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

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

On Tue Jan 27, 2026 at 3:30 AM JST, Gary Guo wrote:
> 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@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);
>
> 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.

Indeed, this is arguably easier to understand.