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

From: Sergey Organov <sorganov@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: 'Dan Carpenter' <dan.carpenter@oracle.com>,
	Joel Stanley <joel@jms.id.au>, Andrew Jeffery <andrew@aj.id.au>,
	"Chia-Wei, Wang" <chiawei_wang@aspeedtech.com>,
	Jae Hyun Yoo <jae.hyun.yoo@intel.com>,
	"John Wang" <wangzhiqiang.bj@bytedance.com>,
	Brad Bishop <bradleyb@fuzziesquirrel.com>,
	Patrick Venture <venture@google.com>,
	"Benjamin Fair" <benjaminfair@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Robert Lippert <rlippert@google.com>,
	"linux-aspeed@lists.ozlabs.org" <linux-aspeed@lists.ozlabs.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kernel-janitors@vger.kernel.org"
	<kernel-janitors@vger.kernel.org>
Subject: Re: [PATCH] soc: aspeed: fix a ternary sign expansion bug
Date: Fri, 23 Apr 2021 13:45:40 +0300	[thread overview]
Message-ID: <877dktuvmz.fsf@osv.gnss.ru> (raw)
In-Reply-To: <59596244622c4a15ac8cc0747332d0be@AcuMS.aculab.com> (David Laight's message of "Thu, 22 Apr 2021 16:21:40 +0000")

David Laight <David.Laight@ACULAB.COM> 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@oracle.com>
>> ---
>>  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