1SETFSUID(2) Linux Programmer's Manual SETFSUID(2)
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6 setfsuid - set user identity used for filesystem checks
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9 #include <sys/fsuid.h>
10 int setfsuid(uid_t fsuid);
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14 On Linux, a process has both a filesystem user ID and an effective user
15 ID. The (Linux-specific) filesystem user ID is used for permissions
16 checking when accessing filesystem objects, while the effective user ID
17 is used for various other kinds of permissions checks (see creden‐
18 tials(7)).
19 Normally, the value of the process's filesystem user ID is the same as
21 the value of its effective user ID. This is so, because whenever a
22 process's effective user ID is changed, the kernel also changes the
23 filesystem user ID to be the same as the new value of the effective
24 user ID. A process can cause the value of its filesystem user ID to
25 diverge from its effective user ID by using setfsuid() to change its
26 filesystem user ID to the value given in fsuid.
27 Explicit calls to setfsuid() and setfsgid(2) are (were) usually used
29 only by programs such as the Linux NFS server that need to change what
30 user and group ID is used for file access without a corresponding
31 change in the real and effective user and group IDs. A change in the
32 normal user IDs for a program such as the NFS server is (was) a secu‐
33 rity hole that can expose it to unwanted signals. (However, this issue
34 is historical; see below.)
35 setfsuid() will succeed only if the caller is the superuser or if fsuid
37 matches either the caller's real user ID, effective user ID, saved set-
38 user-ID, or current filesystem user ID.
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41 On both success and failure, this call returns the previous filesystem
42 user ID of the caller.
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45 This system call is present in Linux since version 1.2.
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48 setfsuid() is Linux-specific and should not be used in programs
49 intended to be portable.
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52 At the time when this system call was introduced, one process could
53 send a signal to another process with the same effective user ID. This
54 meant that if a privileged process changed its effective user ID for
55 the purpose of file permission checking, then it could become vulnera‐
56 ble to receiving signals sent by another (unprivileged) process with
57 the same user ID. The filesystem user ID attribute was thus added to
58 allow a process to change its user ID for the purposes of file permis‐
59 sion checking without at the same time becoming vulnerable to receiving
60 unwanted signals. Since Linux 2.0, signal permission handling is dif‐
61 ferent (see kill(2)), with the result that a process can change its
62 effective user ID without being vulnerable to receiving signals from
63 unwanted processes. Thus, setfsuid() is nowadays unneeded and should
64 be avoided in new applications (likewise for setfsgid(2)).
65 The original Linux setfsuid() system call supported only 16-bit user
67 IDs. Subsequently, Linux 2.4 added setfsuid32() supporting 32-bit IDs.
68 The glibc setfsuid() wrapper function transparently deals with the
69 variation across kernel versions.
70 C library/kernel differences
72 In glibc 2.15 and earlier, when the wrapper for this system call deter‐
73 mines that the argument can't be passed to the kernel without integer
74 truncation (because the kernel is old and does not support 32-bit user
75 IDs), it will return -1 and set errno to EINVAL without attempting the
76 system call.
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79 No error indications of any kind are returned to the caller, and the
80 fact that both successful and unsuccessful calls return the same value
81 makes it impossible to directly determine whether the call succeeded or
82 failed. Instead, the caller must resort to looking at the return value
83 from a further call such as setfsuid(-1) (which will always fail), in
84 order to determine if a preceding call to setfsuid() changed the
85 filesystem user ID. At the very least, EPERM should be returned when
86 the call fails (because the caller lacks the CAP_SETUID capability).
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89 kill(2), setfsgid(2), capabilities(7), credentials(7)
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92 This page is part of release 5.02 of the Linux man-pages project. A
93 description of the project, information about reporting bugs, and the
94 latest version of this page, can be found at
95 https://www.kernel.org/doc/man-pages/.
96Linux 2019-05-09 SETFSUID(2)