linux-kernel - Re: [PATCH 1/2] drm/nouveau/device: avoid usage of list iterator after loop

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Message-ID: <361fb10adaa1cb3ff5360cbb0e36a3c1e99fe3e8.camel@redhat.com>
Date:   Tue, 07 Mar 2023 17:49:08 -0500
From:   Lyude Paul <lyude@...hat.com>
To:     Jakob Koschel <jkl820.git@...il.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Pietro Borrello <borrello@...g.uniroma1.it>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH 1/2] drm/nouveau/device: avoid usage of list iterator
 after loop

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@...il.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