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setfsuid - set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsuid(uid_t fsuid);
The system call setfsuid() sets the user ID that the Linux kernel uses
to check for all accesses to the file system. Normally, the value of
fsuid will shadow the value of the effective user ID. In fact,
whenever the effective user ID is changed, fsuid will also be changed
to the new value of the effective user ID.
Explicit calls to setfsuid() and setfsgid(2) are usually only used by
programs such as the Linux NFS server that need to change what user and
group ID is used for file access without a corresponding change in the
real and effective user and group IDs. A change in the normal user IDs
for a program such as the NFS server is a security hole that can expose
it to unwanted signals. (But see below.)
setfsuid() will only succeed if the caller is the superuser or if fsuid
matches either the real user ID, effective user ID, saved set-user-ID,
or the current value of fsuid.
On success, the previous value of fsuid is returned. On error, the
current value of fsuid is returned.
This system call is present in Linux since version 1.2.
setfsuid() is Linux-specific and should not be used in programs
intended to be portable.
When glibc determines that the argument is not a valid user ID, it will
return -1 and set errno to EINVAL without attempting the system call.
Note that at the time this system call was introduced, a process could
send a signal to a process with the same effective user ID. Today
signal permission handling is slightly different.
The original Linux setfsuid() system call supported only 16-bit user
IDs. Subsequently, Linux 2.4 added setfsuid32() supporting 32-bit IDs.
The glibc setfsuid() wrapper function transparently deals with the
variation across kernel versions.
No error messages of any kind are returned to the caller. At the very
least, EPERM should be returned when the call fails (because the caller
lacks the CAP_SETUID capability).
kill(2), setfsgid(2), capabilities(7), credentials(7)
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description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.