OPENSSL-THREADS(7ossl) OpenSSL OPENSSL-THREADS(7ossl)
NAME
openssl-threads - Overview of thread safety in OpenSSL
DESCRIPTION
In this man page, we use the term thread-safe to indicate that an
object or function can be used by multiple threads at the same time.
OpenSSL can be built with or without threads support. The most
important use of this support is so that OpenSSL itself can use a
single consistent API, as shown in "EXAMPLES" in
CRYPTO_THREAD_run_once(3). Multi-platform applications can also use
this API.
In particular, being configured for threads support does not imply that
all OpenSSL objects are thread-safe. To emphasize: most objects are
not safe for simultaneous use. Exceptions to this should be documented
on the specific manual pages, and some general high-level guidance is
given here.
One major use of the OpenSSL thread API is to implement reference
counting. Many objects within OpenSSL are reference-counted, so
resources are not released, until the last reference is removed.
References are often increased automatically (such as when an X509
certificate object is added into an X509_STORE trust store). There is
often an object_up_ref() function that can be used to increase the
reference count. Failure to match object_up_ref() calls with the right
number of object_free() calls is a common source of memory leaks when a
program exits.
Many objects have set and get API's to set attributes in the object. A
"set0" passes ownership from the caller to the object and a "get0"
returns a pointer but the attribute ownership remains with the object
and a reference to it is returned. A "set1" or "get1" function does
not change the ownership, but instead updates the attribute's reference
count so that the object is shared between the caller and the object;
the caller must free the returned attribute when finished. Functions
that involve attributes that have reference counts themselves, but are
named with just "set" or "get" are historical; and the documentation
must state how the references are handled. Get methods are often
thread-safe as long as the ownership requirements are met and shared
objects are not modified. Set methods, or modifying shared objects,
are generally not thread-safe as discussed below.
Objects are thread-safe as long as the API's being invoked don't modify
the object; in this case the parameter is usually marked in the API as
"const". Not all parameters are marked this way. Note that a "const"
declaration does not mean immutable; for example X509_cmp(3) takes
pointers to "const" objects, but the implementation uses a C cast to
remove that so it can lock objects, generate and cache a DER encoding,
and so on.
Another instance of thread-safety is when updates to an object's
internal state, such as cached values, are done with locks. One
example of this is the reference counting API's described above.
In all cases, however, it is generally not safe for one thread to
mutate an object, such as setting elements of a private or public key,
while another thread is using that object, such as verifying a
signature.
The same API's can usually be used simultaneously on different objects
without interference. For example, two threads can calculate a
signature using two different EVP_PKEY_CTX objects.
For implicit global state or singletons, thread-safety depends on the
facility. The CRYPTO_secure_malloc(3) and related API's have their own
lock, while CRYPTO_malloc(3) assumes the underlying platform allocation
will do any necessary locking. Some API's, such as NCONF_load(3) and
related do no locking at all; this can be considered a bug.
A separate, although related, issue is modifying "factory" objects when
other objects have been created from that. For example, an SSL_CTX
object created by SSL_CTX_new(3) is used to create per-connection SSL
objects by calling SSL_new(3). In this specific case, and probably for
factory methods in general, it is not safe to modify the factory object
after it has been used to create other objects.
SEE ALSO
CRYPTO_THREAD_run_once(3), local system threads documentation.
BUGS
This page is admittedly very incomplete.
COPYRIGHT
Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.3.2 2024-09-04 OPENSSL-THREADS(7ossl)
openssl 3.3.2 - Generated Tue Oct 1 16:26:40 CDT 2024