Re: [PATCH v4 2/6] io_uring/mock: add cmd using vectored regbufs

[Jens Axboe <axboe@kernel.dk>]

On 5/30/25 8:53 AM, Pavel Begunkov wrote:
> On 5/30/25 15:37, Jens Axboe wrote:
>> On 5/30/25 7:40 AM, Pavel Begunkov wrote:
>>> On 5/30/25 14:25, Jens Axboe wrote:
>>>> On 5/30/25 6:51 AM, Pavel Begunkov wrote:
>>>>> +static int io_copy_regbuf(struct iov_iter *reg_iter, void __user *ubuf)
>>>>> +{
>>>>> +    size_t ret, copied = 0;
>>>>> +    size_t buflen = PAGE_SIZE;
>>>>> +    void *tmp_buf;
>>>>> +
>>>>> +    tmp_buf = kzalloc(buflen, GFP_KERNEL);
>>>>> +    if (!tmp_buf)
>>>>> +        return -ENOMEM;
>>>>> +
>>>>> +    while (iov_iter_count(reg_iter)) {
>>>>> +        size_t len = min(iov_iter_count(reg_iter), buflen);
>>>>> +
>>>>> +        if (iov_iter_rw(reg_iter) == ITER_SOURCE) {
>>>>> +            ret = copy_from_iter(tmp_buf, len, reg_iter);
>>>>> +            if (ret <= 0)
>>>>> +                break;
>>>>> +            if (copy_to_user(ubuf, tmp_buf, ret))
>>>>> +                break;
>>>>> +        } else {
>>>>> +            if (copy_from_user(tmp_buf, ubuf, len))
>>>>> +                break;
>>>>> +            ret = copy_to_iter(tmp_buf, len, reg_iter);
>>>>> +            if (ret <= 0)
>>>>> +                break;
>>>>> +        }
>>>>
>>>> Do copy_{to,from}_iter() not follow the same "bytes not copied" return
>>>> value that the copy_{to,from}_user() do? From a quick look, looks like
>>>> they do.
>>>>
>>>> Minor thing, no need for a respin just for that.
>>>
>>> One returns 0 on success the other the number of processed bytes.
>>
>> copy_{to,from}_user() returns bytes NOT processed, and I guess the iter
> 
> Sure, it doesn't contradict it, they follow different semantics.
> 
>> versions return bytes processed. Guess the code is fine, it's more so
>> the API that's a bit wonky on the copy/iter side.
> 
> Which API? This command or copy helpers? copy_{to,from}_user are used
> here in the way they're always used in the kernel, and that's fine,
> they're not supposed to fail for valid input. That's unlike iter
> helpers, which may return a partial result.

Not this command, the copy helpers in the kernel.

-- 
Jens Axboe