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182 lines
7.6 KiB
182 lines
7.6 KiB
=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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BIO_METHOD *BIO_s_bio(void); |
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#define BIO_make_bio_pair(b1,b2) (int)BIO_ctrl(b1,BIO_C_MAKE_BIO_PAIR,0,b2) |
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#define BIO_destroy_bio_pair(b) (int)BIO_ctrl(b,BIO_C_DESTROY_BIO_PAIR,0,NULL) |
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#define BIO_shutdown_wr(b) (int)BIO_ctrl(b, BIO_C_SHUTDOWN_WR, 0, NULL) |
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#define BIO_set_write_buf_size(b,size) (int)BIO_ctrl(b,BIO_C_SET_WRITE_BUF_SIZE,size,NULL) |
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#define BIO_get_write_buf_size(b,size) (size_t)BIO_ctrl(b,BIO_C_GET_WRITE_BUF_SIZE,size,NULL) |
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int BIO_new_bio_pair(BIO **bio1, size_t writebuf1, BIO **bio2, size_t writebuf2); |
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#define BIO_get_write_guarantee(b) (int)BIO_ctrl(b,BIO_C_GET_WRITE_GUARANTEE,0,NULL) |
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size_t BIO_ctrl_get_write_guarantee(BIO *b); |
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#define BIO_get_read_request(b) (int)BIO_ctrl(b,BIO_C_GET_READ_REQUEST,0,NULL) |
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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() 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() 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() less than the amount requested or if the |
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buffer is full request a retry. BIO_ctrl_get_write_guarantee() is a function |
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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() and a response read with BIO_read(), this can occur during an |
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TLS/SSL handshake for example. BIO_write() will succeed and place data in the write |
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buffer. BIO_read() will initially fail and BIO_should_read() will be true. If |
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the application then waits for data to be available on the underlying transport |
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before flushing the write buffer it will never succeed because the request was |
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never sent! |
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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 EXAMPLE |
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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(); |
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... |
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application | TLS-engine |
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| | |
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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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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 non-blocking 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 transfered 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 WARNING |
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As the data is buffered, SSL_operation() may return with a 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)|SSL_set_bio(3)>, L<ssl(3)|ssl(3)>, L<bio(3)|bio(3)>, |
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L<BIO_should_retry(3)|BIO_should_retry(3)>, L<BIO_read(3)|BIO_read(3)> |
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=cut
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