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While stereotyped repetitions are frowned upon in literature, they serve a useful purpose in manual pages, because it is easier for the user to find certain information if it is always presented in the same way. For that reason, this commit harmonizes the varying formulations in the HISTORY section about which functions, flags, etc. were added in which OpenSSL version. It also attempts to make the pod files more grep friendly by avoiding to insert line breaks between the symbol names and the corresponding version number in which they were introduced (wherever possible). Some punctuation and typographical errors were fixed on the way. Reviewed-by: Tim Hudson <tjh@openssl.org> (Merged from https://github.com/openssl/openssl/pull/7854)
153 lines
6.1 KiB
Plaintext
153 lines
6.1 KiB
Plaintext
=pod
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=head1 NAME
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SSL_read_ex, SSL_read, SSL_peek_ex, SSL_peek
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- read bytes from a TLS/SSL connection
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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int SSL_read(SSL *ssl, void *buf, int num);
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int SSL_peek_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);
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int SSL_peek(SSL *ssl, void *buf, int num);
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=head1 DESCRIPTION
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SSL_read_ex() and SSL_read() try to read B<num> bytes from the specified B<ssl>
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into the buffer B<buf>. On success SSL_read_ex() will store the number of bytes
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actually read in B<*readbytes>.
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SSL_peek_ex() and SSL_peek() are identical to SSL_read_ex() and SSL_read()
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respectively except no bytes are actually removed from the underlying BIO during
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the read, so that a subsequent call to SSL_read_ex() or SSL_read() will yield
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at least the same bytes.
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=head1 NOTES
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In the paragraphs below a "read function" is defined as one of SSL_read_ex(),
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SSL_read(), SSL_peek_ex() or SSL_peek().
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If necessary, a read function will negotiate a TLS/SSL session, if not already
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explicitly performed by L<SSL_connect(3)> or L<SSL_accept(3)>. If the
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peer requests a re-negotiation, it will be performed transparently during
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the read function operation. The behaviour of the read functions depends on the
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underlying BIO.
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For the transparent negotiation to succeed, the B<ssl> must have been
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initialized to client or server mode. This is being done by calling
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L<SSL_set_connect_state(3)> or SSL_set_accept_state() before the first
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invocation of a read function.
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The read functions work based on the SSL/TLS records. The data are received in
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records (with a maximum record size of 16kB). Only when a record has been
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completely received, can it be processed (decryption and check of integrity).
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Therefore data that was not retrieved at the last read call can still be
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buffered inside the SSL layer and will be retrieved on the next read
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call. If B<num> is higher than the number of bytes buffered then the read
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functions will return with the bytes buffered. If no more bytes are in the
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buffer, the read functions will trigger the processing of the next record.
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Only when the record has been received and processed completely will the read
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functions return reporting success. At most the contents of one record will
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be returned. As the size of an SSL/TLS record may exceed the maximum packet size
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of the underlying transport (e.g. TCP), it may be necessary to read several
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packets from the transport layer before the record is complete and the read call
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can succeed.
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If B<SSL_MODE_AUTO_RETRY> has been switched off and a non-application data
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record has been processed, the read function can return and set the error to
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B<SSL_ERROR_WANT_READ>.
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In this case there might still be unprocessed data available in the B<BIO>.
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If read ahead was set using L<SSL_CTX_set_read_ahead(3)>, there might also still
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be unprocessed data available in the B<SSL>.
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This behaviour can be controlled using the L<SSL_CTX_set_mode(3)> call.
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If the underlying BIO is B<blocking>, a read function will only return once the
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read operation has been finished or an error occurred, except when a
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non-application data record has been processed and B<SSL_MODE_AUTO_RETRY> is
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not set.
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Note that if B<SSL_MODE_AUTO_RETRY> is set and only non-application data is
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available the call will hang.
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If the underlying BIO is B<non-blocking>, a read function will also return when
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the underlying BIO could not satisfy the needs of the function to continue the
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operation.
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In this case a call to L<SSL_get_error(3)> with the
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return value of the read function will yield B<SSL_ERROR_WANT_READ> or
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B<SSL_ERROR_WANT_WRITE>.
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As at any time it's possible that non-application data needs to be sent,
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a read function can also cause write operations.
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The calling process then must repeat the call after taking appropriate action
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to satisfy the needs of the read function.
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The action depends on the underlying BIO.
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When using a non-blocking socket, nothing is to be done, but select() can be
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used to check for the required condition.
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When using a buffering BIO, like a BIO pair, data must be written into or
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retrieved out of the BIO before being able to continue.
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L<SSL_pending(3)> can be used to find out whether there
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are buffered bytes available for immediate retrieval.
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In this case the read function can be called without blocking or actually
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receiving new data from the underlying socket.
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=head1 RETURN VALUES
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SSL_read_ex() and SSL_peek_ex() will return 1 for success or 0 for failure.
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Success means that 1 or more application data bytes have been read from the SSL
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connection.
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Failure means that no bytes could be read from the SSL connection.
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Failures can be retryable (e.g. we are waiting for more bytes to
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be delivered by the network) or non-retryable (e.g. a fatal network error).
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In the event of a failure call L<SSL_get_error(3)> to find out the reason which
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indicates whether the call is retryable or not.
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For SSL_read() and SSL_peek() the following return values can occur:
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=over 4
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=item E<gt> 0
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The read operation was successful.
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The return value is the number of bytes actually read from the TLS/SSL
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connection.
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=item Z<><= 0
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The read operation was not successful, because either the connection was closed,
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an error occurred or action must be taken by the calling process.
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Call L<SSL_get_error(3)> with the return value B<ret> to find out the reason.
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Old documentation indicated a difference between 0 and -1, and that -1 was
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retryable.
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You should instead call SSL_get_error() to find out if it's retryable.
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=back
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=head1 HISTORY
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The SSL_read_ex() and SSL_peek_ex() functions were added in OpenSSL 1.1.1.
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=head1 SEE ALSO
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L<SSL_get_error(3)>, L<SSL_write_ex(3)>,
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L<SSL_CTX_set_mode(3)>, L<SSL_CTX_new(3)>,
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L<SSL_connect(3)>, L<SSL_accept(3)>
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L<SSL_set_connect_state(3)>,
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L<SSL_pending(3)>,
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L<SSL_shutdown(3)>, L<SSL_set_shutdown(3)>,
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L<ssl(7)>, L<bio(7)>
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=head1 COPYRIGHT
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Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the Apache License 2.0 (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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