxspheres.c:602
(gdb) c
Continuing.
[VGL] !!! Replaced xcb connection 0x61a3a0 with 0x609950 from
xcb_get_extension_data
Breakpoint 7, xcb_glx_query_server_string_string_length (R=R@entry=0x0)
at glx.c:2448
2448 glx.c: No such file or directory.
(gdb) c
Continuing.
Program received signal SIGSEGV, Segmentation fault.
xcb_glx_query_server_string_string_length (R=R@entry=0x0) at glx.c:2448
I.e. xcb_glx_query_server_string fails because the data tries to send
on wrong iovector, and glXQueryServerString isn't prepared to handle a
failure.
This is on an OpenSuSE 13.1 box with Mesa 9.x + Nouveau, and doesn't
happen on modern Mesa or with NVIDIA's binary, but it demonstrates the
pitfalls of hooking an API that is used by the underlying
implementation, not just applications.
Ways I can think of to work around this:
1) just disallow any non-3D-server GLX traffic, and loose the
VGL_PROBEGLX benefits. Hmmm, this looks like it kills the possibility
of stereo support, though I've never seen a stereo user.
2) Introduce a global (TLS-based should work) don't-fake-xcb-for-now
flag that the VGL_PROBEGLX code sets and the XCB hooks honour.
-Nathan
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