doc/man7/openssl-threads.pod

   1 =pod
   2
   3 =head1 NAME
   4
   5 openssl-threads - Overview of thread safety in OpenSSL
   6
   7 =head1 DESCRIPTION
   8
   9 In this man page, we use the term B<thread-safe> to indicate that an
  10 object or function can be used by multiple threads at the same time.
  11
  12 OpenSSL can be built with or without threads support. The most important
  13 use of this support is so that OpenSSL itself can use a single consistent
  14 API, as shown in L<CRYPTO_THREAD_run_once(3)/EXAMPLES>.
  15 Multi-platform applications can also use this API.
  16
  17 In particular, being configured for threads support does not imply that
  18 all OpenSSL objects are thread-safe.
  19 To emphasize: I<most objects are not safe for simultaneous use>.
  20 Exceptions to this should be documented on the specific manual pages, and
  21 some general high-level guidance is given here.
  22
  23 One major use of the OpenSSL thread API is to implement reference counting.
  24 Many objects within OpenSSL are reference-counted, so resources are not
  25 released, until the last reference is removed.
  26 References are often increased automatically (such as when an B<X509>
  27 certificate object is added into an B<X509_STORE> trust store).
  28 There is often an B<I<object>_up_ref>() function that can be used to increase
  29 the reference count.
  30 Failure to match B<I<object>_up_ref>() calls with the right number of
  31 B<I<object>_free>() calls is a common source of memory leaks when a program
  32 exits.
  33
  34 Many objects have set and get API's to set attributes in the object.
  35 A C<set0> passes ownership from the caller to the object and a
  36 C<get0> returns a pointer but the attribute ownership
  37 remains with the object and a reference to it is returned.
  38 A C<set1> or C<get1> function does not change the ownership, but instead
  39 updates the attribute's reference count so that the object is shared
  40 between the caller and the object; the caller must free the returned
  41 attribute when finished.
  42 Functions that involve attributes that have reference counts themselves,
  43 but are named with just C<set> or C<get> are historical; and the documentation
  44 must state how the references are handled.
  45 Get methods are often thread-safe as long as the ownership requirements are
  46 met and shared objects are not modified.
  47 Set methods, or modifying shared objects, are generally not thread-safe
  48 as discussed below.
  49
  50 Objects are thread-safe
  51 as long as the API's being invoked don't modify the object; in this
  52 case the parameter is usually marked in the API as C<const>.
  53 Not all parameters are marked this way.
  54 Note that a C<const> declaration does not mean immutable; for example
  55 L<X509_cmp(3)> takes pointers to C<const> objects, but the implementation
  56 uses a C cast to remove that so it can lock objects, generate and cache
  57 a DER encoding, and so on.
  58
  59 Another instance of thread-safety is when updates to an object's
  60 internal state, such as cached values, are done with locks.
  61 One example of this is the reference counting API's described above.
  62
  63 In all cases, however, it is generally not safe for one thread to
  64 mutate an object, such as setting elements of a private or public key,
  65 while another thread is using that object, such as verifying a signature.
  66
  67 The same API's can usually be used simultaneously on different objects
  68 without interference.
  69 For example, two threads can calculate a signature using two different
  70 B<EVP_PKEY_CTX> objects.
  71
  72 For implicit global state or singletons, thread-safety depends on the facility.
  73 The L<CRYPTO_secure_malloc(3)> and related API's have their own lock,
  74 while L<CRYPTO_malloc(3)> assumes the underlying platform allocation
  75 will do any necessary locking.
  76 Some API's, such as L<NCONF_load(3)> and related do no locking at all;
  77 this can be considered a bug.
  78
  79 A separate, although related, issue is modifying "factory" objects
  80 when other objects have been created from that.
  81 For example, an B<SSL_CTX> object created by L<SSL_CTX_new(3)> is used
  82 to create per-connection B<SSL> objects by calling L<SSL_new(3)>.
  83 In this specific case, and probably for factory methods in general, it is
  84 not safe to modify the factory object after it has been used to create
  85 other objects.
  86
  87 =head1 SEE ALSO
  88
  89 CRYPTO_THREAD_run_once(3),
  90 local system threads documentation.
  91
  92 =head1 BUGS
  93
  94 This page is admittedly very incomplete.
  95
  96 =head1 COPYRIGHT
  97
  98 Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
  99
 100 Licensed under the Apache License 2.0 (the "License").  You may not use
 101 this file except in compliance with the License.  You can obtain a copy
 102 in the file LICENSE in the source distribution or at
 103 L<https://www.openssl.org/source/license.html>.
 104
 105 =cut