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+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:
+        SunOS emulation
+        Solaris emulation
+        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 can not 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