1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
3 .\" %%%LICENSE_START(VERBATIM)
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25 .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
26 .\" Modified 2000-07-01 aeb
27 .\" Modified 2002-07-23 aeb
28 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Added notes on capability requirements
31 .TH SETFSUID 2 2013-08-08 "Linux" "Linux Programmer's Manual"
33 setfsuid \- set user identity used for filesystem checks
35 .B #include <sys/fsuid.h>
37 .BI "int setfsuid(uid_t " fsuid );
41 changes the value of the caller's filesystem user ID\(emthe
42 user ID that the Linux kernel uses to check for all accesses
44 Normally, the value of
45 the filesystem user ID
46 will shadow the value of the effective user ID.
48 effective user ID is changed,
49 the filesystem user ID
50 will also be changed to the new value of the effective user ID.
56 are usually used only by programs such as the Linux NFS server that
57 need to change what user and group ID is used for file access without a
58 corresponding change in the real and effective user and group IDs.
59 A change in the normal user IDs for a program such as the NFS server
60 is a security hole that can expose it to unwanted signals.
64 will succeed only if the caller is the superuser or if
66 matches either the caller's real user ID, effective user ID,
67 saved set-user-ID, or current filesystem user ID.
69 On both success and failure,
70 this call returns the previous filesystem user ID of the caller.
72 This system call is present in Linux since version 1.2.
73 .\" This system call is present since Linux 1.1.44
74 .\" and in libc since libc 4.7.6.
77 is Linux-specific and should not be used in programs intended
80 When glibc determines that the argument is not a valid user ID,
81 it will return \-1 and set \fIerrno\fP to
86 At the time when this system call was introduced, one process
87 could send a signal to another process with the same effective user ID.
88 This meant that if a privilged process changed its effective user ID
89 for the purpose of file permission checking,
90 then it could become vulnerable to receiving signals
91 sent by another (unprivileged) process with the same user ID.
92 The filesystem user ID attribute was thus added to allow a process to
93 change its user ID for the purposes of file permission checking without
94 at the same time becoming vulnerable to receiving unwanted signals.
95 Since Linux 2.0, signal permission handling is different (see
97 with the result that a process change can change its effective user ID
98 without being vulnerable to receiving signals from unwanted processes.
101 is nowadays unneeded and should be avoided in new applications
107 system call supported only 16-bit user IDs.
108 Subsequently, Linux 2.4 added
110 supporting 32-bit IDs.
113 wrapper function transparently deals with the variation across kernel versions.
115 No error indications of any kind are returned to the caller,
116 and the fact that both successful and unsuccessful calls return
117 the same value makes it impossible to directly determine
118 whether the call succeeded or failed.
119 Instead, the caller must resort to looking at the return value
120 from a further call such as
122 (which will always fail), in order to determine if a preceding call to
124 changed the filesystem user ID.
128 should be returned when the call fails (because the caller lacks the
134 .BR capabilities (7),