setfsuid(2)

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

6       setfsuid - set user identity used for file system checks
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SYNOPSIS

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

14       The  system call setfsuid() sets the user ID that the Linux kernel uses
15       to check for all accesses to the file system. Normally,  the  value  of
16       fsuid will shadow the value of the effective user ID. In fact, whenever
17       the effective user ID is changed, fsuid will also be changed to the new
18       value of the effective user ID.
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20       Explicit  calls  to  setfsuid() and setfsgid() are usually only used by
21       programs such as the Linux NFS server that need to change what user and
22       group  ID is used for file access without a corresponding change in the
23       real and effective user and group IDs.  A change in the normal user IDs
24       for a program such as the NFS server is a security hole that can expose
25       it to unwanted signals. (But see below.)
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27       setfsuid() will only succeed if the caller is the superuser or if fsuid
28       matches  either the real user ID, effective user ID, saved set-user-ID,
29       or the current value of fsuid.
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RETURN VALUE

32       On success, the previous value of fsuid is  returned.   On  error,  the
33       current value of fsuid is returned.
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CONFORMING TO

36       setfsuid()  is  Linux  specific  and  should  not  be  used in programs
37       intended to be portable.  It is present since Linux 1.1.44 and in  libc
38       since libc 4.7.6.
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BUGS

41       No  error  messages of any kind are returned to the caller. At the very
42       least, EPERM should be returned when the call fails (because the caller
43       lacks the CAP_SETUID capability).
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NOTES

46       When glibc determines that the argument is not a valid user ID, it will
47       return -1 and set errno to EINVAL without attempting the system call.
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49       Note that at the time this system call was introduced, a process  could
50       send a signal to a process with the same effective user ID.  Today sig‐
51       nal permission handling is slightly different.
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