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29 .Dd September 27, 2017
40 .Nm cred_update_thread
41 .Nd "functions related to user credentials"
48 .Fn crhold "struct ucred *cr"
50 .Fn crfree "struct ucred *cr"
52 .Fn crcopy "struct ucred *dest" "struct ucred *src"
54 .Fn crcopysafe "struct proc *p" "struct ucred *cr"
56 .Fn crdup "struct ucred *cr"
58 .Fn crsetgroups "struct ucred *cr" "int ngrp" "gid_t *groups"
60 .Fn cru2x "struct ucred *cr" "struct xucred *xcr"
62 .Fn cred_update_thread "struct thread *td"
66 family of functions is used to manage user credential structures
72 function allocates memory
73 for a new structure, sets its reference count to 1, and
78 function increases the reference count on the credential.
82 function decreases the reference count on the credential.
83 If the count drops to 0, the storage for the structure is freed.
87 function copies the contents of the source (template)
88 credential into the destination template.
91 structure within the destination is referenced
97 function copies the current credential associated with the process
99 into the newly allocated credential
103 must be held and will be dropped and reacquired as needed to allocate
104 group storage space in
109 function allocates memory for a new structure and copies the
113 The actual copying is performed by
122 variables and allocates space as needed.
123 It also truncates the group list to the current maximum number of
125 No other mechanism should be used to modify the
127 array except for updating the primary group via assignment to
142 it ignores fields in the former that are not present in the latter
145 and appropriately sets fields in the latter that are not present in
151 .Fn cred_update_thread
152 function sets the credentials of
154 to that of its process, freeing its old credential if required.
161 all return a pointer to a
169 structure contains extensible fields.
170 This means that the correct protocol must always be followed to create
171 a fresh and writable credential structure: new credentials must always
172 be derived from existing credentials using
178 In the common case, credentials required for access control decisions are
179 used in a read-only manner.
180 In these circumstances, the thread credential
182 should be used, as it requires no locking to access safely, and remains stable
183 for the duration of the call even in the face of a multi-threaded
184 application changing the process credentials from another thread.
186 During a process credential update, the process lock must be held across
187 check and update, to prevent race conditions.
188 The process credential,
189 .Va td->td_proc->p_ucred ,
190 must be used both for check and update.
191 If a process credential is updated during a system call and checks against
192 the thread credential are to be made later during the same system call,
193 the thread credential must also be refreshed from the process credential
194 so as to prevent use of a stale value.
195 To avoid this scenario, it is recommended that system calls updating the
196 process credential be designed to avoid other authorization functions.
198 If temporarily elevated privileges are required for a thread, the thread
199 credential can by replaced for the duration of an activity, or for
200 the remainder of the system call.
201 However, as a thread credential is often shared, appropriate care should be
202 taken to make sure modifications are made to a writable credential
208 Caution should be exercised when checking authorization for a thread or
209 process perform an operation on another thread or process.
210 As a result of temporary elevation, the target thread credential should
212 be used as the target credential in an access control decision: the process
213 credential associated with the thread,
214 .Va td->td_proc->p_ucred ,
215 should be used instead.
218 accepts a target process, not a target thread, for access control purposes.
222 This manual page was written by
223 .An Chad David Aq Mt davidc@acns.ab.ca .