SGI compares Linux, Unix source code

An open letter to the Linux community published last week by Silicon Graphics indicates that SGI has conducted a comprehensive comparison of the Linux kernel and the Unix System V source code owned by The SCO Group Inc.

According to the letter, authored by SGI Vice President of Software Rich Altmaier, SGI conducted an "exhaustive comparison" of the Linux kernel and the Unix System V source code, which turned up only "trivial" code segments that "may arguably be related" to SCO's software.

The letter also disputed SCO's claims that SGI inappropriately contributed its XFS (eXtensible File System) code to the Linux operating system.

For months SCO has claimed that an exhaustive examination of the Linux source code has revealed software that has been copied line-by-line from its Unix System V code base. The Linux community has denied these allegations, but until this week no one else had claimed to have undertaken a comprehensive comparison of the two operating systems.

SGI's letter was published just as SCO revealed that it had threatened to terminate SGI's Unix license, alleging that the Mountain View, California, computer maker inappropriately contributed source code to Linux. Earlier this year, SCO announced that it had terminated IBM Corp.'s AIX license, citing similar allegations. The Lindon, Utah, company is now engaged in a US$3 billion lawsuit with IBM over the matter.

SGI's code comparison was done during September using the Comparator software created by open source advocate Eric Raymond, as well as some other internally developed tools, according to SGI. It compared source code from the Unix System V release 4.1 software that SGI has licensed from SCO with a version of the Linux kernel released this June, SGI said.

"Our review was focused on the code we contributed to Linux; however, we did run the Comparator code on the Linux 2.4.21 kernel. The process involves using subjective judgment to review similarities identified by the tool," said Greg Estes, SGI's vice president of corporate marketing, in an e-mail response to questions.

The point of SGI's comparison was to search for any potential matches between Unix System V and any contributions that SGI made to the Linux kernel, not to vet the software for the entire community, Estes said in an interview. "We are not making any kind of representation at all about anybody else's contributed code," he said.

SGI first reviewed its open source contributions earlier this summer, and Altmaier's letter concedes that SGI discovered at that time that three "brief fragments" of SGI-contributed code matched the Unix System V code that SGI had licensed from SCO.

"All together, these three small code fragments comprised no more than 200 lines (of code)," wrote Altmaier. "It appears that most or all of the System V fragments we found had previously been placed in the public domain, meaning it is very doubtful that the SCO Group has any proprietary claim to these code fragments," he added.

The code in question was no longer in the core Linux kernel, following the Aug. 25 release of Linux 2.4.22, Altmaier wrote.

Then in September SGI carried out its more comprehensive comparison. "SGI continued our investigation to determine whether any other code in the Linux kernel was even conceivably implicated," Altmaier states in the letter.

This comparison revealed a few examples of line-by-line copying, but did not determine whether the code was owned by SCO or in the public domain, according to the letter. "SGI has discovered a few additional code segments ... that may arguably be related to the Unix code," Altmaier wrote. He added that these segments were "trivial in amount."

SGI declined to reveal any details on the additional code segments it found, but the fact that its analysis appears to reveal no extensive overlap between the code in Linux and System V is good news for Linux users, according to Gartner Inc. analyst George Weiss.

"I think it's very helpful," he said. But more information is needed to fully respond to SCO's copyright allegations, he added. "I don't know if the job is complete from this letter," Weiss said.

It would be more helpful if other SCO licensees like Hewlett-Packard Co. or IBM Corp. performed similar analyses and went public with their results, Weiss said.

Such a thorough vetting of the Linux code might answer questions about line-by-line copying, but it would not counter all of SCO's charges, he added. SCO claims that Linux also contains derivative works built on top of its System V Unix, such as the XFS code that SGI contributed to Linux, as well as "obfuscated" code that is almost identical to SCO's Unix. These claims would not be answered by the kind of analysis that SGI has done, Weiss said.

Weiss praised the tack SGI has taken with its letter, saying that Altmaier's response has helped mitigate SCO's allegations. "I thought it was one of the best responses (to SCO) that I had seen. Instead of getting deeply offensive and heaping abuse on SCO, they took a more productive approach, attempting to see what the claims might be," he said.

The fact that SGI has replaced the three code fragments in question does not satisfy SCO, according to Blake Stowell, a SCO spokesman. "These releases have already taken place in Linux," he said. "You still have all these machines out there that haven't applied patches that are still benefitting from this Unix System V code."

Any line-by-line contribution of SCO's code to Linux was "not trivial," he added.

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