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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Jan 23, 2026 at 9:12 PM JST, 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`.

Another dangerous one! Well found.

>
> 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@nvidia.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);
>  
> -        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])

I guess we can also use `&[]` here (maybe convert the whole file in the
same patch).