[Mesa-dev] [PATCH 21/30] egl/x11: don't crash if dri2_dpy->conn is NULL

[Emil Velikov]

On 25 August 2016 at 18:06, Kristian Høgsberg <hoegsberg at gmail.com> wrote:
> On Thu, Aug 25, 2016 at 9:18 AM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>> From: Emil Velikov <emil.velikov at collabora.com>
>>
>> The dri3 version of commits 60e9c35b3a0 and 6de9a03bed4.
>>
>> CC: <mesa-stable at lists.freedesktop.org>
>> Signed-off-by: Emil Velikov <emil.velikov at collabora.com>
>> ---
>>  src/egl/drivers/dri2/platform_x11.c | 14 +++++---------
>>  1 file changed, 5 insertions(+), 9 deletions(-)
>>
>> diff --git a/src/egl/drivers/dri2/platform_x11.c b/src/egl/drivers/dri2/platform_x11.c
>> index 2a5fc0b..ede12b6 100644
>> --- a/src/egl/drivers/dri2/platform_x11.c
>> +++ b/src/egl/drivers/dri2/platform_x11.c
>> @@ -1313,15 +1313,13 @@ dri2_initialize_x11_dri3(_EGLDriver *drv, _EGLDisplay *disp)
>>        dri2_dpy->screen = DefaultScreen(dpy);
>>     }
>>
>> -   if (xcb_connection_has_error(dri2_dpy->conn)) {
>> +   if (!dri2_dpy->conn || xcb_connection_has_error(dri2_dpy->conn)) {
>>        _eglLog(_EGL_WARNING, "DRI3: xcb_connect failed");
>>        goto cleanup_dpy;
>>     }
>
> When is dri2_dpy->conn ever NULL? xcb_connect() returns a valid
> connection or an error object, and I don't see how XGetXCBConnection()
> would ever return a NULL connection either.
>
It's been a while since I looked into these...
- xcb_connect's man page is perfectly clear. Does not return NULL,
even on failure.
- XGetXCBConnection's one is pretty quiet. Looking at the XCB wiki [1]
there is a NULL check, which (IMHO) is due to the fact that one cannot
guarantee xcb_connect-like behaviour.

That said I'm inclined to keep the patch as-is, regardless of how
(un)likely it is to trigger.

Thanks
Emil
[1] https://xcb.freedesktop.org/MixingCalls