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From:   Walter Harms <wharms@bfs.de>
To:     David Laight <David.Laight@ACULAB.COM>,
        'Sergey Organov' <sorganov@gmail.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: AW: [PATCH] soc: aspeed: fix a ternary sign expansion bug
Thread-Topic: [PATCH] soc: aspeed: fix a ternary sign expansion bug
Thread-Index: AQHXN1ea5N8/zdZIw0CYuTs8hCrx1qrAlvAAgAFWXQf//+CxgIAAIfA0
Date:   Fri, 23 Apr 2021 11:03:30 +0000
Message-ID: <ebe4a1a6dd0748e28e6ca19aec20223e@bfs.de>
References: <YIE90PSXsMTa2Y8n@mwanda>
        <59596244622c4a15ac8cc0747332d0be@AcuMS.aculab.com>
 <877dktuvmz.fsf@osv.gnss.ru>,<265e2d3accc74c89b5bab22eadb43808@AcuMS.aculab.com>
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as indepentent observer,
i would go for Dans solution:

ret =3D kfifo_to_user();
/* if an error occurs just return */
if (ret)
   return ret;

/* otherwise return the copied number of bytes */

return copied;

there is no need for any deeper language knowledge,

jm2c
re,
 wh
________________________________________
Von: David Laight <David.Laight@ACULAB.COM>
Gesendet: Freitag, 23. April 2021 12:54:59
An: 'Sergey Organov'
Cc: 'Dan Carpenter'; Joel Stanley; Andrew Jeffery; Chia-Wei, Wang; Jae Hyun=
 Yoo; John Wang; Brad Bishop; Patrick Venture; Benjamin Fair; Greg Kroah-Ha=
rtman; Robert Lippert; linux-aspeed@lists.ozlabs.org; linux-kernel@vger.ker=
nel.org; kernel-janitors@vger.kernel.org
Betreff: RE: [PATCH] soc: aspeed: fix a ternary sign expansion bug

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From: Sergey Organov
> Sent: 23 April 2021 11:46
>
> 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() functi=
on
> >> 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 direct=
ly
> >> 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/aspee=
d/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, c=
har __user *buffer,
> >>                    return -EINTR;
> >>    }
> >>    ret =3D 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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