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

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Message-Id: <20230301-drm-nouveau-avoid-iter-after-loop-v1-2-0702ec23f970@gmail.com>
Date:   Wed, 01 Mar 2023 18:25:38 +0100
From:   Jakob Koschel <jkl820.git@...il.com>
To:     Ben Skeggs <bskeggs@...hat.com>, Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...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>, Jakob Koschel <jkl820.git@...il.com>
Subject: [PATCH 2/2] drm/nouveau/clk: avoid usage of list iterator after
 loop

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/subdev/clk/base.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
index da07a2fbef06..871127dfe1d7 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/clk/base.c
@@ -269,14 +269,17 @@ nvkm_pstate_prog(struct nvkm_clk *clk, int pstatei)
 	struct nvkm_subdev *subdev = &clk->subdev;
 	struct nvkm_fb *fb = subdev->device->fb;
 	struct nvkm_pci *pci = subdev->device->pci;
-	struct nvkm_pstate *pstate;
+	struct nvkm_pstate *pstate = NULL, *iter;
 	int ret, idx = 0;
 
-	list_for_each_entry(pstate, &clk->states, head) {
-		if (idx++ == pstatei)
+	list_for_each_entry(iter, &clk->states, head) {
+		if (idx++ == pstatei) {
+			pstate = iter;
 			break;
+		}
 	}
 
+	BUG_ON(!pstate);
 	nvkm_debug(subdev, "setting performance state %d\n", pstatei);
 	clk->pstate = pstatei;
 

-- 
2.34.1