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

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Message-ID: <877dktuvmz.fsf@osv.gnss.ru>
Date:   Fri, 23 Apr 2021 13:45:40 +0300
From:   Sergey Organov <sorganov@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Dan Carpenter' <dan.carpenter@...cle.com>,
        Joel Stanley <joel@....id.au>,
        Andrew Jeffery <andrew@...id.au>,
        "Chia-Wei, Wang" <chiawei_wang@...eedtech.com>,
        Jae Hyun Yoo <jae.hyun.yoo@...el.com>,
        "John Wang" <wangzhiqiang.bj@...edance.com>,
        Brad Bishop <bradleyb@...ziesquirrel.com>,
        Patrick Venture <venture@...gle.com>,
        "Benjamin Fair" <benjaminfair@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Robert Lippert <rlippert@...gle.com>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: [PATCH] soc: aspeed: fix a ternary sign expansion bug

David Laight <David.Laight@...LAB.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@...cle.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