Changeset - 9a142ebfd720
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Bradley Kuhn (bkuhn) - 6 years ago 2016-08-09 05:05:59
bkuhn@ebb.org
Add link to partial source release.
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www/conservancy/static/copyleft-compliance/vmware-code-similarity.html
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<h1 id="similarity-analysis-and-contribution-analysis-of-christoph-hellwigs-linux-code-as-found-in-vmware-esxi-5.5">Similarity Analysis and Contribution Analysis of Christoph Hellwig's Linux Code as found in VMware ESXi 5.5</h1>
 
<p>This analysis verifies by reproducible analysis a set of specific contributions that are clearly made by Christoph Hellwig to Linux, and shows how those contributions appear in the VMware ESXi 5.5 product.</p>
 
<p>This analysis was prepared and written by <a href="https://sfconservancy.org/about/staff/#bkuhn">Bradley M. Kuhn</a>.</p>
 
<h1 id="understanding-code-similarity-and-cloning">Understanding Code Similarity and &quot;Cloning&quot;</h1>
 
<p>Software is often modified in various ways; indeed, Linux developers form a community that encourages and enables modification by many parties. Given this development model, communities often find it valuable to determine when software source code moves from one place to another with only minor modifications. Various scientifically-vetted techniques can be used to identify &quot;clones&quot; -- a portion of code that is substantially similar to pre-existing source code. The specific area of academic research is called &quot;code cloning detection&quot; or &quot;code duplication detection&quot;. The area has been under active research since the mid-1990s <a href="#fn1" class="footnoteRef" id="fnref1"><sup>1</sup></a>. In 2002, Japanese researchers published a tool called CCFinder <a href="#fn2" class="footnoteRef" id="fnref2"><sup>2</sup></a>, which, in its updated incarnation (called CCFinderX), is widely used and referenced by academic researchers in the field <a href="#fn3" class="footnoteRef" id="fnref3"><sup>3</sup></a> and has specifically been used to explore reuses of code in GPL'd software such as Linux <a href="#fn4" class="footnoteRef" id="fnref4"><sup>4</sup></a>.</p>
 
<p>CCFinderX uses a token-based clone detection method and a suffix-tree matching algorithm; both techniques have been highly vetted and considered in the academic literature. The techniques are considered viable and useful in detecting clones. Many academic papers on the subject have been peer-reviewed and published, and nearly every newly published paper compares its new techniques of clone detection to the seminal results found by CCFinderX. For purposes of our analysis, we have therefore chosen to use CCFinderX. These results can be easily reproduced since CCFinderX is, itself, also Open Source software.</p>
 
<h1 id="establishing-a-baseline-of-the-ccfinderx-tool">Establishing A Baseline of the CCFinderX Tool</h1>
 
<p>CCFinderX offers many statistics for clone detection. After expert analysis, we concluded that most relevant to this situation is the &quot;ratio of similarity&quot; between the existing code and the new code. To establish a baseline, we considered two different comparisons of Free and Open Source Software (FOSS). First, we compared the Linux kernel, Version 4.5.2, to the FreeBSD kernel, Version 10.3.0. This comparison was inspired by the similar 2002 study <a href="#fn5" class="footnoteRef" id="fnref5"><sup>5</sup></a> of these two large C programs. The hypothesis remained that CCFinderX would encounter a small percentage of code similarity, since the FreeBSD and Linux projects collaborate on some subprojects and willingly share code under the 3-Clause BSD license for those parts. (These collaborations are public and well-documented.)</p>
 
<p>The experiment confirmed the hypothesis. We found that a 3.68% &quot;ratio of similarity&quot; when comparing code from Linux to the FreeBSD kernel.</p>
 
<p>Next, we compared the source code of the Linux Kernel 4.5.2 to the LLVM+Clang system, version 3.8.0. These two projects are each a large program written in the C programming language, but they are not known to actively share code. We would expect some very minimal similarity simply due to chance, but something much lower than the 3.68% found between Linux and FreeBSD's kernel.</p>
 
