[Mesa-dev] glxinfo proposed change

[Ian Romanick]

On 01/07/2013 06:35 AM, Jorge Ramirez Ortiz, HCL Europe wrote:
>> -----Original Message-----
>> From: Dan Nicholson [mailto:dbn.lists at gmail.com]
>> Sent: Monday, January 07, 2013 2:31 PM
>> To: Jorge Ramirez Ortiz, HCL Europe
>> Cc: mesa-dev at lists.freedesktop.org
>> Subject: Re: [Mesa-dev] glxinfo proposed change
>>
>> On Mon, Jan 7, 2013 at 5:06 AM, Jorge Ramirez Ortiz,  HCL Europe
>> <Jorge.Ramirez-Ortiz at hcl.com> wrote:
>>> Hi all,
>>>
>>> Does this patch make sense? glVersion can be NULL and not having the
>> changes below could cause a SIGSEGV
>>>
>>> [jramirez at calypso-2 mesa-demos.git (tmp *)]$ git diff src/xdemos/glxinfo.c
>>> diff --git a/src/xdemos/glxinfo.c b/src/xdemos/glxinfo.c
>>> index aa6430d..97b6539 100644
>>> --- a/src/xdemos/glxinfo.c
>>> +++ b/src/xdemos/glxinfo.c
>>> @@ -639,7 +639,7 @@ print_screen_info(Display *dpy, int scrnum, Bool
>> allowDirect, Bool limits, Bool
>>>         printf("OpenGL renderer string: %s\n", glRenderer);
>>>         printf("OpenGL version string: %s\n", glVersion);
>>>   #ifdef GL_VERSION_2_0
>>> -      if (glVersion[0] >= '2' && glVersion[1] == '.') {
>>> +      if (glVersion && glVersion[0] >= '2' && glVersion[1] == '.') {
>>>            char *v = (char *) glGetString(GL_SHADING_LANGUAGE_VERSION);
>>>            printf("OpenGL shading language version string: %s\n", v);
>>>         }
>>
>> Normally, this is a good idea. However, glVersion is already
>> dereferenced in the printf on the previous line. So, any actual
>> checking for NULL glVersion should at least come earlier in the
>> function. On the other hand, returning NULL from
>> glGetString(GL_VERSION) might be so rare that bothering to guard for
>> it would be more effort than it's worth.
>>
>
> Bear in mind that printf of a null string wont SIGSEGV.
> Only trying to access glVersion[] elements presents -and indeed is - a risk and therefore a critical bug (for my usual standards that is).

The critical bug is that glGetString is not allowed to return NULL. 
Your GL is horrifically broken in that case, and no application will 
work either.