237 lines
8.8 KiB
Plaintext
237 lines
8.8 KiB
Plaintext
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.\" Automatically generated by Pod::Man 2.27 (Pod::Simple 3.28)
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\{\
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.rm #[ #] #H #V #F C
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.\" ========================================================================
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.\"
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.IX Title "OPENSSL-THREADS 7ossl"
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.TH OPENSSL-THREADS 7ossl "2024-01-30" "3.2.1" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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openssl\-threads \- Overview of thread safety in OpenSSL
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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In this man page, we use the term \fBthread-safe\fR to indicate that an
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object or function can be used by multiple threads at the same time.
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.PP
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OpenSSL can be built with or without threads support. The most important
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use of this support is so that OpenSSL itself can use a single consistent
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\&\s-1API,\s0 as shown in \*(L"\s-1EXAMPLES\*(R"\s0 in \fICRYPTO_THREAD_run_once\fR\|(3).
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Multi-platform applications can also use this \s-1API.\s0
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.PP
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In particular, being configured for threads support does not imply that
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all OpenSSL objects are thread-safe.
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To emphasize: \fImost objects are not safe for simultaneous use\fR.
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Exceptions to this should be documented on the specific manual pages, and
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some general high-level guidance is given here.
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.PP
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One major use of the OpenSSL thread \s-1API\s0 is to implement reference counting.
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Many objects within OpenSSL are reference-counted, so resources are not
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released, until the last reference is removed.
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References are often increased automatically (such as when an \fBX509\fR
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certificate object is added into an \fBX509_STORE\fR trust store).
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There is often an \fB\f(BIobject\fB_up_ref\fR() function that can be used to increase
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the reference count.
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Failure to match \fB\f(BIobject\fB_up_ref\fR() calls with the right number of
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\&\fB\f(BIobject\fB_free\fR() calls is a common source of memory leaks when a program
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exits.
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.PP
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Many objects have set and get \s-1API\s0's to set attributes in the object.
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A \f(CW\*(C`set0\*(C'\fR passes ownership from the caller to the object and a
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\&\f(CW\*(C`get0\*(C'\fR returns a pointer but the attribute ownership
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remains with the object and a reference to it is returned.
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A \f(CW\*(C`set1\*(C'\fR or \f(CW\*(C`get1\*(C'\fR function does not change the ownership, but instead
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updates the attribute's reference count so that the object is shared
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between the caller and the object; the caller must free the returned
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attribute when finished.
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Functions that involve attributes that have reference counts themselves,
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but are named with just \f(CW\*(C`set\*(C'\fR or \f(CW\*(C`get\*(C'\fR are historical; and the documentation
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must state how the references are handled.
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Get methods are often thread-safe as long as the ownership requirements are
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met and shared objects are not modified.
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Set methods, or modifying shared objects, are generally not thread-safe
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as discussed below.
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.PP
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Objects are thread-safe
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as long as the \s-1API\s0's being invoked don't modify the object; in this
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case the parameter is usually marked in the \s-1API\s0 as \f(CW\*(C`const\*(C'\fR.
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Not all parameters are marked this way.
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Note that a \f(CW\*(C`const\*(C'\fR declaration does not mean immutable; for example
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\&\fIX509_cmp\fR\|(3) takes pointers to \f(CW\*(C`const\*(C'\fR objects, but the implementation
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uses a C cast to remove that so it can lock objects, generate and cache
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a \s-1DER\s0 encoding, and so on.
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.PP
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Another instance of thread-safety is when updates to an object's
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internal state, such as cached values, are done with locks.
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One example of this is the reference counting \s-1API\s0's described above.
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.PP
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In all cases, however, it is generally not safe for one thread to
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mutate an object, such as setting elements of a private or public key,
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while another thread is using that object, such as verifying a signature.
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.PP
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The same \s-1API\s0's can usually be used simultaneously on different objects
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without interference.
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For example, two threads can calculate a signature using two different
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\&\fB\s-1EVP_PKEY_CTX\s0\fR objects.
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.PP
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For implicit global state or singletons, thread-safety depends on the facility.
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The \fICRYPTO_secure_malloc\fR\|(3) and related \s-1API\s0's have their own lock,
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while \fICRYPTO_malloc\fR\|(3) assumes the underlying platform allocation
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will do any necessary locking.
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Some \s-1API\s0's, such as \fINCONF_load\fR\|(3) and related do no locking at all;
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this can be considered a bug.
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.PP
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A separate, although related, issue is modifying \*(L"factory\*(R" objects
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when other objects have been created from that.
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For example, an \fB\s-1SSL_CTX\s0\fR object created by \fISSL_CTX_new\fR\|(3) is used
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to create per-connection \fB\s-1SSL\s0\fR objects by calling \fISSL_new\fR\|(3).
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In this specific case, and probably for factory methods in general, it is
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not safe to modify the factory object after it has been used to create
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other objects.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fICRYPTO_THREAD_run_once\fR\|(3),
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local system threads documentation.
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.SH "BUGS"
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.IX Header "BUGS"
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This page is admittedly very incomplete.
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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2021 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the Apache License 2.0 (the \*(L"License\*(R"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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