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

[Alexandre Courbot]

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.