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tridge offers a new patch to Linux's VFAT filesystem
Sunday, June 28 2009 @ 11:59 PM EDT

tridge has done it again, offering a patch to Linux's VFAT filesystem that retains support for long names, while carefully avoiding ever having both a long and a short name for the same file. As before, media containing the old long/short-name combination VFAT format are still supported. His comment on LKML:
Date Sat, 27 Jun 2009 05:19:33 +1000
From tridge@samba ...

This is a new patch for VFAT long filename support, replacing the one that I posted last month. It retains a lot more functionality then the previous patch.

A FAQ will be posted immediately after this patch to answer the questions that were raised from the previous discussion.

Cheers, Tridge

And here is the FAQ. Note particularly what can and can't be safely discussed regarding patents on LKML. Here's the prior patch, for completeness.

To make sure we are all up to speed, let's look at the entirety of the FAQ:
Date Sat, 27 Jun 2009 05:22:54 +1000
Subject FAQ on 2nd VFAT patch
From tridge@samba ...

Following the feedback from the first patch, we have put together a short FAQ that tries to address the kinds of questions that were raised during that discussion. The Linux Foundation has organized for a patent attorney, John Lanza, to be available to answer legal questions that come up, to the extent that he is able to. John is CCd on this email.

Cheers, Tridge


Q1. Has this patch undergone legal review?

A1. Both the original patch and the new patch that we posted today have been through legal review by several lawyers who specialize in this area. We can't post all of the details of those reviews, but John Lanza is CCd on this email, and hopefully he will be able to answer any questions that arise or if he can't answer some question he may be able to explain why he can't answer it. John is a patent attorney who represents the Linux Foundation.

Q2. What can we safely post on LKML about patent infringement?

A2. Almost nothing. Even a statement of the form "If A were the case, then B would result in infringement of patent C" is extremely dangerous. It allows a patent attorney to argue that truth or falsity of A is a legal question of fact, even if A is an obvious technical fallacy (remember that the patent holder can call on its own expert witnesses). The existence of a legal question of fact can be used to defeat a pre-trial summary judgment motion for noninfringement and dismissal of the lawsuit. The defeat of pre-trial summary judgment motions results in the suit going to trial, which is always quite expensive. The patent holder can be expected to choose a target that cannot afford a trial, resulting in a settlement or capitulation. The patent holder can then use the fact that target #1 settled to apply more pressure on targets #2, #3, and so on.

This can happen even if you later clearly state that A is an obvious technical fallacy. The patent holder would just say something like, "Your Honor, J. Random Hacker admitted that B is the case, and we will prove that he was mistaken when he later retracted that admission and tried to claim that A was an obvious technical fallacy. We will demonstrate, by Mr. Hacker's own words, that our valuable patent C is in fact infringed." The patent holder might well lose that point later at trial, but only if the target can afford to go to trial in the first place.

In short, anything posted on LKML is on the public record and can therefore be read and archived and used by patent holders to convince a judge to dismiss summary judgment motions in order to threaten a costly trial and try to force a settlement.

Q3. Why should we apply a patch to avoid a patent that might be invalid?

A3. It takes a lot of time in the courts (and thus, a lot of money) to legally prove that a patent is invalid, and in the meantime the patent remains at least a nuisance and in practice still dangerous. Any damage done during this time period, to both persons and organizations, might well be permanent -- it might never be possible to repair that damage, even if the patent should eventually be invalidated. On the other hand, a patch that avoids both the patent and regressions in function and performance can greatly reduce the danger of such damage.

One of the specific dangers that needs to be addressed is illustrated by the ITC action that Microsoft took against TomTom: microsoft-files-new-337-complaint-against-tomtom-regarding- certain-portable-navigation-devices/

That ITC action asked for TomTom's products to be seized at US borders, and might have effectively shutdown TomTom as a going concern in the US. If Microsoft did the same thing to another Linux vendor then even though the vendor might eventually win they could still be severely damaged just by having their market disrupted or frightened.

The way to avoid this is to ensure that the Linux kernel is so obviously non-infringing that the case does not even go to trial. That means you have to have an extremely clear explanation of how the patent does not apply to your code. The aim of the patches we have posted is to ensure that we would meet that standard.

Q4. Suppose we accept such a patch. How do we deal with the possibility that future bug fixes undo the patch?

A4. This is a very real concern. In this case, I hope that the explanation below provides Hirofumi-san the information needed to avoid this possibility. If some future patch looks to him like it might re-introduce the problem, he could check with one of the people who submitted this patch.

This is similar to what a subsystem maintainer does when they check with the owner of some exotic piece of hardware whether a proposed patch would break the kernel on that hardware.

Q5. Why do you believe that this patch avoids the patent?

A5. It is not appropriate for us to provide the full details of the legal reasoning on a public forum, but we can sketch out some facts here which may be helpful. Then if you have any more specific questions you can direct them to John Lanza. You should note that this sketch is not a legal opinion, because legal opinions are given to direct clients, not to a public forum like lkml.

The claims of both of the VFAT patents "create" or "store" a long filename entry. The first patch we posted changed the code so that long filename entries are never created or stored. That patch left unchanged the code for reading long filename entries, and so the patch kept the functionality of reading existing long filenames. The result is a patch that loses some functionality (creating or storing long filenames), but did no creating or storing of long filenames.

The 2nd patch we just posted takes a different approach. The claims of both of the VFAT patents involve the creation (or storing) of both a long filename and a short filename for a file. The 2nd patch only creates/stores either a short filename or a long filename for a file, but never both. The 11 bytes created by vfat_build_dummy_83_buffer() to pad the field for short filenames cannot be used to access the file, and contain bytes which are invalid in FAT and VFAT filenames, and therefore are not filenames as that term is and has been used in the technical community.

Q6. Why the random values for the field in which the short filename would normally be stored?

A6. The patch includes some comments that explain why we chose those particular byte values. Basically we needed to minimise the chance of triggering a bug in WindowsXP where some values would cause XP to crash with a blue screen (Vista and Windows7 do not seem to have this bug). The values were also chosen to have a quite low chance of causing any problems with chkdsk.exe on Windows.

Q7. What if I need to access the files with an old version of DOS or Windows that does not understand long filenames?

A7. In this case, you can use the msdos filesystem to enforce 8.3 format names. We believe that these old versions are sufficiently rare that this is not a real concern.

Q8. The first patch was configured off by default, while this patch is configured on by default. Why the difference in behavior?

A8. The first patch disabled significant function, while this new patch allows all the filenames allowed by the VFAT format. If accepted, it is of course up to the maintainers to decide what the default should be in mainline, or even if the patch should be unconditional. We are happy with any of these three choices (configurable default disabled, configurable default enabled, or unconditionally disabled).

There is an ancient saying that when a wise man sees trouble ahead, he avoids it, but a fool walks right into it. The community sees trouble ahead, so it makes good sense to avoid it.

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