Hi Mauro,
On Saturday, 26 May 2018 14:28:18 EEST Mauro Carvalho Chehab wrote:
> Em Sat, 26 May 2018 03:24:00 +0300 Laurent Pinchart escreveu:
[snip]
> > I've reproduced the issue and created a minimal test case.
> >
> > 1. struct vsp1_pipeline;
> > 2.
> > 3. struct vsp1_entity {
> > 4. struct vsp1_pipeline *pipe;
> > 5. struct vsp1_entity *sink;
> > 6. unsigned int source_pad;
> > 7. };
> > 8.
> > 9. struct vsp1_pipeline {
> > 10. struct vsp1_entity *brx;
> > 11. };
> > 12.
> > 13. struct vsp1_brx {
> > 14. struct vsp1_entity entity;
> > 15. };
> > 16.
> > 17. struct vsp1_device {
> > 18. struct vsp1_brx *bru;
> > 19. struct vsp1_brx *brs;
> > 20. };
> > 21.
> > 22. unsigned int frob(struct vsp1_device *vsp1, struct vsp1_pipeline
> > *pipe)
> > 23. {
> > 24. struct vsp1_entity *brx;
> > 25.
> > 26. if (pipe->brx)
> > 27. brx = pipe->brx;
> > 28. else if (!vsp1->bru->entity.pipe)
> > 29. brx = &vsp1->bru->entity;
> > 30. else
> > 31. brx = &vsp1->brs->entity;
> > 32.
> > 33. if (brx != pipe->brx)
> > 34. pipe->brx = brx;
> > 35.
> > 36. return pipe->brx->source_pad;
> > 37. }
> >
> > The reason why smatch complains is that it has no guarantee that vsp1->brs
> > is not NULL. It's quite tricky:
> >
> > - On line 26, smatch assumes that pipe->brx can be NULL
> > - On line 27, brx is assigned a non-NULL value (as pipe->brx is not NULL
> > due to line 26)
> > - On line 28, smatch assumes that vsp1->bru is not NULL
> > - On line 29, brx is assigned a non-NULL value (as vsp1->bru is not NULL
> > due to line 28)
> > - On line 31, brx is assigned a possibly NULL value (as there's no
> > information regarding vsp1->brs)
> > - On line 34, pipe->brx is not assigned a non-NULL value if brx is NULL
> > - On line 36 pipe->brx is dereferenced
> >
> > The problem comes from the fact that smatch assumes that vsp1->brs isn't
> > NULL. Adding a "(void)vsp1->brs->entity;" statement on line 25 makes the
> > warning disappear.
> >
> > So how do we know that vsp1->brs isn't NULL in the original code ?
> >
> > if (pipe->num_inputs > 2)
> > brx = &vsp1->bru->entity;
> > else if (pipe->brx && !drm_pipe->force_brx_release)
> > brx = pipe->brx;
> > else if (!vsp1->bru->entity.pipe)
> > brx = &vsp1->bru->entity;
> > else
> > brx = &vsp1->brs->entity;
> >
> > A VSP1 instance can have no brs, so in general vsp1->brs can be NULL.
> > However, when that's the case, the following conditions are fulfilled.
> >
> > - drm_pipe->force_brx_release will be false
> > - either pipe->brx will be non-NULL, or vsp1->bru->entity.pipe will be
> > NULL
> >
> > The fourth branch should thus never be taken.
>
> I don't think that adding a forth branch there would solve.
>
> The thing is that Smatch knows that pipe->brx can be NULL, as the function
> explicly checks if pipe->brx != NULL.
>
> When Smatch handles this if:
>
> if (brx != pipe->brx) {
>
> It wrongly assumes that this could be false if pipe->brx is NULL.
> I don't know why, as Smatch should know that brx can't be NULL.
brx can be NULL here if an only if vsp1->brs is NULL (as the entity field is
first in the vsp1->brs structure, so &vsp1->brs->entity has the same address
as vsp1->brs).
vsp1->brs can be NULL on some devices, but in that case we have the following
guarantees:
- drm_pipe->force_brx_release will always be FALSE
- either pipe->brx will be non-NULL or vsp1->bru->entity.pipe will be NULL
So the fourth branch is never taken.
