Re: [PATCH] rust: dma: return zero for Coherent reads past EOF

[Gary Guo]

On Fri Jul 24, 2026 at 2:19 AM BST, Alexandre Courbot wrote:
> On Fri Jul 24, 2026 at 9:46 AM JST, Gary Guo wrote:
>> On Fri Jul 24, 2026 at 1:18 AM BST, Alexandre Courbot wrote:
>>> On Fri Jul 24, 2026 at 8:32 AM JST, Gary Guo wrote:
>>>> On Mon Jul 20, 2026 at 8:21 PM BST, Younes Akhouayri via B4 Relay wrote:
>>>>> From: Younes Akhouayri <git@xxxxxxxxx>
>>>>>
>>>>> Coherent<T>::write_to_slice() calculates a zero-byte copy when the file
>>>>> offset is beyond the allocation, but still calls
>>>>> UserSliceWriter::write_dma(). The latter rejects offsets beyond the
>>>>> allocation even when the copy length is zero, so a debugfs read past EOF
>>>>> returns -ERANGE.
>>>>>
>>>>> Return before calling write_dma() when the offset is at or beyond the
>>>>> allocation, matching simple_read_from_buffer() EOF semantics.
>>>>>
>>>>> Fixes: 016818513936 ("rust: dma: implement BinaryWriter for Coherent<[u8]>")
>>>>> Signed-off-by: Younes Akhouayri <git@xxxxxxxxx>
>>>>> ---
>>>>>  rust/kernel/dma.rs | 4 ++++
>>>>>  1 file changed, 4 insertions(+)
>>>>>
>>>>> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
>>>>> index 200def84fb69..36b72fb9ab46 100644
>>>>> --- a/rust/kernel/dma.rs
>>>>> +++ b/rust/kernel/dma.rs
>>>>> @@ -1005,6 +1005,10 @@ fn write_to_slice(
>>>>>              return Ok(0);
>>>>>          };
>>>>>  
>>>>> +        if offset_val >= self.size() {
>>>>> +            return Ok(0);
>>>>> +        }
>>>>> +
>>>>>          let count = self.size().saturating_sub(offset_val).min(writer.len());
>>>>
>>>> This saturating_sub is redundant now and should just use -.
>>>
>>> Or even better, you could use `checked_sub` to perform the test and the
>>> subtraction in one go, while avoiding potentially panicking operands.
>>
>> I think the code is clearer with separate EOF check length calc. This also needs
>> to return `Ok` so it's not like that you can use `?`.
>
> Is it clearer? The `checked_sub` is efficient and makes it clear the
> code won't panic. And `ok_or` takes care of converting to the right
> error nicely.

There's no error here.

Best,
Gary