Ubuntu Manpage: ucred, crget, crhold, crfree, crcopy, crdup, cru2x

Provided by: freebsd-manpages_12.2-1_all

NAME

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

SYNOPSIS

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

       struct ucred *
       crget(void);

       struct ucred *
       crhold(struct ucred *cr);

       void
       crfree(struct ucred *cr);

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

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

       struct ucred *
       crdup(struct ucred *cr);

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

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

DESCRIPTION

       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 cr_groups[0].

       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., cr_version).

RETURN VALUES

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

USAGE NOTES

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.

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