1SETFSUID(2)                Linux Programmer's Manual               SETFSUID(2)
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NAME

6       setfsuid - set user identity used for filesystem checks
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SYNOPSIS

9       #include <sys/fsuid.h>
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11       int setfsuid(uid_t fsuid);
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DESCRIPTION

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)).
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20       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.
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28       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.)
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36       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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RETURN VALUE

41       On  both success and failure, this call returns the previous filesystem
42       user ID of the caller.
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VERSIONS

45       This system call is present in Linux since version 1.2.
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CONFORMING TO

48       setfsuid() is Linux-specific and should not be  used  in  programs  in‐
49       tended to be portable.
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NOTES

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 ef‐
62       fective  user ID without being vulnerable to receiving signals from un‐
63       wanted processes.  Thus, setfsuid() is nowadays unneeded and should  be
64       avoided in new applications (likewise for setfsgid(2)).
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66       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.
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71   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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BUGS

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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SEE ALSO

89       kill(2), setfsgid(2), capabilities(7), credentials(7)
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COLOPHON

92       This  page  is  part of release 5.12 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/.
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99Linux                             2021-03-22                       SETFSUID(2)
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