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 04:43:23 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
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 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:

@@ -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)