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SETRESUID(2)                                                            Linux Programmer's Manual                                                           SETRESUID(2)

NAME
       setresuid, setresgid - set real, effective, and saved user or group ID

SYNOPSIS
       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <unistd.h>

       int setresuid(uid_t ruid, uid_t euid, uid_t suid);
       int setresgid(gid_t rgid, gid_t egid, gid_t sgid);

DESCRIPTION
       setresuid() sets the real user ID, the effective user ID, and the saved set-user-ID of the calling process.

       An  unprivileged  process  may change its real UID, effective UID, and saved set-user-ID, each to one of: the current real UID, the current effective UID, or the
       current saved set-user-ID.

       A privileged process (on Linux, one having the CAP_SETUID capability) may set its real UID, effective UID, and saved set-user-ID to arbitrary values.

       If one of the arguments equals -1, the corresponding value is not changed.

       Regardless of what changes are made to the real UID, effective UID, and saved set-user-ID, the filesystem UID is always set to the same value  as  the  (possibly
       new) effective UID.

       Completely analogously, setresgid() sets the real GID, effective GID, and saved set-group-ID of the calling process (and always modifies the filesystem GID to be
       the same as the effective GID), with the same restrictions for unprivileged processes.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set to indicate the error.

       Note: there are cases where setresuid() can fail even when the caller is UID 0; it is a grave security error to omit checking for a failure  return  from  setre‐
       suid().

ERRORS
       EAGAIN The  call would change the caller's real UID (i.e., ruid does not match the caller's real UID), but there was a temporary failure allocating the necessary
              kernel data structures.

       EAGAIN ruid does not match the caller's real UID and this call would bring the number of processes  belonging  to  the  real  user  ID  ruid  over  the  caller's
              RLIMIT_NPROC resource limit.  Since Linux 3.1, this error case no longer occurs (but robust applications should check for this error); see the description
              of EAGAIN in execve(2).

       EINVAL One or more of the target user or group IDs is not valid in this user namespace.

       EPERM  The calling process is not privileged (did not have the necessary capability in its user namespace) and tried to change the IDs to  values  that  are  not
              permitted.  For setresuid(), the necessary capability is CAP_SETUID; for setresgid(), it is CAP_SETGID.

VERSIONS
       These calls are available under Linux since Linux 2.1.44.

CONFORMING TO
       These calls are nonstandard; they also appear on HP-UX and some of the BSDs.

NOTES
       Under HP-UX and FreeBSD, the prototype is found in <unistd.h>.  Under Linux, the prototype is provided by glibc since version 2.3.2.

       The  original  Linux  setresuid() and setresgid() system calls supported only 16-bit user and group IDs.  Subsequently, Linux 2.4 added setresuid32() and setres‐
       gid32(), supporting 32-bit IDs.  The glibc setresuid() and setresgid() wrapper functions transparently deal with the variations across kernel versions.

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread attribute.  However, POSIX requires that all threads in a process share the same  credentials.   The
       NPTL  threading  implementation  handles  the  POSIX  requirements by providing wrapper functions for the various system calls that change process UIDs and GIDs.
       These wrapper functions (including those for setresuid() and setresgid()) employ a signal-based technique to ensure that when one thread changes credentials, all
       of the other threads in the process also change their credentials.  For details, see nptl(7).

SEE ALSO
       getresuid(2), getuid(2), setfsgid(2), setfsuid(2), setreuid(2), setuid(2), capabilities(7), credentials(7), user_namespaces(7)

Linux                                                                          2021-03-22                                                                   SETRESUID(2)