man2/setfsuid.2

   1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
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  23 .\" Created   1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
  24 .\" Modified  2000-07-01 aeb
  25 .\" Modified  2002-07-23 aeb
  26 .\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  27 .\"     Added notes on capability requirements
  28 .\"
  29 .TH SETFSUID 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
  30 .SH NAME
  31 setfsuid \- set user identity used for file system checks
  32 .SH SYNOPSIS
  33 .B #include <unistd.h>
  34 /* glibc uses <sys/fsuid.h> */
  35 .sp
  36 .BI "int setfsuid(uid_t " fsuid );
  37 .SH DESCRIPTION
  38 The system call
  39 .BR setfsuid ()
  40 sets the user ID that the Linux kernel uses to check for all accesses
  41 to the file system.
  42 Normally, the value of
  43 .I fsuid
  44 will shadow the value of the effective user ID.
  45 In fact, whenever the
  46 effective user ID is changed,
  47 .I fsuid
  48 will also be changed to the new value of the effective user ID.
  49
  50 Explicit calls to
  51 .BR setfsuid ()
  52 and
  53 .BR setfsgid ()
  54 are usually only used by programs such as the Linux NFS server that
  55 need to change what user and group ID is used for file access without a
  56 corresponding change in the real and effective user and group IDs.
  57 A change in the normal user IDs for a program such as the NFS server
  58 is a security hole that can expose it to unwanted signals. (But see below.)
  59
  60 .BR setfsuid ()
  61 will only succeed if the caller is the superuser or if
  62 .I fsuid
  63 matches either the real user ID, effective user ID, saved set-user-ID, or
  64 the current value of
  65 .IR fsuid .
  66 .SH "RETURN VALUE"
  67 On success, the previous value of
  68 .I fsuid
  69 is returned.
  70 On error, the current value of
  71 .I fsuid
  72 is returned.
  73 .SH "CONFORMING TO"
  74 .BR setfsuid ()
  75 is Linux specific and should not be used in programs intended
  76 to be portable.
  77 It is present since Linux 1.1.44 and in libc since libc 4.7.6.
  78 .SH BUGS
  79 No error messages of any kind are returned to the caller.
  80 At the very
  81 least,
  82 .B EPERM
  83 should be returned when the call fails (because the caller lacks the
  84 .B CAP_SETUID
  85 capability).
  86 .SH NOTES
  87 When glibc determines that the argument is not a valid user ID,
  88 it will return \-1 and set \fIerrno\fP to EINVAL without attempting
  89 the system call.
  90 .LP
  91 Note that at the time this system call was introduced, a process
  92 could send a signal to a process with the same effective user ID.
  93 Today signal permission handling is slightly different.
  94 .SH "SEE ALSO"
  95 .BR kill (2),
  96 .BR setfsgid (2),
  97 .BR capabilities (7)