<p>Indeed, when the same test is run to compare Linux to the LLVM+Clang system, the &quot;ratio of similarity&quot; was 0.075%.</p>
 
<h1 id="general-comparison-of-linux-kernel-to-vmware-sources">General Comparison of Linux Kernel to VMware sources</h1>
 
<p>With the baseline established, we now begin relevant comparisons. First, we compare the Linux kernel version 2.6.34 to the sources released by VMware in their (partial) source release. the &quot;ratio of similarity&quot; between Linux 2.6.34 and VMware's partial source release is 20.72%. There is little question that much of VMware's kernel has come from Linux.</p>
 
<p>With the baseline established, we now begin relevant comparisons. First, we compare the Linux kernel version 2.6.34 to the sources <a href="http://k.sfconservancy.org/vmkdrivers">released by VMware in their (partial) source release</a>. The &quot;ratio of similarity&quot; between Linux 2.6.34 and VMware's partial source release is 20.72%. There is little question that much of VMware's kernel has come from Linux.</p>
 
<h1 id="methodology-of-showing-hellwigs-contributions-in-vmware-esxi-5.5-sources">Methodology Of Showing Hellwig's Contributions in VMware ESXi 5.5 Sources</h1>
 
<p>The following describes a methodology to show Hellwig's contributions to Linux, and how they compare to code found in VMware ESXi 5.5.</p>
 
<h2 id="extracting-hellwigs-contributions-from-linux-historical-repository">Extracting Hellwig's Contributions From Linux Historical Repository</h2>
 
<p>Excellent records exist of contributions made to Linux from 2002-02-04 through present date. From 2002-02-04 through 2005-04-03, Bitkeeper was used to store revision control history of Linux. Each improvement contributed to Linux has information regarding who placed the contribution in Linux, and a comment field in which the contributor can credit others, such as by noting that the contribution actually came from someone else.</p>
 
<p>I extracted from the historical Linux tree the identifying number of all commits that are either made with Hellwig in the official Author field, or where the person in the Author field left notes clearly indicating that the contribution was done by Hellwig. For the latter, the following regular expression search against the log file was used:</p>
 
<pre><code>(Submitted\s+by|original\s+patch|patch\s+(from|by)|originally\s+(from|by)).*Hellwig</code></pre>
 
<p>Specifically, I used <a href="https://github.com/conservancy/gpl-compliance-tools/blob/master/commit-id-list-matching-regex.plx">a script</a> to extract a list of commit ids from the <a href="git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git">historical Linux repository</a>. This method found 1,012 separate occasions of contribution by Hellwig from 2002-02-04 through 2005-04-03.</p>
 
<p>After finding these separate occasions of contribution, I then extracted the source code lines that Hellwig added or changed in each contribution in this repository. I did so by carefully cross-referencing the commits that Hellwig performed with the output of <code>git blame</code>. I specifically <a href="https://github.com/conservancy/gpl-compliance-tools/blob/master/extract-code-added-in-commits.plx">wrote a script</a> to carefully extracted only lines that Hellwig changed or added in that repository, and placed only those contributions identifiable as Hellwig's into new files whose named matched the original filenames. This created a corpus of code that can be verifiable as added or changed by Hellwig and no one else.</p>
 
<p>Here are the specific commands I ran:</p>
 
<pre><code>$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git linux-historical
 
$ ./commit-id-list-matching-regex.plx `pwd`/linux-historical/.git Hellwig &#39;(Submitted\s+by|originals+patch|patch\s+from|originally\s+by).*&#39; &gt; hellwig-historical.ids
 
$ ./extract-code-added-in-commits.plx --repository=`pwd`/linux-historical --output-dir=`pwd`/hellwig-historical --central-commit e7e173af42dbf37b1d946f9ee00219cb3b2bea6a --progress --blame-opts=-M --blame-opts=-C &lt; ./hellwig-historical.ids
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