uterm: video: fbdev: disable double buffering

Kernel fbdev drivers are really broken in respect to virtual framebuffer
sizes. They all report success when allocating and even mmap()'ing them
but memory access will result in SIGBUS or even SIGSEGV.

Therefore, we disable it by default now. We might consider re-enabling it,
but there is really no need to work too much on fbdev backends.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

diff --git a/src/uterm_video_fbdev.c b/src/uterm_video_fbdev.c
index b6263d4..ed17ae1 100644 (file)
--- a/src/uterm_video_fbdev.c
+++ b/src/uterm_video_fbdev.c
@@ -119,8 +119,12 @@ static int display_activate_force(struct uterm_display *disp,
         * (even mmap() accepts it) but the actual size that we can access
         * without segfaults is the _real_ framebuffer. Therefore, disable
         * double-buffering for it.
-        * TODO: fix this kernel-side! */
-       if (!strcmp(finfo->id, "udlfb")) {
+        * TODO: fix this kernel-side!
+        * TODO: There are so many broken fbdev drivers that just accept any
+        * virtual FB sizes and then break mmap that we now disable
+        * double-buffering entirely. We might instead add a white-list or
+        * optional command-line argument to re-enable it. */
+       if (true || !strcmp(finfo->id, "udlfb")) {
                disp->flags &= ~DISPLAY_DBUF;
                vinfo->yres_virtual = vinfo->yres;
        }
@@ -174,9 +178,9 @@ static int display_activate_force(struct uterm_display *disp,
            (disp->flags & DISPLAY_DBUF &&
             vinfo->yres_virtual < vinfo->yres * 2) ||
            vinfo->yres_virtual < vinfo->yres) {
-               log_error("device %s has weird buffer sizes",
-                         disp->fbdev.node);
-               return -EFAULT;
+               log_warning("device %s has weird virtual buffer sizes (%d %d %d %d)",
+                           disp->fbdev.node, vinfo->xres, vinfo->xres_virtual,
+                           vinfo->yres, vinfo->yres_virtual);
        }
 
        if (vinfo->bits_per_pixel != 32 &&