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Documentation / highuid.txt

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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 EST.

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