Reorder fields mbedtls_ssl_handshake_params in order to save code on Arm
Thumb builds. The general idea is to put often-used fields in the direct
access window of 128 elements from the beginning of the structure.
The reordering is a human selection based on a report of field offset and
use counts, and informed by measuring the code size with various
arrangements. Some notes:
* I moved most byte-sized fields at the beginning where they're sure to be
in the direct access window.
* I moved buffering earlier because it can be around the threshold depending
on the configuration, and it's accessed in a lot of places.
* I moved several fields, including update_checksum and friends, early so
that they're guaranteed to be in the early access window.
* I tried moving randbytes or premaster to the early access window, but
I couldn't find a placement which would save code size, presumably because
they're bumping too many other fields, and they're mostly accessed through
memcpy and friends which translates to instructions that don't have an
offset for free anyway.
Results (arm-none-eabi-gcc 7.3.1, build_arm_none_eabi_gcc_m0plus build):
library/ssl_cli.o: 20200 -> 20104 (diff: 96)
library/ssl_msg.o: 25978 -> 25942 (diff: 36)
library/ssl_srv.o: 22691 -> 22467 (diff: 224)
library/ssl_tls.o: 23570 -> 23390 (diff: 180)
Results (same architecture, config-suite-b.h + MBEDTLS_ECDH_LEGACY_CONTEXT +
MBEDTLS_ECP_RESTARTABLE):
library/ssl_cli.o: 3012 -> 2928 (diff: 84)
library/ssl_msg.o: 2932 -> 2924 (diff: 8)
library/ssl_srv.o: 3288 -> 3232 (diff: 56)
library/ssl_tls.o: 6032 -> 5904 (diff: 128)
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Replace bitfields mbedtls_ssl_handshake_params by bytes. This saves some
code size, and since the bitfields weren't group, this doesn't increase the
RAM usage.
Replace several ints that only store values in the range 0..255 by uint8_t.
This can increase or decrease the code size depending on the architecture
and on how the field is used. I chose changes that save code size on Arm
Thumb builds and may potentially save more after field reordering.
Leave the bitfields in struct mbedtls_ssl_hs_buffer alone: replacing them by
uint8_t slightly increases the code size.
Results (arm-none-eabi-gcc 7.3.1, build_arm_none_eabi_gcc_m0plus build):
library/ssl_srv.o: 22735 -> 22691 (diff: 44)
library/ssl_tls.o: 23566 -> 23570 (diff: -4)
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move fields around to have fewer accesses outside the 128-element Thumb
direct access window.
In psa_crypto.c's global_data, put the state fields first (-20).
In psa_crypto_slot_management.c's global_data, keep the key slots first
(otherwise it's +24).
In mbedtls_psa_random_context_t, swapping entropy and drbg makes no
difference (at least when the DRBG is mbedtls_ctr_drbg_context).
Results (arm-none-eabi-gcc 7.3.1, build_arm_none_eabi_gcc_m0plus build):
library/psa_crypto.o: 16166 -> 16146 (diff: 20)
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move fields around to have fewer accesses outside the 128-element Thumb
direct access window.
In psa_hkdf_key_derivation_t, move the large fields (output_block, prk,
hmac) after the state bit-fields. Experimentally, it's slightly better
to put hmac last.
In aead_operation_t, tag_length was outside the window. The details depend
on the sizes of contexts included in ctx. Make the large ctx be the last
field.
In mbedtls_psa_hmac_operation_t, the opad field is outside the window when
SHA-512 is enabled. Moving opad before hash_ctx only saves 4 bytes and made
the structure clumsy, so I left it alone.
Results (arm-none-eabi-gcc 7.3.1, build_arm_none_eabi_gcc_m0plus build):
library/psa_crypto.o: 16246 -> 16166 (diff: 80)
library/psa_crypto_aead.o: 952 -> 928 (diff: 24)
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This comment is about how the functions are implemented, not about their
public interface, so it doesn't belong in the header file.
It applies to everything in constant_time.c so moved there.
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
The equality checker functions always return 0 or 1 value,
thus the type of return value can be the same dispite of the
size of the parameters.
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
Tne unpadding part of `mbedtls_rsa_rsaes_pkcs1_v15_decrypt` function is
contant-time therefore it moved to a separate function to be prepared
for moving to the contant-time module.
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
Add a constant-time function with size_t parameter for choosing
between two integer values, like the ?: ternary operator.
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
All function declaration provided by ssl_invasive.h is needed only for
testing purposes and all of them are provided by constant_time.h as well.
Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
