202 lines
8.0 KiB
Plaintext
202 lines
8.0 KiB
Plaintext
=pod
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=head1 NAME
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BIO_s_bio, BIO_make_bio_pair, BIO_destroy_bio_pair, BIO_shutdown_wr,
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BIO_set_write_buf_size, BIO_get_write_buf_size, BIO_new_bio_pair,
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BIO_get_write_guarantee, BIO_ctrl_get_write_guarantee, BIO_get_read_request,
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BIO_ctrl_get_read_request, BIO_ctrl_reset_read_request - BIO pair BIO
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=head1 SYNOPSIS
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#include <openssl/bio.h>
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const BIO_METHOD *BIO_s_bio(void);
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int BIO_make_bio_pair(BIO *b1, BIO *b2);
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int BIO_destroy_bio_pair(BIO *b);
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int BIO_shutdown_wr(BIO *b);
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int BIO_set_write_buf_size(BIO *b, long size);
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size_t BIO_get_write_buf_size(BIO *b, long size);
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int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2);
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int BIO_get_write_guarantee(BIO *b);
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size_t BIO_ctrl_get_write_guarantee(BIO *b);
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int BIO_get_read_request(BIO *b);
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size_t BIO_ctrl_get_read_request(BIO *b);
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int BIO_ctrl_reset_read_request(BIO *b);
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=head1 DESCRIPTION
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BIO_s_bio() returns the method for a BIO pair. A BIO pair is a pair of source/sink
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BIOs where data written to either half of the pair is buffered and can be read from
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the other half. Both halves must usually by handled by the same application thread
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since no locking is done on the internal data structures.
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Since BIO chains typically end in a source/sink BIO it is possible to make this
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one half of a BIO pair and have all the data processed by the chain under application
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control.
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One typical use of BIO pairs is to place TLS/SSL I/O under application control, this
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can be used when the application wishes to use a non standard transport for
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TLS/SSL or the normal socket routines are inappropriate.
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Calls to BIO_read_ex() will read data from the buffer or request a retry if no
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data is available.
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Calls to BIO_write_ex() will place data in the buffer or request a retry if the
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buffer is full.
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The standard calls BIO_ctrl_pending() and BIO_ctrl_wpending() can be used to
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determine the amount of pending data in the read or write buffer.
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BIO_reset() clears any data in the write buffer.
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BIO_make_bio_pair() joins two separate BIOs into a connected pair.
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BIO_destroy_pair() destroys the association between two connected BIOs. Freeing
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up any half of the pair will automatically destroy the association.
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BIO_shutdown_wr() is used to close down a BIO B<b>. After this call no further
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writes on BIO B<b> are allowed (they will return an error). Reads on the other
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half of the pair will return any pending data or EOF when all pending data has
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been read.
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BIO_set_write_buf_size() sets the write buffer size of BIO B<b> to B<size>.
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If the size is not initialized a default value is used. This is currently
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17K, sufficient for a maximum size TLS record.
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BIO_get_write_buf_size() returns the size of the write buffer.
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BIO_new_bio_pair() combines the calls to BIO_new(), BIO_make_bio_pair() and
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BIO_set_write_buf_size() to create a connected pair of BIOs B<bio1>, B<bio2>
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with write buffer sizes B<writebuf1> and B<writebuf2>. If either size is
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zero then the default size is used. BIO_new_bio_pair() does not check whether
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B<bio1> or B<bio2> do point to some other BIO, the values are overwritten,
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BIO_free() is not called.
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BIO_get_write_guarantee() and BIO_ctrl_get_write_guarantee() return the maximum
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length of data that can be currently written to the BIO. Writes larger than this
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value will return a value from BIO_write_ex() less than the amount requested or
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if the buffer is full request a retry. BIO_ctrl_get_write_guarantee() is a
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function whereas BIO_get_write_guarantee() is a macro.
