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Date:   Thu, 22 Apr 2021 09:24:59 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     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>,
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        Patrick Venture <venture@google.com>,
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        Robert Lippert <rlippert@google.com>,
        linux-aspeed@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        kernel-janitors@vger.kernel.org
Subject: Re: [PATCH] soc: aspeed: fix a ternary sign expansion bug
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On Thu, Apr 22, 2021 at 12:11:44PM +0300, Dan Carpenter wrote:
> 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.

Hmm...  Let's grep for kfifo_to_user() - smells like a possible recurring bug...
Yup -

samples/kfifo/bytestream-example.c:138: ret = kfifo_to_user(&test, buf, count, &copied);
samples/kfifo/inttype-example.c:131:    ret = kfifo_to_user(&test, buf, count, &copied);
samples/kfifo/record-example.c:145:     ret = kfifo_to_user(&test, buf, count, &copied);

All three are exactly like that one.