Re: [PATCH] soc: aspeed: fix a ternary sign expansion bug

[Sergey Organov]

David Laight <David.Laight@xxxxxxxxxx> writes:

> From: Dan Carpenter
>> Sent: 22 April 2021 10:12
>> 
>> The intent here was to return negative error codes but it actually
>> returns positive values.  The problem is that type promotion with
>> ternary operations is quite complicated.
>> 
>> "ret" is an int.  "copied" is a u32.  And the snoop_file_read() function
>> returns long.  What happens is that "ret" is cast to u32 and becomes
>> positive then it's cast to long and it's still positive.
>> 
>> Fix this by removing the ternary so that "ret" is type promoted directly
>> to long.
>> 
>> Fixes: 3772e5da4454 ("drivers/misc: Aspeed LPC snoop output using misc chardev")
>> Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>> ---
>>  drivers/soc/aspeed/aspeed-lpc-snoop.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> 
>> diff --git a/drivers/soc/aspeed/aspeed-lpc-snoop.c b/drivers/soc/aspeed/aspeed-lpc-snoop.c
>> index 210455efb321..eceeaf8dfbeb 100644
>> --- a/drivers/soc/aspeed/aspeed-lpc-snoop.c
>> +++ b/drivers/soc/aspeed/aspeed-lpc-snoop.c
>> @@ -94,8 +94,10 @@ static ssize_t snoop_file_read(struct file *file, char __user *buffer,
>>  			return -EINTR;
>>  	}
>>  	ret = kfifo_to_user(&chan->fifo, buffer, count, &copied);
>> +	if (ret)
>> +		return ret;
>> 
>> -	return ret ? ret : copied;
>> +	return copied;
>
> I wonder if changing it to:
> 	return ret ? ret + 0L : copied;
>
> Might make people think in the future and not convert it back
> as an 'optimisation'.

It rather made me think: "what the heck is going on here?!"

Shouldn't it better be:

 	return ret ? ret : (long)copied;

or even:

        return ret ?: (long)copied;

?

-- Sergey Organov