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Let MBEDTLS_SSL_MAX_CONTENT_LEN to be split into outward & inward sizes

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For the situation where the mbedTLS device has limited RAM, but the
other end of the connection doesn't support the max_fragment_length
extension. To be spec-compliant, mbedTLS has to keep a 16384 byte
incoming buffer. However the outgoing buffer can be made smaller without
breaking spec compliance, and we save some RAM.

See comments in include/mbedtls/config.h for some more details.

(The lower limit of outgoing buffer size is the buffer size used during
handshake/cert negotiation. As the handshake is half-duplex it might
even be possible to store this data in the "incoming" buffer during the
handshake, which would save even more RAM - but it would also be a lot
hackier and error-prone. I didn't really explore this possibility, but
thought I'd mention it here in case someone sees this later on a mission
to jam mbedTLS into an even tinier RAM footprint.)
This commit is contained in:
Angus Gratton 2016-05-25 20:56:48 +10:00
parent 2dbecc04cc
commit d8213d00db
7 changed files with 198 additions and 87 deletions
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63
include/mbedtls/ssl_internal.h
View file

@ -143,32 +143,73 @@
#define MBEDTLS_SSL_PADDING_ADD 0
#endif
#define MBEDTLS_SSL_PAYLOAD_LEN ( MBEDTLS_SSL_MAX_CONTENT_LEN \
+ MBEDTLS_SSL_COMPRESSION_ADD \
+ MBEDTLS_MAX_IV_LENGTH \
+ MBEDTLS_SSL_MAC_ADD \
+ MBEDTLS_SSL_PADDING_ADD \
)
#define MBEDTLS_SSL_PAYLOAD_OVERHEAD ( MBEDTLS_SSL_COMPRESSION_ADD + \
MBEDTLS_MAX_IV_LENGTH + \
MBEDTLS_SSL_MAC_ADD + \
MBEDTLS_SSL_PADDING_ADD \
)
#define MBEDTLS_SSL_IN_PAYLOAD_LEN ( MBEDTLS_SSL_PAYLOAD_OVERHEAD + \
( MBEDTLS_SSL_IN_CONTENT_LEN ) )
#define MBEDTLS_SSL_OUT_PAYLOAD_LEN ( MBEDTLS_SSL_PAYLOAD_OVERHEAD + \
( MBEDTLS_SSL_OUT_CONTENT_LEN ) )
/* Maximum length we can advertise as our max content length for
RFC 6066 max_fragment_length extension negotiation purposes
(the lesser of both sizes, if they are unequal.)
*/
#define MBEDTLS_TLS_EXT_ADV_CONTENT_LEN ( \
(MBEDTLS_SSL_IN_CONTENT_LEN > MBEDTLS_SSL_OUT_CONTENT_LEN) \
? ( MBEDTLS_SSL_OUT_CONTENT_LEN ) \
: ( MBEDTLS_SSL_IN_CONTENT_LEN ) \
)
/*
* Check that we obey the standard's message size bounds
*/
#if MBEDTLS_SSL_MAX_CONTENT_LEN > 16384
#error Bad configuration - record content too large.
#error "Bad configuration - record content too large."
#endif
#if MBEDTLS_SSL_PAYLOAD_LEN > 16384 + 2048
#error Bad configuration - protected record payload too large.
#if MBEDTLS_SSL_IN_CONTENT_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN
#error "Bad configuration - incoming record content should not be larger than MBEDTLS_SSL_MAX_CONTENT_LEN."
#endif
#if MBEDTLS_SSL_OUT_CONTENT_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN
#error "Bad configuration - outgoing record content should not be larger than MBEDTLS_SSL_MAX_CONTENT_LEN."
#endif
#if MBEDTLS_SSL_IN_PAYLOAD_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN + 2048
#error "Bad configuration - incoming protected record payload too large."
#endif
#if MBEDTLS_SSL_OUT_PAYLOAD_LEN > MBEDTLS_SSL_MAX_CONTENT_LEN + 2048
#error "Bad configuration - outgoing protected record payload too large."
#endif
/* Calculate buffer sizes */
/* Note: Even though the TLS record header is only 5 bytes
long, we're internally using 8 bytes to store the
implicit sequence number. */
#define MBEDTLS_SSL_HEADER_LEN 13
#define MBEDTLS_SSL_BUFFER_LEN \
( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_PAYLOAD_LEN ) )
#define MBEDTLS_SSL_IN_BUFFER_LEN \
( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_IN_PAYLOAD_LEN ) )
#define MBEDTLS_SSL_OUT_BUFFER_LEN \
( ( MBEDTLS_SSL_HEADER_LEN ) + ( MBEDTLS_SSL_OUT_PAYLOAD_LEN ) )
#ifdef MBEDTLS_ZLIB_SUPPORT
/* Compression buffer holds both IN and OUT buffers, so should be size of the larger */
#define MBEDTLS_SSL_COMPRESS_BUFFER_LEN ( \
( MBEDTLS_SSL_IN_BUFFER_LEN > MBEDTLS_SSL_OUT_BUFFER_LEN ) \
? MBEDTLS_SSL_IN_BUFFER_LEN \
: MBEDTLS_SSL_OUT_BUFFER_LEN \
)
#endif
/*
* TLS extension flags (for extensions with outgoing ServerHello content