Increase screen pixmap size beyond initial video RAM if needed

If FBBase is not backed by grant tables, it's now possible to reallocate
it. This is especially useful when new external display is connected and
the overall resolution is increased.

Note the realloc() call may change FBBase address. In practice, FBBase
is given to fbScreenInit, and it does save it at some point
(fbScreenInit->fbFinishScreenInit->miScreenInit->miScreenDevPrivateInit),
but then it's used only to attach it to the screen pixmap, which is
updated via ModifyPixmapHeader() call few lines below anyway.

This change adjusts also pScrn->videoRam. But fortunately X server seems
to use it only at startup, and only to validate initial modes list.

Fixes QubesOS/qubes-issues#7448

Author: marmarek

xf86-video-dummy/src/dummy_driver.c

@@ -406,13 +406,41 @@ Bool DUMMYAdjustScreenPixmap(ScrnInfoPtr pScrn, int width, int height)
                 "Failed to get the screen pixmap.\n");
         return FALSE;
     }
-    if (cbLine > UINT32_MAX || cbLine * height > pScrn->videoRam * 1024)
-    {
+    if (cbLine > UINT32_MAX) {
         xf86DrvMsg(pScrn->scrnIndex, X_ERROR,
-                "Unable to set up a virtual screen size of %dx%d with %d Kb of video memory available.  Please increase the video memory size.\n",
-                width, height, pScrn->videoRam);
+                "Unable to set up a virtual screen size of %dx%d, cbLine "
+                "overflow\n",
+                width, height);
         return FALSE;
     }
+    if (cbLine * height > pScrn->videoRam * 1024) {
+        if (!dPtr->FBBasePriv) {
+            /* If there is no backing grant entries, it's easy enough to extend
+             */
+            pointer *newFBBase;
+            size_t new_size = (cbLine * height + 1023) & ~1023;
+
+            newFBBase = realloc(dPtr->FBBase, new_size);
+            if (!newFBBase) {
+                xf86DrvMsg(pScrn->scrnIndex, X_ERROR,
+                        "Unable to set up a virtual screen size of %dx%d, "
+                        "cannot allocate memory (%zu bytes)\n",
+                        width, height, new_size);
+                return FALSE;
+            }
+            memset((char*)newFBBase + pScrn->videoRam * 1024,
+                   0,
+                   new_size - pScrn->videoRam * 1024);
+            dPtr->FBBase = newFBBase;
+            pScrn->videoRam = new_size / 1024;
+        } else {
+            xf86DrvMsg(pScrn->scrnIndex, X_ERROR,
+                    "Unable to set up a virtual screen size of %dx%d with %d Kb of video memory available.  Please increase the video memory size.\n",
+                    width, height, pScrn->videoRam);
+            return FALSE;
+        }
+    }
+
     pScreen->ModifyPixmapHeader(pPixmap, width, height,
             pScrn->depth, xf86GetBppFromDepth(pScrn, pScrn->depth), cbLine,
             dPtr->FBBase);