1.\" 2.\" Copyright (C) 2001 Chad David <davidc@acns.ab.ca>. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice(s), this list of conditions and the following disclaimer as 9.\" the first lines of this file unmodified other than the possible 10.\" addition of one or more copyright notices. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice(s), this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 16.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 17.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 18.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY 19.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 20.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 21.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 22.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH 25.\" DAMAGE. 26.\" 27.\" $FreeBSD$ 28.\" 29.Dd January 23, 2019 30.Dt UCRED 9 31.Os 32.Sh NAME 33.Nm ucred , 34.Nm crget , 35.Nm crhold , 36.Nm crfree , 37.Nm crcopy , 38.Nm crdup , 39.Nm cru2x 40.Nd "functions related to user credentials" 41.Sh SYNOPSIS 42.In sys/param.h 43.In sys/ucred.h 44.Ft "struct ucred *" 45.Fn crget void 46.Ft "struct ucred *" 47.Fn crhold "struct ucred *cr" 48.Ft void 49.Fn crfree "struct ucred *cr" 50.Ft void 51.Fn crcopy "struct ucred *dest" "struct ucred *src" 52.Ft "struct ucred *" 53.Fn crcopysafe "struct proc *p" "struct ucred *cr" 54.Ft "struct ucred *" 55.Fn crdup "struct ucred *cr" 56.Ft void 57.Fn crsetgroups "struct ucred *cr" "int ngrp" "gid_t *groups" 58.Ft void 59.Fn cru2x "struct ucred *cr" "struct xucred *xcr" 60.Sh DESCRIPTION 61The 62.Nm 63family of functions is used to manage user credential structures 64.Pq Vt "struct ucred" 65within the kernel. 66.Pp 67The 68.Fn crget 69function allocates memory 70for a new structure, sets its reference count to 1, and 71initializes its lock. 72.Pp 73The 74.Fn crhold 75function increases the reference count on the credential. 76.Pp 77The 78.Fn crfree 79function decreases the reference count on the credential. 80If the count drops to 0, the storage for the structure is freed. 81.Pp 82The 83.Fn crcopy 84function copies the contents of the source (template) 85credential into the destination template. 86The 87.Vt uidinfo 88structure within the destination is referenced 89by calling 90.Xr uihold 9 . 91.Pp 92The 93.Fn crcopysafe 94function copies the current credential associated with the process 95.Fa p 96into the newly allocated credential 97.Fa cr . 98The process lock on 99.Fa p 100must be held and will be dropped and reacquired as needed to allocate 101group storage space in 102.Fa cr . 103.Pp 104The 105.Fn crdup 106function allocates memory for a new structure and copies the 107contents of 108.Fa cr 109into it. 110The actual copying is performed by 111.Fn crcopy . 112.Pp 113The 114.Fn crsetgroups 115function sets the 116.Va cr_groups 117and 118.Va cr_ngroups 119variables and allocates space as needed. 120It also truncates the group list to the current maximum number of 121groups. 122No other mechanism should be used to modify the 123.Va cr_groups 124array except for updating the primary group via assignment to 125.Va cr_groups[0] . 126.Pp 127The 128.Fn cru2x 129function converts a 130.Vt ucred 131structure to an 132.Vt xucred 133structure. 134That is, 135it copies data from 136.Fa cr 137to 138.Fa xcr ; 139it ignores fields in the former that are not present in the latter 140(e.g., 141.Va cr_uidinfo ) , 142and appropriately sets fields in the latter that are not present in 143the former 144(e.g., 145.Va cr_version ) . 146.Sh RETURN VALUES 147.Fn crget , 148.Fn crhold , 149.Fn crdup , 150and 151.Fn crcopysafe 152all return a pointer to a 153.Vt ucred 154structure. 155.Sh USAGE NOTES 156As of 157.Fx 5.0 , 158the 159.Vt ucred 160structure contains extensible fields. 161This means that the correct protocol must always be followed to create 162a fresh and writable credential structure: new credentials must always 163be derived from existing credentials using 164.Fn crget , 165.Fn crcopy , 166and 167.Fn crcopysafe . 168.Pp 169In the common case, credentials required for access control decisions are 170used in a read-only manner. 171In these circumstances, the thread credential 172.Va td_ucred 173should be used, as it requires no locking to access safely, and remains stable 174for the duration of the call even in the face of a multi-threaded 175application changing the process credentials from another thread. 176.Pp 177During a process credential update, the process lock must be held across 178check and update, to prevent race conditions. 179The process credential, 180.Va td->td_proc->p_ucred , 181must be used both for check and update. 182If a process credential is updated during a system call and checks against 183the thread credential are to be made later during the same system call, 184the thread credential must also be refreshed from the process credential 185so as to prevent use of a stale value. 186To avoid this scenario, it is recommended that system calls updating the 187process credential be designed to avoid other authorization functions. 188.Pp 189If temporarily elevated privileges are required for a thread, the thread 190credential can be replaced for the duration of an activity, or for 191the remainder of the system call. 192However, as a thread credential is often shared, appropriate care should be 193taken to make sure modifications are made to a writable credential 194through the use of 195.Fn crget 196and 197.Fn crcopy . 198.Pp 199Caution should be exercised when checking authorization for a thread or 200process perform an operation on another thread or process. 201As a result of temporary elevation, the target thread credential should 202.Em never 203be used as the target credential in an access control decision: the process 204credential associated with the thread, 205.Va td->td_proc->p_ucred , 206should be used instead. 207For example, 208.Xr p_candebug 9 209accepts a target process, not a target thread, for access control purposes. 210.Sh SEE ALSO 211.Xr uihold 9 212.Sh AUTHORS 213This manual page was written by 214.An Chad David Aq Mt davidc@acns.ab.ca . 215