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 2017-09-15 "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 At the time when this system call was introduced, one process
81 could send a signal to another process with the same effective user ID.
82 This meant that if a privileged process changed its effective user ID
83 for the purpose of file permission checking,
84 then it could become vulnerable to receiving signals
85 sent by another (unprivileged) process with the same user ID.
86 The filesystem user ID attribute was thus added to allow a process to
87 change its user ID for the purposes of file permission checking without
88 at the same time becoming vulnerable to receiving unwanted signals.
89 Since Linux 2.0, signal permission handling is different (see
91 with the result that a process change can change its effective user ID
92 without being vulnerable to receiving signals from unwanted processes.
95 is nowadays unneeded and should be avoided in new applications
101 system call supported only 16-bit user IDs.
102 Subsequently, Linux 2.4 added
104 supporting 32-bit IDs.
107 wrapper function transparently deals with the variation across kernel versions.
108 .SS C library/kernel differences
109 In glibc 2.15 and earlier,
110 when the wrapper for this system call determines that the argument can't be
111 passed to the kernel without integer truncation (because the kernel
112 is old and does not support 32-bit user IDs),
113 they will return \-1 and set \fIerrno\fP to
118 No error indications of any kind are returned to the caller,
119 and the fact that both successful and unsuccessful calls return
120 the same value makes it impossible to directly determine
121 whether the call succeeded or failed.
122 Instead, the caller must resort to looking at the return value
123 from a further call such as
125 (which will always fail), in order to determine if a preceding call to
127 changed the filesystem user ID.
131 should be returned when the call fails (because the caller lacks the
137 .BR capabilities (7),