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Can FAT patch avoid Microsoft lawsuits?
Jul. 02, 2009

Andrew Tridgell has published a patch that could make the Linux implementation of the FAT filesystem impervious to Microsoft patent claims of the kind that forced a settlement from TomTom. The patch alters the VFAT code so that it does not generate both short and long filenames, says Tridgell.

The Microsoft FAT patent claims lay at the heart of its lawsuit against Dutch personal navigation device (PND) vendor TomTom earlier this year, which resulted in a TomTom settlement in late March. The settlement included an agreement to remove the FAT code from Linux-based products such as the Go 940 Live PND (pictured at right).

The Tridgell patch, which was published on, attempts to configure the Linux VFAT implementation so that it works around Microsoft's patent language. The patent describes how to implement a "common name space for long and short filenames."

The CONFIG_VFAT_FS_DUALNAMES patch works around this characterization, in essence, by using either long or short filenames, but not both at the same time. The patch builds upon initial VFAT patch that Samba developer Tridgell posted in May, which disabled the creation of files with long filenames entirely. As explained in a story by Ryan Paul in ArsTechnica, the Linux community reviewed the patch and responded with a general consensus that it would not be a sufficiently practical. Tridgell quickly revised the patch to make it more flexible by also supporting long file names.

The new patch causes the filesystem to generate only a short name in situations when the total filename fits within the 11-character limit, explains Paul. When the filename is longer, it only generates a long name and "populates the short name value with 11 invalid characters so that it is ignored by the operating system," says the story.

As Tridgell puts it, "we avoid the creation of 8.3 short filenames for files on VFAT filesystems that require a long name." (The 8.3 format refers to the old MS-DOS naming requirements of 8-character filenames followed by three-character extensions, a naming convention that has followed us into the.COM era.)

In a subsequent FAQ issued by Tridgell after posting the code (see link farther below), Tridgell is careful not to make any legal claims about the patch. According to Paul, however, the Linux Foundation's patent lawyers have reviewed the second patch and have concluded that it will "effectively evade the common namespace method described by Microsoft's patents."

Paul also suggests that the patch has a good chance of being accepted in the mainline kernel, although it may not end up being enabled by default. Meanwhile, the patch may not work properly when filesystem data is accessed from older versions of DOS or Windows, says the story, but workarounds are said to be possible.

OIN's Linux Defenders logo

Better to fight than switch?

After the TomTom settlement, the Linux Foundation joined the Software Freedom Law Center (SFLC), the Open Invention Network (OIN), and many others in arguing that the Microsoft patents were invalid. However, LF executive director Jim Zemlin also said his organization would help companies excise the Microsoft FAT filesystem from their systems to avoid the fate of TomTom, and that the industry should look for alternatives to FAT. The Tridgell patch may well save the industry from spending the time and money to develop and implement an alternative, or at least give it some breathing room while it does.

Meanwhile, the Linux community is pushing forward in attempts to have the patents overturned. A month after the TomTom settlement, OIN announced that three of the eight patents cited in Microsoft's lawsuit against TomTom have been posted for prior art review by the Linux community. The evidence is being compiled to convince the U.S. Patent and Trademark Office that the patents are invalid. The three patents cited by Microsoft that cover the FAT filesystem and related technology -- U.S. patents 5579517, 5758352, and 6256642 -- have been posted on the Post-Issue Peer-to-Patent website associated with Linux Defenders.


Tridgell's CONFIG_VFAT_FS_DUALNAMES patch may be found here, and his FAQ explaining the patch should be here.

The ArsTechnica story on Tridgell's patch may be found here.

-- Eric Brown

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