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1 [May be out of date. Last significant update: Jan 2005.]
3 In general, it's assumed that the library initialization function (if
4 initialization isn't delayed) and the library finalization function
5 are run in some thread-safe fashion, with no other parts of the
6 library in question in use. (If dlopen or dlsym in one thread starts
7 running the initializer, and then dlopen/dlsym in another thread
8 returns and lets you start accessing functions or data in the library
9 before the initializer is finished, that really seems like a
10 dlopen/dlsym bug.)
12 It's also assumed that if library A depends on library B, then library
13 B's initializer runs first, and its finalizer last, whether loading
14 dynamically at run time or at process startup/exit. (It appears that
15 AIX 4.3.3 may violate this, at least when we use gcc's
16 constructor/destructor attributes in shared libraries.)
18 Support for freeing the heap storage allocated by a library has NOT,
19 in general, been written. There are hooks, but often they ignore some
20 of the library's local storage, mutexes, etc.
22 If shared library finalization code doesn't get run at all at dlclose
23 time, or if we can't use it because the execution order is wrong, then
24 you'll get memory leaks. Deal with it.
26 Several debugging variables that are not part of our official API are
27 not protected by mutexes. In general, the only way to set them is by
28 changing the sources and recompiling, which obviously has no run-time
29 thread safety issues, or by stopping the process under a debugger,
30 which we blithely assert is "safe enough".
32 Debug code that we don't normally enable may be less thread safe than
33 might be desired. For example, multiple printf calls may be made,
34 with the assumption that the output will not be intermixed with output
35 from some other thread. Offhand, I'm not aware of any cases where
36 debugging code is "really" unsafe, as in likely to crash the program
37 or produce insecure results.
39 Various libraries may call assert() and abort(). This should only be
40 for "can't happen" cases, and indicate programming errors. In some
41 cases, the compiler may be able to infer that the "can't happen" cases
42 really can't happen, and drop the calls, but in many cases, this is
43 not possible.
45 There are cases (e.g., in the com_err library) where errors arising
46 when dealing with other errors are handled by calling abort, for lack
47 of anything better. We should probably clean those up someday.
49 Various libraries call getenv(). This is perfectly safe, as long as
50 nothing is calling setenv or putenv or what have you, while multiple
51 threads are executing. Of course, that severely curtails the ability
52 to control our libraries through that "interface".
54 Various libraries call the ctype functions/macros (isupper, etc). It
55 is assumed that the program does not call setlocale, or does so only
56 while the program is still single-threaded or while calls into the
57 Kerberos libraries are not in progress.
59 The Windows thread safety support is unfinished.
61 I'm assuming that structure fields that are never written to (e.g.,
62 after a structure has been initialized and *then* made possibly
63 visible to multiple threads) are safe to read from one thread while
64 another field is being updated by another thread. If that's not the
65 case, some more work is needed (and I'd like details on why it's not
74 The callback hook support (set_com_err_hook, reset_com_err_hook, and
75 calls to com_err and com_err_va) uses a mutex to protect the handle on
76 the hook function. As a side effect of this, if a callback function
77 is registered which pops up a window and waits for the users'
78 acknowledgement, then other errors cannot be reported by other threads
79 until after the acknowledgement. This could be fixed with
80 multiple-reader-one-writer type locks, but that's a bit more
83 The string returned by error_message may be per-thread storage. It
84 can be passed off between threads, but it shouldn't be in use by any
85 thread by the time the originating thread calls error_message again.
87 Error tables must no longer be in use (including pointers returned by
88 error_message) when the library containing them is unloaded.
90 Temporary: A flag variable has been created in error_message.c which
91 is used to try to catch cases where remove_error_table is called after
92 the library finalization function. This generally indicates
93 out-of-order execution of the library finalization functions. The
94 handling of this flag is not thread-safe, but if the finalization
95 function is called, other threads should in theory be finished with
96 this library anyways.
