On Wed, 2009-08-05 at 11:01 +0200, Thomas Hellström wrote:
> Michel Dänzer wrote:
> > On Wed, 2009-08-05 at 10:50 +0200, Thomas Hellström wrote:
> >
> >> Michel Dänzer wrote:
> >>
> >>> From: Michel Dänzer <daen...@vmware.com>
> >>>
> >>> Make sure bo->vm_node is valid, to prevent crashes in the fault handler,
> >>> and
> >>> adjust vma->vm_pgoff according to it, so userspace attempts to access
> >>> /dev/fb*
> >>> mappings don't always result in SIGBUS.
> >>>
> >>> Signed-off-by: Michel Dänzer <daen...@vmware.com>
> >>> ---
> >>> drivers/gpu/drm/ttm/ttm_bo_vm.c | 6 ++++++
> >>> 1 files changed, 6 insertions(+), 0 deletions(-)
> >>>
> >>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>> b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>> index 27b146c..57d0c94 100644
> >>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> >>> @@ -282,7 +282,13 @@ int ttm_fbdev_mmap(struct vm_area_struct *vma,
> >>> struct ttm_buffer_object *bo)
> >>> if (vma->vm_pgoff != 0)
> >>> return -EACCES;
> >>>
> >>> + if (!bo->vm_node) {
> >>> + printk(KERN_ERR TTM_PFX "bo->vm_node == NULL\n");
> >>> + return -EACCES;
> >>> + }
> >>>
> >>>
> >> Michel,
> >> Did you actually hit a bo with an invalid vm_node?
> >>
> >
> > See patch 3 - the radeon driver used to create the fbcon BO as
> > kernel-only, so when I tried passing that to ttm_fbdev_mmap()
> > all /dev/fb* userspace mappings mysteriously caught SIGBUS on access.
Actually, it was even worse - the TTM fault handler crashed because it
unconditionally dereferences bo->vm_node.
> > It took me a while to track that down, this is basically a debugging aid
> > for bugs like that.
> >
>
> Ah, OK. But since that is really a kernel bug, we should trap it with
> BUG_ON().
Good point, will do.
--
Earthling Michel Dänzer | http://www.vmware.com
Libre software enthusiast | Debian, X and DRI developer
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