The above conditions come from outside this function, and smatch can't know
about them. However, I don't know whether the problems comes from smatch
assuming that vsp1->brs can be NULL, or from somewhere else.
> On such case, the next code to be executed would be:
>
> format.pad = pipe->brx->source_pad;
>
> With would be trying to de-ref a NULL pointer.
>
> There are two ways to fix it:
>
> 1) with my patch.
>
> It is based to the fact that, if pipe->brx is null, then brx won't be
> NULL. So, the logic that "Switch BRx if needed." will always be called:
>
> diff --git a/drivers/media/platform/vsp1/vsp1_drm.c
> b/drivers/media/platform/vsp1/vsp1_drm.c index 095dc48aa25a..cb6b60843400
> 100644
> --- a/drivers/media/platform/vsp1/vsp1_drm.c
> +++ b/drivers/media/platform/vsp1/vsp1_drm.c
> @@ -185,7 +185,7 @@ static int vsp1_du_pipeline_setup_brx(struct vsp1_device
> *vsp1, brx = &vsp1->brs->entity;
>
> /* Switch BRx if needed. */
> - if (brx != pipe->brx) {
> + if (brx != pipe->brx || !pipe->brx) {
> struct vsp1_entity *released_brx = NULL;
>
> /* Release our BRx if we have one. */
>
> The code with switches BRx ensures that pipe->brx won't be null, as
> in the end, it sets:
>
> pipe->brx = brx;
>
> And brx can't be NULL.
The reason I don't like this is because the problem originally comes from the
fact that smatch assumes that vsp1->brs can be NULL when it can't. I'd rather
modify the code in a way that explicitly tests for vsp1->brs. However, smatch
won't accept that happily :-/ I tried
if (pipe->num_inputs > 2)
brx = &vsp1->bru->entity;
else if (pipe->brx && !drm_pipe->force_brx_release)
brx = pipe->brx;
else if (!vsp1->bru->entity.pipe)
brx = &vsp1->bru->entity;
else if (vsp1->brs)
brx = &vsp1->brs->entity;
else
return -EINVAL;
and I still get the same warning. I had to write the following (which is
obviously not correct) to silence the warning.
if (pipe->num_inputs > 2)
brx = &vsp1->bru->entity;
else if (pipe->brx)
brx = pipe->brx;
else if (!vsp1->bru->entity.pipe)
brx = &vsp1->bru->entity;
else {
(void)vsp1->brs->entity;
brx = &vsp1->brs->entity;
}
Both the (void)vsp1->brs->entity and the removal of the !drm_pipe-
>force_brx_release were needed, any of those on its own didn't fix the
problem.
> From my PoV, this patch has the advantage of explicitly showing
> to humans that the code inside the if statement will always be
> executed when pipe->brx is NULL.
>
> -
>
> Another way to solve would be to explicitly check if pipe->brx is still
> null before de-referencing:
>
> diff --git a/drivers/media/platform/vsp1/vsp1_drm.c
> b/drivers/media/platform/vsp1/vsp1_drm.c index edb35a5c57ea..9fe063d6df31
> 100644
> --- a/drivers/media/platform/vsp1/vsp1_drm.c
> +++ b/drivers/media/platform/vsp1/vsp1_drm.c
> @@ -327,6 +327,9 @@ static int vsp1_du_pipeline_setup_brx(struct vsp1_device
> *vsp1, list_add_tail(&pipe->brx->list_pipe, &pipe->entities);
> }
>
> + if (!pipe->brx)
> + return -EINVAL;
> +
> /*
> * Configure the format on the BRx source and verify that it matches the
> * requested format. We don't set the media bus code as it is configured
>
> The right fix would be, instead, to fix Smatch to handle the:
>
> if (brx != pipe->brx)
>
> for the cases where one var can be NULL while the other can't be NULL,
> but, as I said before, I suspect that this can be a way more complex.
I'm not sure smatch is faulty here, or at least not when it interprets the brx
!= pipe->brx check. The problem seems to come from the fact that is believes
brx can be NULL.
--
Regards,
Laurent Pinchart
