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List:       linux-kernel
Subject:    AW: [PATCH] soc: aspeed: fix a ternary sign expansion bug
From:       Walter Harms <wharms () bfs ! de>
Date:       2021-04-23 11:03:30
Message-ID: ebe4a1a6dd0748e28e6ca19aec20223e () bfs ! de
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as indepentent observer,
i would go for Dans solution:

ret = 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-Hartman; Robert \
Lippert; linux-aspeed@lists.ozlabs.org; linux-kernel@vger.kernel.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() 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;

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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