Notes on the change from 16-bit UIDs to 32-bit UIDs:
  
  - kernel code MUST take into account __kernel_uid_t and __kernel_uid32_t
    when communicating between user and kernel space in an ioctl or data
    structure.
  
  - kernel code should use uid_t and gid_t in kernel-private structures and
    code.
  
  What's left to be done for 32-bit UIDs on all Linux architectures:
  
  - Disk quotas have an interesting limitation that is not related to the
    maximum UID/GID. They are limited by the maximum file size on the
    underlying filesystem, because quota records are written at offsets
    corresponding to the UID in question.
    Further investigation is needed to see if the quota system can cope
    properly with huge UIDs. If it can deal with 64-bit file offsets on all 
    architectures, this should not be a problem.
  
  - Decide whether or not to keep backwards compatibility with the system
    accounting file, or if we should break it as the comments suggest
    (currently, the old 16-bit UID and GID are still written to disk, and
    part of the former pad space is used to store separate 32-bit UID and
    GID)
  
  - Need to validate that OS emulation calls the 16-bit UID
    compatibility syscalls, if the OS being emulated used 16-bit UIDs, or
    uses the 32-bit UID system calls properly otherwise.
  
    This affects at least:

  	iBCS on Intel
  
  	sparc32 emulation on sparc64
  	(need to support whatever new 32-bit UID system calls are added to
  	sparc32)
  
  - Validate that all filesystems behave properly.
  
    At present, 32-bit UIDs _should_ work for:
  	ext2
  	ufs
  	isofs
  	nfs
  	coda
  	udf
  
    Ioctl() fixups have been made for:
  	ncpfs
  	smbfs
  
    Filesystems with simple fixups to prevent 16-bit UID wraparound:
  	minix
  	sysv
  	qnx4
  
    Other filesystems have not been checked yet.

  - The ncpfs and smpfs filesystems cannot presently use 32-bit UIDs in

    all ioctl()s. Some new ioctl()s have been added with 32-bit UIDs, but
    more are needed. (as well as new user<->kernel data structures)
  
  - The ELF core dump format only supports 16-bit UIDs on arm, i386, m68k,
    sh, and sparc32. Fixing this is probably not that important, but would
    require adding a new ELF section.
  
  - The ioctl()s used to control the in-kernel NFS server only support
    16-bit UIDs on arm, i386, m68k, sh, and sparc32.
  
  - make sure that the UID mapping feature of AX25 networking works properly
    (it should be safe because it's always used a 32-bit integer to
    communicate between user and kernel)
  
  
  Chris Wing
  wingc@umich.edu
  
  last updated: January 11, 2000