98 Statics: error_message.c, com_err.c, covered above.
102 libprofile (and its use in libkrb5)
104 Does no checks to see if it's opened multiple instances of the same
105 file under different names. Does not guard against trying to open a
106 file while another thread or process is in the process of replacing
107 it, or two threads trying to update a file at the same time. The
108 former should be pretty safe on UNIX with atomic rename, but on
109 Windows there's a race condition; there's a window (so to speak) where
110 the filename does not correspond to an actual file.
112 Statics: prof_file.c, a list of opened config files and their parse
113 trees, and a mutex to protect it.
119 Uses of the Yarrow code from the krb5 crypto interface are protected
120 by a single mutex. Initialization of the Yarrow state will be done
121 once, the first time these routines are called. Calls directly to the
122 Yarrow functions are not protected.
124 Uses ctype macros; what happens if the locale is changed in a
125 multi-threaded program?
127 Debug var in pbkdf2.c.
129 Statics: pbkdf2.c: debug variable.
131 Statics: prng.c: Global Yarrow data and mutex.
133 Statics: crypto_libinit.c: library initializer aux data.
141 Uses: ctype macros
143 Uses: getaddrinfo, getnameinfo. According to current specifications,
144 getaddrinfo should be thread-safe; some implementations are not, and
145 we're not attempting to figure out which ones. NetBSD 1.6, for
146 example, had an unsafe implementation.
148 Uses: res_ninit, res_nsearch. If these aren't available, the non-'n'
149 versions will be used, and they are sometimes not thread-safe.
151 Uses: mkstemp, mktemp -- Are these, or our uses of them, likely to be
154 Uses: sigaction
156 The use of sigaction is in the code prompting for a password; we try
157 to catch the keyboard interrupt character being used and turn it into
158 an error return from that function. THIS IS NOT THREAD-SAFE.
160 Uses: tcgetattr, tcsetattr. This is also in the password-prompting
161 code. These are fine as long as no other threads are accessing the
162 same terminal at the same time.
164 Uses: fopen. This is thread-safe, actually, but a multi-threaded
165 server is likely to be using lots of file descriptors. On 32-bit
166 Solaris platforms, fopen will not work if the next available file
167 descriptor number is 256 or higher. This can cause the keytab code to
170 Statics: prompter.c: interrupt flag
172 Statics: ccdefops.c: default operations table pointer
174 Statics: ktdefname.c: variable to override default keytab name, NO
175 LOCKING. DON'T TOUCH THESE VARIABLES, at least in threaded programs.
177 Statics: conv_creds.c: debug variable
179 Statics: sendto_kdc.c: debug variable, in export list for KDC
181 Statics: parse.c: default realm cache, changed to not cache
183 Statics: krb5_libinit.c: lib init aux data
185 Statics: osconfig.c: various internal variables, probably should be const
187 Statics: init_ctx.c: "brand" string; not written.
189 Statics: cc_memory.c: list of caches, with mutex.
191 Statics: c_ustime.c: last timestamp, to implement "microseconds must
192 always increment"
194 Statics: ktbase.c, ccbase.c, rc_base.c: type registries and mutexes.
202 Uses: ctype macros
204 Statics: acquire_cred.c: name of keytab to use, and mutex.
206 Statics: gssapi_krb5.c:
208 Statics: init_sec_context.c:
210 Statics: set_ccache.c:
212 Statics: gssapi_generic.c: OID definitions, non-const by
213 specification. We probably could make them const anyways.
215 The keytab name saved away by krb5_gss_register_acceptor_identity is
216 global and protected by a mutex; the ccache name stored by
217 gss_krb5_ccache_name is per-thread. This inconsistency is due to the
218 anticipated usage patterns.
220 The old ccache name returned by gss_krb5_ccache_name if the last
221 parameter is not a null pointer is also stored per-thread, and will be
222 discarded at the next call to that routine from the same thread, or at
223 thread termination.
225 Needs work: check various objects for thread safety
231 New version is in place. Ignore it for now?
238 Skip these for now. We may want the KDC libraries to be thread-safe
239 eventually, so the KDC can take better advantage of hyperthreaded or
240 multiprocessor systems.
247 Used by single-threaded programs only (but see above re KDC). Don't
248 bother for now.