If potentially no valid element is found, 'pstate' would contain an
invalid pointer past the iterator loop. To ensure 'pstate' is always
valid, we only set it if the correct element was found. That allows
adding a BUG_ON in case the code works incorrectly, exposing currently
undetectable potential bugs.

Additionally, Linus proposed to avoid any use of the list iterator
variable after the loop, in the attempt to move the list iterator
variable declaration into the marcro to avoid any potential misuse after
the loop [1].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jkl820.git@gmail.com>
---
 drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
index ce774579c89d..7c9dd91e98ee 100644
--- a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
+++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
@@ -72,7 +72,7 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
 	} *args = data;
 	struct nvkm_clk *clk = ctrl->device->clk;
 	const struct nvkm_domain *domain;
-	struct nvkm_pstate *pstate;
+	struct nvkm_pstate *pstate = NULL, *iter;
 	struct nvkm_cstate *cstate;
 	int i = 0, j = -1;
 	u32 lo, hi;
@@ -103,11 +103,14 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
 		return -EINVAL;
 
 	if (args->v0.state != NVIF_CONTROL_PSTATE_ATTR_V0_STATE_CURRENT) {
-		list_for_each_entry(pstate, &clk->states, head) {
-			if (i++ == args->v0.state)
+		list_for_each_entry(iter, &clk->states, head) {
+			if (i++ == args->v0.state) {
+				pstate = iter;
 				break;
+			}
 		}
 
+		BUG_ON(!pstate);
 		lo = pstate->base.domain[domain->name];
 		hi = lo;
 		list_for_each_entry(cstate, &pstate->list, head) {

-- 
2.34.1

On Wed, 2023-03-01 at 18:25 +0100, Jakob Koschel wrote:
> If potentially no valid element is found, 'pstate' would contain an
> invalid pointer past the iterator loop. To ensure 'pstate' is always
> valid, we only set it if the correct element was found. That allows
> adding a BUG_ON in case the code works incorrectly, exposing currently
> undetectable potential bugs.
> 
> Additionally, Linus proposed to avoid any use of the list iterator
> variable after the loop, in the attempt to move the list iterator
> variable declaration into the marcro to avoid any potential misuse after
> the loop [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jkl820.git@gmail.com>
> ---
>  drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
> index ce774579c89d..7c9dd91e98ee 100644
> --- a/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
> +++ b/drivers/gpu/drm/nouveau/nvkm/engine/device/ctrl.c
> @@ -72,7 +72,7 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
>  	} *args = data;
>  	struct nvkm_clk *clk = ctrl->device->clk;
>  	const struct nvkm_domain *domain;
> -	struct nvkm_pstate *pstate;
> +	struct nvkm_pstate *pstate = NULL, *iter;
>  	struct nvkm_cstate *cstate;
>  	int i = 0, j = -1;
>  	u32 lo, hi;
> @@ -103,11 +103,14 @@ nvkm_control_mthd_pstate_attr(struct nvkm_control *ctrl, void *data, u32 size)
>  		return -EINVAL;
>  
>  	if (args->v0.state != NVIF_CONTROL_PSTATE_ATTR_V0_STATE_CURRENT) {
> -		list_for_each_entry(pstate, &clk->states, head) {
> -			if (i++ == args->v0.state)
> +		list_for_each_entry(iter, &clk->states, head) {
> +			if (i++ == args->v0.state) {
> +				pstate = iter;
>  				break;
> +			}
>  		}
>  
> +		BUG_ON(!pstate);

Let's replace this with

		if (WARN_ON_ONCE(!pstate))
			return -EINVAL;

>  		lo = pstate->base.domain[domain->name];
>  		hi = lo;
>  		list_for_each_entry(cstate, &pstate->list, head) {
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat