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

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Message-ID: <265e2d3accc74c89b5bab22eadb43808@AcuMS.aculab.com>
Date:   Fri, 23 Apr 2021 10:54:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sergey Organov' <sorganov@...il.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

From: Sergey Organov
> Sent: 23 April 2021 11:46
> 
> 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;

Or change the return type to int ?

The problem is that ?: doesn't behave the way most people expect.
The two possible values have to be converted to the same type.

Together with the broken decision of the original ANSI C committee
to change from K&R's 'sign preserving' to 'value preserving'
integer promotions causes grief here and elsewhere.
(Not no mention breaking existing code!)

	David

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