path: root/Documentation/highuid.txt
diff options
authorLinus Torvalds <>2005-04-16 15:20:36 -0700
committerLinus Torvalds <>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/highuid.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
Diffstat (limited to 'Documentation/highuid.txt')
1 files changed, 79 insertions, 0 deletions
diff --git a/Documentation/highuid.txt b/Documentation/highuid.txt
new file mode 100644
index 000000000000..2c33926b9099
--- /dev/null
+++ b/Documentation/highuid.txt
@@ -0,0 +1,79 @@
+Notes on the change from 16-bit UIDs to 32-bit UIDs:
+- 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
+ structure.
+- kernel code should use uid_t and gid_t in kernel-private structures and
+ code.
+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
+ GID)
+- 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:
+ SunOS emulation
+ Solaris emulation
+ iBCS on Intel
+ sparc32 emulation on sparc64
+ (need to support whatever new 32-bit UID system calls are added to
+ sparc32)
+- 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 can not 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)
+Chris Wing
+last updated: January 11, 2000