man/man9/ucred.9

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.Dd January 23, 2019
.Dt UCRED 9
.Os
.Sh NAME
.Nm ucred ,
.Nm crget ,
.Nm crhold ,
.Nm crfree ,
.Nm crcopy ,
.Nm crdup ,
.Nm cru2x
.Nd "functions related to user credentials"
.Sh SYNOPSIS
.In sys/param.h
.In sys/ucred.h
.Ft "struct ucred *"
.Fn crget void
.Ft "struct ucred *"
.Fn crhold "struct ucred *cr"
.Ft void
.Fn crfree "struct ucred *cr"
.Ft void
.Fn crcopy "struct ucred *dest" "struct ucred *src"
.Ft "struct ucred *"
.Fn crcopysafe "struct proc *p" "struct ucred *cr"
.Ft "struct ucred *"
.Fn crdup "struct ucred *cr"
.Ft void
.Fn crsetgroups "struct ucred *cr" "int ngrp" "gid_t *groups"
.Ft void
.Fn cru2x "struct ucred *cr" "struct xucred *xcr"
.Sh DESCRIPTION
The
.Nm
family of functions is used to manage user credential structures
.Pq Vt "struct ucred"
within the kernel.
.Pp
The
.Fn crget
function allocates memory
for a new structure, sets its reference count to 1, and
initializes its lock.
.Pp
The
.Fn crhold
function increases the reference count on the credential.
.Pp
The
.Fn crfree
function decreases the reference count on the credential.
If the count drops to 0, the storage for the structure is freed.
.Pp
The
.Fn crcopy
function copies the contents of the source (template)
credential into the destination template.
The
.Vt uidinfo
structure within the destination is referenced
by calling
.Xr uihold 9 .
.Pp
The
.Fn crcopysafe
function copies the current credential associated with the process
.Fa p
into the newly allocated credential
.Fa cr .
The process lock on
.Fa p
must be held and will be dropped and reacquired as needed to allocate
group storage space in
.Fa cr .
.Pp
The
.Fn crdup
function allocates memory for a new structure and copies the
contents of
.Fa cr
into it.
The actual copying is performed by
.Fn crcopy .
.Pp
The
.Fn crsetgroups
function sets the
.Va cr_groups
and
.Va 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
.Va cr_groups
array except for updating the primary group via assignment to
.Va cr_groups[0] .
.Pp
The
.Fn cru2x
function converts a
.Vt ucred
structure to an
.Vt xucred
structure.
That is,
it copies data from
.Fa cr
to
.Fa xcr ;
it ignores fields in the former that are not present in the latter
(e.g.,
.Va cr_uidinfo ) ,
and appropriately sets fields in the latter that are not present in
the former
(e.g.,
.Va cr_version ) .
.Pp
.Sh RETURN VALUES
.Fn crget ,
.Fn crhold ,
.Fn crdup ,
and
.Fn crcopysafe
all return a pointer to a
.Vt ucred
structure.
.Sh USAGE NOTES
As of
.Fx 5.0 ,
the
.Vt 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
.Fn crget ,
.Fn crcopy ,
and
.Fn crcopysafe .
.Pp
In the common case, credentials required for access control decisions are
used in a read-only manner.
In these circumstances, the thread credential
.Va 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.
.Pp
During a process credential update, the process lock must be held across
check and update, to prevent race conditions.
The process credential,
.Va 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.
.Pp
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
.Fn crget
and
.Fn crcopy .
.Pp
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
.Em never
be used as the target credential in an access control decision: the process
credential associated with the thread,
.Va td->td_proc->p_ucred ,
should be used instead.
For example,
.Xr p_candebug 9
accepts a target process, not a target thread, for access control purposes.
.Sh SEE ALSO
.Xr uihold 9
.Sh AUTHORS
This manual page was written by
.An Chad David Aq Mt davidc@acns.ab.ca .