UCRED(9)                  BSD Kernel Developer's Manual                 UCRED(9)

     ucred, crget, crhold, crfree, crcopy, crdup, cru2x, cred_update_thread —
     functions related to user credentials

     #include <sys/param.h>
     #include <sys/ucred.h>

     struct ucred *

     struct ucred *
     crhold(struct ucred *cr);

     crfree(struct ucred *cr);

     crcopy(struct ucred *dest, struct ucred *src);

     struct ucred *
     crcopysafe(struct proc *p, struct ucred *cr);

     struct ucred *
     crdup(struct ucred *cr);

     crsetgroups(struct ucred *cr, int ngrp, gid_t *groups);

     cru2x(struct ucred *cr, struct xucred *xcr);

     cred_update_thread(struct thread *td);

     The ucred family of functions is used to manage user credential structures
     (struct ucred) within the kernel.

     The crget() function allocates memory for a new structure, sets its
     reference count to 1, and initializes its lock.

     The crhold() function increases the reference count on the credential.

     The crfree() function decreases the reference count on the credential.  If
     the count drops to 0, the storage for the structure is freed.

     The crcopy() function copies the contents of the source (template)
     credential into the destination template.  The uidinfo structure within the
     destination is referenced by calling uihold(9).

     The crcopysafe() function copies the current credential associated with the
     process p into the newly allocated credential cr.  The process lock on p
     must be held and will be dropped and reacquired as needed to allocate group
     storage space in cr.

     The crdup() function allocates memory for a new structure and copies the
     contents of cr into it.  The actual copying is performed by crcopy().

     The crsetgroups() function sets the cr_groups and cr_ngroups variables and
     allocates space as needed.  It also truncates the group list to the current
     maximum number of groups.  No other mechanism should be used to modify the
     cr_groups array except for updating the primary group via assignment to

     The cru2x() function converts a ucred structure to an xucred structure.
     That is, it copies data from cr to xcr; it ignores fields in the former
     that are not present in the latter (e.g., cr_uidinfo), and appropriately
     sets fields in the latter that are not present in the former (e.g.,

     The cred_update_thread() function sets the credentials of td to that of its
     process, freeing its old credential if required.

     crget(), crhold(), crdup(), and crcopysafe() all return a pointer to a
     ucred structure.

     As of FreeBSD 5.0, the ucred structure contains extensible fields.  This
     means that the correct protocol must always be followed to create a fresh
     and writable credential structure: new credentials must always be derived
     from existing credentials using crget(), crcopy(), and crcopysafe().

     In the common case, credentials required for access control decisions are
     used in a read-only manner.  In these circumstances, the thread credential
     td_ucred should be used, as it requires no locking to access safely, and
     remains stable for the duration of the call even in the face of a multi-
     threaded application changing the process credentials from another thread.

     During a process credential update, the process lock must be held across
     check and update, to prevent race conditions.  The process credential,
     td->td_proc->p_ucred, must be used both for check and update.  If a process
     credential is updated during a system call and checks against the thread
     credential are to be made later during the same system call, the thread
     credential must also be refreshed from the process credential so as to
     prevent use of a stale value.  To avoid this scenario, it is recommended
     that system calls updating the process credential be designed to avoid
     other authorization functions.

     If temporarily elevated privileges are required for a thread, the thread
     credential can by replaced for the duration of an activity, or for the
     remainder of the system call.  However, as a thread credential is often
     shared, appropriate care should be taken to make sure modifications are
     made to a writable credential through the use of crget() and crcopy().

     Caution should be exercised when checking authorization for a thread or
     process perform an operation on another thread or process.  As a result of
     temporary elevation, the target thread credential should never be used as
     the target credential in an access control decision: the process credential
     associated with the thread, td->td_proc->p_ucred, should be used instead.
     For example, p_candebug(9) accepts a target process, not a target thread,
     for access control purposes.


     This manual page was written by Chad David <davidc@acns.ab.ca>.

BSD                            September 27, 2017                            BSD