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Notes on the change from 16-bit UIDs to 32-bit UIDs
:Author: Chris Wing <>
:Last updated: January 11, 2000
- 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
- kernel code should use uid_t and gid_t in kernel-private structures and
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
- 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
- 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)