linux-kernel - Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in iomap_seek

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Date:   Mon, 5 Jul 2021 12:05:33 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next 1/1] iomap: Fix a false positive of UBSAN in
 iomap_seek_data()



On 2021/7/5 11:43, Matthew Wilcox wrote:
> On Mon, Jul 05, 2021 at 11:29:44AM +0800, Leizhen (ThunderTown) wrote:
>> I've thought about it, and that "if" statement can be removed as follows:
> 
> I think this really misses Christoph's point.  He's looking for
> something more like this:

Yes, I know that. I need to get rid of the "if" judgment first, and then I can
modify iomap_seek_data() according to Christoph's point. Otherwise there are too
many conversions from "length" to "size - offset" and the code doesn't look clear.

> 
> @@ -83,27 +83,23 @@ loff_t
>  iomap_seek_data(struct inode *inode, loff_t offset, const struct iomap_ops *ops)
>  {
>         loff_t size = i_size_read(inode);
> -       loff_t length = size - offset;
>         loff_t ret;
> 
>         /* Nothing to be found before or beyond the end of the file. */
>         if (offset < 0 || offset >= size)
>                 return -ENXIO;
> 
> -       while (length > 0) {
> +       while (offset < size) {
>                 ret = iomap_apply(inode, offset, length, IOMAP_REPORT, ops,
>                                   &offset, iomap_seek_data_actor);
>                 if (ret < 0)
>                         return ret;
>                 if (ret == 0)
> -                       break;
> +                       return offset;
> 
>                 offset += ret;
> -               length -= ret;
>         }
> 
> -       if (length <= 0)
> -               return -ENXIO;
> -       return offset;
> +       return -ENXIO;
>  }
>  EXPORT_SYMBOL_GPL(iomap_seek_data);
> 
> (not even slightly tested)
> 
> .
>