Improve mapping of attributes to UNIX permissions

[piranna]

ATTR_SYSTEM attribute could be used as a lightweight user access control, only allowing to PID 0 (root) access files and folders that has this attribute enabled. This could be enabled by a mount flag, and would be as simple as unsetting the group and other permissions when enabled.

[piranna]

I have been thinking, the ATTR_HIDDEN would also be used to disable the UNIX read permission, similar to how the ATTR_READONLY flag is used for the write permission. This way, the ATTR_SYSTEM would only change the file ownership to root when calling the getattr() function (stat), leaving the other ones to the pid of the user that mounted the filesystem.

[dorimanx]

If you think its can bring improvement for all users, send pull request with all defined and tested.

[piranna]

According to https://en.wikipedia.org/wiki/Design_of_the_FAT_file_system#Directory_entry, seems several OSes (DR-DOS, OpenDOS, Multiuser-DOS and other similar ones) make use of reserved bytes 0x12-0x15 to store both user IDs and UNIX-like file permissions as a extension to FAT filesystem. I've not been able to find a reference to actual usage of this reserved bytes on Microsoft systems, except that this reserved bytes are also being used by VFAT long names support, but this entries would be easily detected by checking the file attributes, so I think it would be feasable to add support for real POSIX permissions here. What do you think?

[piranna]

Main problem here with UIDs and GIDs is that they are stored on a single byte, while standard UNIXes use two bytes, so it would need to give a base offset as parameter (Debian/Ubuntu regular users UIDs and GIDs start at 1000 for default user), or a mapping file between both systems.

[piranna]

ExFAT don't provide space in the same place to implement the DR-DOS POSIX-like permissions, but has a lot of reserved bytes at the end of its structure that could be used for this task, allowing real UIDs, GIDs and permissions. On the other hand, the ATTR_DEVICE bit could be used as a replacement for execution bit in a simple way doing the same thing as ATTR_VOLUME is being used for symlinks, and being this change compatible with all FAT versions.