Root/Documentation/highuid.txt

1Notes on the change from 16-bit UIDs to 32-bit UIDs:
2
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.
6
7- kernel code should use uid_t and gid_t in kernel-private structures and
8  code.
9
10What's left to be done for 32-bit UIDs on all Linux architectures:
11
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.
19
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)
25
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.
29
30  This affects at least:
31    iBCS on Intel
32
33    sparc32 emulation on sparc64
34    (need to support whatever new 32-bit UID system calls are added to
35    sparc32)
36
37- Validate that all filesystems behave properly.
38
39  At present, 32-bit UIDs _should_ work for:
40    ext2
41    ufs
42    isofs
43    nfs
44    coda
45    udf
46
47  Ioctl() fixups have been made for:
48    ncpfs
49    smbfs
50
51  Filesystems with simple fixups to prevent 16-bit UID wraparound:
52    minix
53    sysv
54    qnx4
55
56  Other filesystems have not been checked yet.
57
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)
61
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.
65
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.
68
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)
72
73
74Chris Wing
75wingc@umich.edu
76
77last updated: January 11, 2000
78

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