1 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
2 .\" and Copyright (C) 2013, 2019, Michael Kerrisk <mtk.manpages@gmail.com>
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26 .\" Created 1995-08-06 Thomas K. Dyas <tdyas@eden.rutgers.edu>
27 .\" Modified 2000-07-01 aeb
28 .\" Modified 2002-07-23 aeb
29 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
30 .\" Added notes on capability requirements
32 .TH SETFSUID 2 2021-03-22 "Linux" "Linux Programmer's Manual"
34 setfsuid \- set user identity used for filesystem checks
37 .B #include <sys/fsuid.h>
39 .BI "int setfsuid(uid_t " fsuid );
42 On Linux, a process has both a filesystem user ID and an effective user ID.
43 The (Linux-specific) filesystem user ID is used
44 for permissions checking when accessing filesystem objects,
45 while the effective user ID is used for various other kinds
46 of permissions checks (see
49 Normally, the value of the process's filesystem user ID
50 is the same as the value of its effective user ID.
51 This is so, because whenever a process's effective user ID is changed,
52 the kernel also changes the filesystem user ID to be the same as
53 the new value of the effective user ID.
54 A process can cause the value of its filesystem user ID to diverge
55 from its effective user ID by using
57 to change its filesystem user ID to the value given in
64 are (were) usually used only by programs such as the Linux NFS server that
65 need to change what user and group ID is used for file access without a
66 corresponding change in the real and effective user and group IDs.
67 A change in the normal user IDs for a program such as the NFS server
68 is (was) a security hole that can expose it to unwanted signals.
69 (However, this issue is historical; see below.)
72 will succeed only if the caller is the superuser or if
74 matches either the caller's real user ID, effective user ID,
75 saved set-user-ID, or current filesystem user ID.
77 On both success and failure,
78 this call returns the previous filesystem user ID of the caller.
80 This system call is present in Linux since version 1.2.
81 .\" This system call is present since Linux 1.1.44
82 .\" and in libc since libc 4.7.6.
85 is Linux-specific and should not be used in programs intended
88 At the time when this system call was introduced, one process
89 could send a signal to another process with the same effective user ID.
90 This meant that if a privileged process changed its effective user ID
91 for the purpose of file permission checking,
92 then it could become vulnerable to receiving signals
93 sent by another (unprivileged) process with the same user ID.
94 The filesystem user ID attribute was thus added to allow a process to
95 change its user ID for the purposes of file permission checking without
96 at the same time becoming vulnerable to receiving unwanted signals.
97 Since Linux 2.0, signal permission handling is different (see
99 with the result that a process can change its effective user ID
100 without being vulnerable to receiving signals from unwanted processes.
103 is nowadays unneeded and should be avoided in new applications
109 system call supported only 16-bit user IDs.
110 Subsequently, Linux 2.4 added
112 supporting 32-bit IDs.
115 wrapper function transparently deals with the variation across kernel versions.
116 .SS C library/kernel differences
117 In glibc 2.15 and earlier,
118 when the wrapper for this system call determines that the argument can't be
119 passed to the kernel without integer truncation (because the kernel
120 is old and does not support 32-bit user IDs),
121 it will return \-1 and set \fIerrno\fP to
126 No error indications of any kind are returned to the caller,
127 and the fact that both successful and unsuccessful calls return
128 the same value makes it impossible to directly determine
129 whether the call succeeded or failed.
130 Instead, the caller must resort to looking at the return value
131 from a further call such as
133 (which will always fail), in order to determine if a preceding call to
135 changed the filesystem user ID.
139 should be returned when the call fails (because the caller lacks the
145 .BR capabilities (7),