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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	===================================================
2	Notes on the change from 16-bit UIDs to 32-bit UIDs
3	===================================================
5	:Author: Chris Wing <wingc@umich.edu>
6	:Last updated: January 11, 2000
8	- kernel code MUST take into account __kernel_uid_t and __kernel_uid32_t
9	  when communicating between user and kernel space in an ioctl or data
10	  structure.
12	- kernel code should use uid_t and gid_t in kernel-private structures and
13	  code.
15	What's left to be done for 32-bit UIDs on all Linux architectures:
17	- Disk quotas have an interesting limitation that is not related to the
18	  maximum UID/GID. They are limited by the maximum file size on the
19	  underlying filesystem, because quota records are written at offsets
20	  corresponding to the UID in question.
21	  Further investigation is needed to see if the quota system can cope
22	  properly with huge UIDs. If it can deal with 64-bit file offsets on all 
23	  architectures, this should not be a problem.
25	- Decide whether or not to keep backwards compatibility with the system
26	  accounting file, or if we should break it as the comments suggest
27	  (currently, the old 16-bit UID and GID are still written to disk, and
28	  part of the former pad space is used to store separate 32-bit UID and
29	  GID)
31	- Need to validate that OS emulation calls the 16-bit UID
32	  compatibility syscalls, if the OS being emulated used 16-bit UIDs, or
33	  uses the 32-bit UID system calls properly otherwise.
35	  This affects at least:
37		- iBCS on Intel
39		- sparc32 emulation on sparc64
40		  (need to support whatever new 32-bit UID system calls are added to
41		  sparc32)
43	- Validate that all filesystems behave properly.
45	  At present, 32-bit UIDs _should_ work for:
47		- ext2
48		- ufs
49		- isofs
50		- nfs
51		- coda
52		- udf
54	  Ioctl() fixups have been made for:
56		- ncpfs
57		- smbfs
59	  Filesystems with simple fixups to prevent 16-bit UID wraparound:
61		- minix
62		- sysv
63		- qnx4
65	  Other filesystems have not been checked yet.
67	- The ncpfs and smpfs filesystems cannot presently use 32-bit UIDs in
68	  all ioctl()s. Some new ioctl()s have been added with 32-bit UIDs, but
69	  more are needed. (as well as new user<->kernel data structures)
71	- The ELF core dump format only supports 16-bit UIDs on arm, i386, m68k,
72	  sh, and sparc32. Fixing this is probably not that important, but would
73	  require adding a new ELF section.
75	- The ioctl()s used to control the in-kernel NFS server only support
76	  16-bit UIDs on arm, i386, m68k, sh, and sparc32.
78	- make sure that the UID mapping feature of AX25 networking works properly
79	  (it should be safe because it's always used a 32-bit integer to
80	  communicate between user and kernel)
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