ucred(9)

NAME

ucred, crget, crhold, crfree, crshared, crcopy, crdup,

cru2x,
cred_update_thread - 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);
int
crshared(struct ucred *cr);
void
crcopy(struct ucred *dest, struct ucred *src);
struct ucred *
crdup(struct ucred *cr);
void
cru2x(struct ucred *cr, struct xucred *xcr);
void
cred_update_thread(struct thread *td);

DESCRIPTION

The ucred family of functions is used to manage user creden

tial 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 crshared() function returns true if the credential is

shared. A credential is considered to be shared if its reference count is

greater than
one.

The crcopy() function copies the contents of the source

(template) credential into the destination template. The uidinfo struc

ture within the
destination is referenced by calling uihold(9).

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 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).

The cred_update_thread() function sets the credentials of td

to that of
its process, freeing its old credential if required.

RETURN VALUES

crget(), crhold() and crdup() all return a pointer to a

ucred structure.

crshared() returns 0 if the credential has a reference count

greater than
1; otherwise, 1 is returned.

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 al

ways be derived
from existing credentials using crget() and crcopy().

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. Primitives such as suser(9) will assume the use of

td_ucred
unless explicitly specified using suser_cred(9).

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 up

date. If a process credential is updated during a system call and checks

against the
thread credential are to be made later during the same sys

tem 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 creden

tial is often
shared, appropriate care should be taken to make sure modi

fications 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 pro

cess, not a target thread, for access control purposes.

SEE ALSO

uihold(9)

AUTHORS

This manual page was written by Chad David <davidc@ac

ns.ab.ca>.

BSD March 3, 2002

BSD