[FFmpeg-devel] [PATCH v6] avdevice/xcbgrab: check return values of xcb query functions

[Moritz Barsnick]

On Mon, Jul 20, 2020 at 09:18:55 +0200, Alexander Strasser wrote:
> On 2020-07-19 19:47 -0400, Andriy Gelman wrote:
> > > > This check seems dead code. Looking at xcb sources, cursor is just an offset in
> > > > memory from ci so I don't think it can be null here.

> > But anyway, this part of the patch doesn't really have anything to do with
> > ticket #7312, and should be in a separate patch.
>
> Yes, it's definitely something that was changed in this patch
> at all. So it's better not to touch it in this patch.

Okay, so I "fixed" dead code. You guys can remove the dead code
yourselves then, if you like. ;-)

New patch for the original issue attached, not touching the dead code.

Thanks,
Moritz
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From e44b7f03354add2272a2739e04aafb38b7ce027f Mon Sep 17 00:00:00 2001
From: Moritz Barsnick <barsnick at gmx.net>
Date: Wed, 5 Aug 2020 14:06:53 +0200
Subject: [PATCH] avdevice/xcbgrab: check return values of xcb query functions

Fixes #7312, segmentation fault on close of X11 server

xcb_query_pointer_reply() and xcb_get_geometry_reply() can return NULL
if e.g. the X server closes or the connection is lost. This needs to
be checked in order to cleanly exit, because the returned pointers are
dereferenced later.

Signed-off-by: Moritz Barsnick <barsnick at gmx.net>
---
 libavdevice/xcbgrab.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/libavdevice/xcbgrab.c b/libavdevice/xcbgrab.c
index 6f6b2dbf15..8ef2a30d02 100644
--- a/libavdevice/xcbgrab.c
+++ b/libavdevice/xcbgrab.c
@@ -425,7 +425,16 @@ static int xcbgrab_read_packet(AVFormatContext *s, AVPacket *pkt)
         pc  = xcb_query_pointer(c->conn, c->screen->root);
         gc  = xcb_get_geometry(c->conn, c->screen->root);
         p   = xcb_query_pointer_reply(c->conn, pc, NULL);
+        if (!p) {
+            av_log(s, AV_LOG_ERROR, "Failed to query xcb pointer\n");
+            return AVERROR_EXTERNAL;
+        }
         geo = xcb_get_geometry_reply(c->conn, gc, NULL);
+        if (!geo) {
+            av_log(s, AV_LOG_ERROR, "Failed to get xcb geometry\n");
+            free(p);
+            return AVERROR_EXTERNAL;
+        }
     }

     if (c->follow_mouse && p->same_screen)
--
2.26.2