tree ad0b7f2faf6420554a4c848a9d76544bffee957e
parent 014f683ca95a987c203a24beb84b48840e73a3ec
author Hanno Becker <hanno.becker@arm.com> 1612767270 +0000
committer Hanno Becker <hanno.becker@arm.com> 1617024018 +0100

Don't invalidate MPS reader buffers upon commit call

Previously, the semantics of mbedtls_mps_reader_commit() was to invalidate
all buffers previously fetched via mbedtls_mps_reader_get(), forbidding
any further use by the 'consumer'. This was in fact a necessary constraint
for the current implementation, which did some memory moving in
mbedtls_mps_reader_commit().

This commit simplifies the reader's semantics and implementation in
the following way:

- API: A call to mbedtls_mps_reader_commit() does no longer invalidate
       the buffers previously obtained via mbedtls_mps_reader_get().
       Instead, they can continue to be used until
       mbedtls_mps_reader_reclaim() is called.

       Calling mbedtls_mps_reader_commit() now only sets a marker
       indicating which parts of the data received through
       mbedtls_mps_reader_get() need not be backed up once
       mbedtls_mps_reader_reclaim() is called. Allowing the user
       to call mbedtls_mbedtls_reader_commit() multiple times
       before mbedtls_mps_reader_reclaim() is mere convenience:
       We'd get exactly the same functionality if instead of
       mbedtls_mps_reader_commit(), there was an additional argument
       to mbedtls_mps_reader_reclaim() indicating how much data
       to retain. However, the present design is more convenient
       for the user and doesn't appear to introduce any unnecessary
       complexity (anymore), so we stick with it for now.

- Implementation: mbedtls_mps_reader_commit() is now a 1-liner,
                  setting the 'commit-marker', but doing nothing else.

                  Instead, the complexity of mbedtls_mp_reader_reclaim()
                  slightly increases because it has to deal with creating
                  backups from both the accumulator and the current
                  fragment. In the previous implementation, which shifted
                  the accumulator content with every call to
                  mbedtls_mps_reader_commit(), only the backup from the
                  fragment was necessary; with the new implementation
                  which doesn't shift anything in
                  mbedtls_mps_reader_commit(), we need to do the
                  accumulator shift in mbedtls_mps_reader_reclaim().

Signed-off-by: Hanno Becker <hanno.becker@arm.com>