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

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Date:   Fri, 23 Apr 2021 00:07:04 +0000
From:   Joel Stanley <joel@....id.au>
To:     David Laight <David.Laight@...lab.com>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        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

On Thu, 22 Apr 2021 at 16:21, David Laight <David.Laight@...lab.com> wrote:
>
> 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'.

I think the change that Dan posted is clear.

Thanks Dan! I'll get it queued up.

Cheers,

Joel