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BIO_get_read_request() and BIO_ctrl_get_read_request() return the
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amount of data requested, or the buffer size if it is less, if the
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last read attempt at the other half of the BIO pair failed due to an
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empty buffer. This can be used to determine how much data should be
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written to the BIO so the next read will succeed: this is most useful
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in TLS/SSL applications where the amount of data read is usually
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meaningful rather than just a buffer size. After a successful read
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this call will return zero. It also will return zero once new data
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has been written satisfying the read request or part of it.
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Note that BIO_get_read_request() never returns an amount larger
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than that returned by BIO_get_write_guarantee().
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BIO_ctrl_reset_read_request() can also be used to reset the value returned by
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BIO_get_read_request() to zero.
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=head1 NOTES
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Both halves of a BIO pair should be freed. That is even if one half is implicit
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freed due to a BIO_free_all() or SSL_free() call the other half needs to be freed.
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When used in bidirectional applications (such as TLS/SSL) care should be taken to
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flush any data in the write buffer. This can be done by calling BIO_pending()
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on the other half of the pair and, if any data is pending, reading it and sending
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it to the underlying transport. This must be done before any normal processing
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(such as calling select() ) due to a request and BIO_should_read() being true.
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To see why this is important consider a case where a request is sent using
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BIO_write_ex() and a response read with BIO_read_ex(), this can occur during an
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TLS/SSL handshake for example. BIO_write_ex() will succeed and place data in the
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write buffer. BIO_read_ex() will initially fail and BIO_should_read() will be
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true. If the application then waits for data to be available on the underlying
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transport before flushing the write buffer it will never succeed because the
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request was never sent!
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BIO_eof() is true if no data is in the peer BIO and the peer BIO has been
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shutdown.
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BIO_make_bio_pair(), BIO_destroy_bio_pair(), BIO_shutdown_wr(),
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BIO_set_write_buf_size(), BIO_get_write_buf_size(),
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BIO_get_write_guarantee(), and BIO_get_read_request() are implemented
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as macros.
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=head1 RETURN VALUES
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BIO_new_bio_pair() returns 1 on success, with the new BIOs available in
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B<bio1> and B<bio2>, or 0 on failure, with NULL pointers stored into the
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locations for B<bio1> and B<bio2>. Check the error stack for more information.
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[XXXXX: More return values need to be added here]
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=head1 EXAMPLES
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The BIO pair can be used to have full control over the network access of an
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application. The application can call select() on the socket as required
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without having to go through the SSL-interface.
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BIO *internal_bio, *network_bio;
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...
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BIO_new_bio_pair(&internal_bio, 0, &network_bio, 0);
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SSL_set_bio(ssl, internal_bio, internal_bio);
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SSL_operations(); /* e.g. SSL_read and SSL_write */
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...
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application | TLS-engine
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+----------> SSL_operations()
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| /\ ||
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| || \/
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| BIO-pair (internal_bio)
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| BIO-pair (network_bio)
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| || /\
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| \/ ||
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+-----------< BIO_operations()
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socket
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...
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SSL_free(ssl); /* implicitly frees internal_bio */
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BIO_free(network_bio);
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...
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As the BIO pair will only buffer the data and never directly access the
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connection, it behaves nonblocking and will return as soon as the write
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buffer is full or the read buffer is drained. Then the application has to
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flush the write buffer and/or fill the read buffer.
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Use the BIO_ctrl_pending(), to find out whether data is buffered in the BIO
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and must be transferred to the network. Use BIO_ctrl_get_read_request() to
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find out, how many bytes must be written into the buffer before the
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SSL_operation() can successfully be continued.
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=head1 WARNINGS
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As the data is buffered, SSL_operation() may return with an ERROR_SSL_WANT_READ
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condition, but there is still data in the write buffer. An application must
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not rely on the error value of SSL_operation() but must assure that the
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write buffer is always flushed first. Otherwise a deadlock may occur as
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the peer might be waiting for the data before being able to continue.
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=head1 SEE ALSO
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L<SSL_set_bio(3)>, L<ssl(7)>, L<bio(7)>,
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L<BIO_should_retry(3)>, L<BIO_read_ex(3)>
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=head1 COPYRIGHT
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Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). 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 LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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