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Date:   Mon, 11 Nov 2019 00:38:15 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Jan Stancek <jstancek@redhat.com>
Cc:     "Darrick J. Wong" <darrick.wong@oracle.com>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        LTP List <ltp@lists.linux.it>,
        Linux-Next Mailing List <linux-next@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, chrubis <chrubis@suse.cz>,
        open list <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Mark Brown <broonie@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        lkft-triage@lists.linaro.org,
        Christoph Hellwig <hch@infradead.org>,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        Theodore Ts'o <tytso@mit.edu>
Subject: Re: LTP: diotest4.c:476: read to read-only space. returns 0: Success
Message-ID: <20191111083815.GA29540@infradead.org>
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On Mon, Nov 11, 2019 at 03:19:40AM -0500, Jan Stancek wrote:
> > > loff_t length,
> > >                 if (pad)
> > >                         iomap_dio_zero(dio, iomap, pos, fs_block_size -
> > >                         pad);
> > >         }
> > > -       return copied ? copied : ret;
> > > +       return copied ? (loff_t) copied : ret;
> > 
> > I'm a little confused on this proposed fix -- why does casting size_t
> > (aka unsigned long) to loff_t (long long) on a 32-bit system change the
> > test outcome?
> 
> Ternary operator has a return type and an attempt is made to convert
> each of operands to the type of the other. So, in this case "ret"
> appears to be converted to type of "copied" first. Both have size of
> 4 bytes on 32-bit x86:

Sounds like we should use a good old if here to avoid that whole problem
spacE:

	if (copied)
		return copied;
	return ret;

> size_t copied = 0;
> int ret = -14;
> long long actor_ret = copied ? copied : ret;
> 
> On x86_64: actor_ret == -14;
> On x86   : actor_ret == 4294967282
> 
> > Does this same diotest4 failure happen with XFS?  I ask
> > because XFS has been using iomap for directio for ages.
> 
> Yes, it fails on XFS too.

Is this a new test?  If not why was this never reported?  Sounds like
we should add this test case to xfstests.