Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 Notes 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 10 What'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 74 Chris Wing 75 firstname.lastname@example.org 76 77 last updated: January 11, 2000