установить личность пользователя (set user identity)
Имя (Name)
setuid - set user identity
Синопсис (Synopsis)
#include <unistd.h>
int setuid(uid_t uid);
Описание (Description)
setuid() sets the effective user ID of the calling process. If
the calling process is privileged (more precisely: if the process
has the CAP_SETUID capability in its user namespace), the real
UID and saved set-user-ID are also set.
Under Linux, setuid() is implemented like the POSIX version with
the _POSIX_SAVED_IDS feature. This allows a set-user-ID (other
than root) program to drop all of its user privileges, do some
un-privileged work, and then reengage the original effective user
ID in a secure manner.
If the user is root or the program is set-user-ID-root, special
care must be taken: setuid() checks the effective user ID of the
caller and if it is the superuser, all process-related user ID's
are set to uid. After this has occurred, it is impossible for
the program to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root
privileges, assume the identity of an unprivileged user, and then
regain root privileges afterward cannot use setuid(). You can
accomplish this with seteuid(2).
Возвращаемое значение (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 setuid() can fail even when the
caller is UID 0; it is a grave security error to omit checking
for a failure return from setuid().
Ошибки (Error)
EAGAIN The call would change the caller's real UID (i.e., uid
does not match the caller's real UID), but there was a
temporary failure allocating the necessary kernel data
structures.
EAGAIN uid does not match the real user ID of the caller and this
call would bring the number of processes belonging to the
real user ID uid 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 The user ID specified in uid is not valid in this user
namespace.
EPERM The user is not privileged (Linux: does not have the
CAP_SETUID capability in its user namespace) and uid does
not match the real UID or saved set-user-ID of the calling
process.
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008, SVr4. Not quite compatible with the
4.4BSD call, which sets all of the real, saved, and effective
user IDs.
Примечание (Note)
Linux has the concept of the filesystem user ID, normally equal
to the effective user ID. The setuid() call also sets the
filesystem user ID of the calling process. See setfsuid(2).
If uid is different from the old effective UID, the process will
be forbidden from leaving core dumps.
The original Linux setuid() system call supported only 16-bit
user IDs. Subsequently, Linux 2.4 added setuid32() supporting
32-bit IDs. The glibc setuid() wrapper function transparently
deals with the variation 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 the one for setuid()) 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).
Смотри также (See also)
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),
credentials(7), user_namespaces(7)
