Microsoft is contributing thousands of lines of code to the Linux kernel, with the blessing of open-source leaders. If that isn't enough of a shocker, the company is making the contribution under the same free-software license that Microsoft CEO Steve Ballmer once questioned in language so strong it remains one of his most controversial remarks.

The company is making the announcement this morning. The release of driver code under the GNU General Public License, version 2.0, is far from the first overture Microsoft has made to the open-source software community in recent years. But symbolically, at least, it's a major development in the company's historically tumultuous relationship with the Linux operating system.

Microsoft's motives will no doubt be scrutinized closely as a result.

Why the decision? "If you want to participate in the community, if you want your code to be with that community, you have to engage with them in the way they work, and they work under GPLv2," said Tom Hanrahan, director of Microsoft's Open Source Technology Center.

The contribution of code will make it possible to run a variety of Linux distributions as guests on Microsoft's Hyper-V virtualization technology for Windows Server 2008. Virtualization lets multiple operating systems run on the same server, reducing hardware and management costs. Microsoft says customers are pushing for that type of flexibility, and for that reason it says the move is right for its business.

Sam Ramji (L) and Tom Hanrahan

"In this economic downturn, if we’re able to provide more value by giving you a consistent platform to take all of your applications to -- including the operating system that’s hosting that application that you’re comfortable with -- we’ve done something really good for you," said Sam Ramji, Microsoft senior director of platform strategy.

[Related Post: Video: Inside Microsoft's Open-Source Technology Center.]

Microsoft's move validates the GPL and the Linux kernel development process and should resolve any remaining questions about that process as a legitimate way to make software or do business, said Jim Zemlin, executive director of the Linux Foundation. If anyone has questions about the company's motivations, he said, it's important to note that the move is good for its business.

"It's good to see Microsoft participating, but they're not saving the whales here," Zemlin said. "They're doing this for business reasons. They want their products to be more competitive and compatible with Linux -- not because they love Linux or open source, but because they need this for their products to succeed in the marketplace."

The process started when Microsoft began offering, on its own, drivers to let companies use Hyper-V on Windows Server to run two widely used Linux distributions, Novell Suse Linux Enterprise Server and Red Hat Enterprise Linux, under pacts with those companies.

Greg Kroah-Hartman, a Linux kernel developer and Novell fellow who oversees the Linux Driver Project, approached Microsoft and worked with the company on its contribution after a Linux community member alerted him to the existence of Microsoft's standalone drivers. It makes more sense to bring the code into the open-source process than to offer it as a separate download, he explained.

"Any Linux user can use this code, because it will be in the main Linux kernel tree," Kroah-Hartman said. "It will be maintained over time by Microsoft, working with the kernel community, so it will let anybody use Linux on top of Hyper-V much, much easier."

Microsoft's longstanding objections to the General Public License are based in part on a provision requiring derivative works, or programs containing GPL code, to be offered under the same free-software license as the original. In a 2001 document, for example, Microsoft warned that "a business that combines and distributes GPL code with its own proprietary code may be obligated to share with the rest of the world valuable intellectual property (including patent) rights in both code bases on a royalty free basis."

Ballmer put it more bluntly in a 2001 interview with the Chicago Sun-Times. "Linux is a cancer that attaches itself in an intellectual property sense to everything it touches," the Microsoft CEO said at the time. "That's the way that the license works."

Open source leaders have long taken issue with Microsoft's harsh characterizations. Asking Microsoft to explain the GPL "is a little like asking Joe Stalin to explain the US Constitution," wrote Eben Moglen, lawyer for the Free Software Foundation, in a 2001 article.

Microsoft remains controversial in the open-source community, thanks in part to actions such as its since-settled lawsuit against Tom-Tom for alleged patent infringement in the navigation company's implementation of the Linux kernel.

Linux is still a competitive threat to the Redmond company's Windows business. But Microsoft has come to acknowledge the key role of Linux in corporate systems, and become more pragmatic in its approach. Microsoft says its business customers have increasingly been looking for ways to run a variety of systems, including Windows and Linux, inside their data centers and other IT systems.

The desire for Windows and Linux interoperability was one of the main reasons given for Microsoft's 2006 collaboration agreement with Novell, which also included a patent provisions that drew fire for appearing to support Microsoft's claims that Linux violates its patents.

At the same time, Microsoft taken a more active role in the open-source community -- working on open-source projects and hiring a number of prominent open-source figures, including Hanrahan, the former Linux Foundation engineering director. The company is announcing its contribution to Linux under the GPL this morning at the O'Reilly OSCON 2009 Open Source Convention.

Although Microsoft has toned down its language, officials say the company's fundamental stance on the GPL hasn't changed. In that way, it's notable that Microsoft isn't planning to incorporate the newly GPL-based driver code directly into Windows or Hyper-V. It's contributing the code to Linux, and continuing to offer the Novell and Red Hat drivers as a standalone download.

That separation appears designed to protect Microsoft from possible claims that the free-software terms of the GPL would then apply to Windows Server and Hyper-V, as well.