Re: [PATCH] Added CONFIG_VFAT_FS_DUALNAMES option

[Eric W. Biederman]

tridge@xxxxxxxxx writes:

> Hi Eric,
>
>  > This hunk allowing the examination of the long name if there is not a
>  > short name is something else entirely.
>  > 
>  > You don't describe it in your summary, and don't make the case that
>  > this is the correct behavior.
>
> Sorry for not explaining that part of the patch in my posting.
>
> The reason I initially included it was that during the development of
> this patch we were experimenting with variations of the
> vfat_build_dummy_83_buffer() function which put a wide varity of
> different values in the 11 bytes where a short filename would normally
> go.
>
> The most technically attractive varient was to put 11 spaces in
> there. That has the advantage that Windows then returns the long name
> to the GetShortPathName() API call, which is also what Windows does on
> a NTFS filesystem when the NTFS filesystem is configured not to have
> 8.3 names. So it would have been very nice to have this same behaviour
> on VFAT, instead of getting a single space back from
> GetShortPathName() as we get with the patch I posted.
>
> The reason we didn't go with the 11 spaces varient is that it causes
> WindowsXP to bluescreen. This is just a bug in WinXP, and is not
> present in Vista or Windows7.
>
> The 2nd technical problem with 11 spaces is that current Linux kernels
> would not see the long names, as the last_u logic in fat_search_long()
> will skip files where the the 11 bytes don't have at least one
> non-space byte before the first nul byte in either the 8 byte prefix
> or the 3 byte extension.
>
> The last_u logic is at odds with the behaviour of other operating
> systems for VFAT. On all windows varients that I have tested (and on
> MacOSX), the long name is still used no matter what values are in the
> 11 bytes. You can stick all nul bytes, all spaces, or anything else
> you like in there and Windows will still allow you to operate on the
> file by the long name and the filesystem will work perfectly.
>
> So that part of the patch is bringing us inline with the behaviour of
> other VFAT implementations.
>
> More importantly, it gives us the opportunity in the future to move to
> the techically better 11 spaces solution when either Microsoft fixes
> their WindowsXP crash bug, or WindowsXP has such a low market share
> that nobody cares any more (say like WfWg and maybe Win95 has now).
>
> If we don't put in this change now then we won't be able to move to
> the technically better solution later as we wouldn't have given Linux
> distros a chance to include support for the 11 spaces in distros. By
> putting that change in now, when (if?) we change to 11 spaces later we
> will not be breaking recent Linux kernels.
>
>  > This should probably be split out into a separate patch if it is the
>  > right thing to do, or drop it if (as it looks) there is no point in
>  > that change.
>
> hopefully the above explanation makes it clear why the last_u change
> should go together with the vfat_build_slots() change.

It is part of the same investigation certainly.

Given what you have said our interpretation of vfat has a bug,
and that small change is a candidate for -stable.  If it could
be it's own patch.

That change seems much less controversial and obviously correct
than the other.

Eric
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