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

[Gary Guo]

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 `?`.

Best,
Gary