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suser, suser_cred - check if credentials have superuser privilege
suser(struct thread *td);
suser_cred(struct ucred *cred, int flag);
The suser() and suser_cred() functions check if the credentials given
include superuser powers.
The suser() function is the most common, and should be used unless
special circumstances dictate otherwise.
The suser_cred() function should be used when the credentials to be
checked are not the thread’s own, when there is no thread, when superuser
powers should be extended to imprisoned roots, or when the credential to
be checked is the real user rather than the effective user.
By default, a process does not command superuser powers if it has been
imprisoned by the jail(2) system call. There are cases however where
this is appropriate, and this can be done by passing SUSER_ALLOWJAIL in
the flag argument to the suser_cred() function. It is important to
review carefully in each case that this does not weaken the prison.
Generally, only where the action is protected by chroot(2) implicit in
the jail(2) call should such powers be granted.
By default, the credential checked is the effective user. There are
cases where it is instead necessary to check the real user (for example,
when determining if resource limits should be applied), and this can be
done by passing the SUSER_RUID flag in the flag argument to the
The suser() and suser_cred() functions note the fact that superuser
powers have been used in the process structure of the process specified.
Because part of their function is to notice whether superuser powers have
been used, the functions should only be called after other permission
possibilities have been exhausted.
The suser() and suser_cred() functions return 0 if the user has superuser
powers and EPERM otherwise. This is the reverse logic of some other
implementations of suser() in which a TRUE response indicates superuser
The suser() and suser_cred() functions do not, in fact, record that
superuser privileges have been used, and have not done so since August