linux-kernel - Re: [PATCH v2 4/4] gpu: nova-core: gsp: fix improper indexing in driver_read

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Message-Id: <DFYQGXHPBZP7.O34DZ6RTDGU0@garyguo.net>
Date: Mon, 26 Jan 2026 18:30:40 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Eliot Courtney" <ecourtney@...dia.com>, "Danilo Krummrich"
 <dakr@...nel.org>, "Alexandre Courbot" <acourbot@...dia.com>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, "Alistair Popple" <apopple@...dia.com>
Cc: <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] gpu: nova-core: gsp: fix improper indexing in
 driver_read_area

On Fri Jan 23, 2026 at 12:12 PM GMT, Eliot Courtney wrote:
> The current code indexes into `after_rx` using `tx` which is an index
> for the whole buffer, not the split buffer `after_rx`.
>
> Also add more rigorous no-panic proofs.
>
> Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
> Signed-off-by: Eliot Courtney <ecourtney@...dia.com>
> ---
>  drivers/gpu/nova-core/gsp/cmdq.rs | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
> index aa8758fc7723..c26396fda29c 100644
> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
> @@ -1,7 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
>  use core::{
> -    cmp,
>      mem,
>      sync::atomic::{
>          fence,
> @@ -267,10 +266,20 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>          // PANIC: per the invariant of `cpu_read_ptr`, `rx` is `< MSGQ_NUM_PAGES`.
>          let (before_rx, after_rx) = gsp_mem.gspq.msgq.data.split_at(rx);

This code doesn't need splitting as it doesn't have the uniqueness issue that
mutable references have. While you're at it, probably it's chance to simplify
the code.

>  
> -        match tx.cmp(&rx) {
> -            cmp::Ordering::Equal => (&after_rx[0..0], &after_rx[0..0]),
> -            cmp::Ordering::Greater => (&after_rx[..tx], &before_rx[0..0]),
> -            cmp::Ordering::Less => (after_rx, &before_rx[..tx]),
> +        // The area starting at `rx` and ending at `tx - 1` modulo MSGQ_NUM_PAGES, inclusive,
> +        // belongs to the driver for reading.
> +        if rx <= tx {
> +            // The area is contiguous.
> +            // PANIC:
> +            // - The index `tx - rx` is non-negative because `rx <= tx` in this branch.
> +            // - The index does not exceed `after_rx.len()` (which is `MSGQ_NUM_PAGES - rx`)
> +            //   because `tx < MSGQ_NUM_PAGES` by the `gsp_write_ptr` invariant.
> +            (&after_rx[..(tx - rx)], &after_rx[0..0])

This can be just `(&data[rx..tx], &[])` without the split.

> +        } else {
> +            // The area is discontiguous.
> +            // PANIC: `tx` does not exceed `before_rx.len()` (which equals `rx`) because
> +            //   `tx < rx` in this branch.
> +            (after_rx, &before_rx[..tx])

This can be just `(&data[rx..], &data[..tx])` without the split.

Best,
Gary


>          }
>      }
>