Created attachment 27243 [details] [review]
Patch to fix GetGLXScreenConfigs().

While testing rendering fallback functionality in an application that supports both OpenGL and software rendering, I discovered a bug in Mesa that causes the program to crash if it's running on an X server that doesn't support GLX at all (e.g. vncserver).

A backtrace reveals the source:

#0  GetGLXScreenConfigs (dpy=<value optimized out>, scrn=0) at glxcmds.c:167
        priv = (__GLXdisplayPrivate * const) 0x0
#1  0xb7a40fa7 in CreateContext (dpy=0x9c3ad98, vis=<value optimized out>, fbconfig=0x0, shareList=0x0, allowDirect=1, contextID=0, use_glx_1_3=0, 
    renderType=0) at glxcmds.c:364
        gc = (GLXContext) 0xb79e2ff4
        screen = -1208638311
        psc = <value optimized out>
#2  0xb7a415c0 in glXCreateContext (dpy=0x9c3ad98, vis=0x99ce4c0, shareList=0x0, allowDirect=1) at glxcmds.c:487

The GetGLXScreenConfigs() function is as follows:

static __GLXscreenConfigs *
GetGLXScreenConfigs(Display *dpy, int scrn)
{
    __GLXdisplayPrivate * const priv = __glXInitialize(dpy);

    return (priv->screenConfigs != NULL) ? &priv->screenConfigs[scrn] : NULL;
}

__glXInitialize() returns NULL because the X server doesn't support GLX; however, the following statement doesn't check if priv is NULL, which results in a null pointer dereference. The solution is to check both priv and priv->screenConfigs for NULL.

Mesa version is 7.5-rc4.