Goal

Reduce the extension bitfield size to the actual size needed to hold all supported extensions

Why

S2N_SUPPORTED_EXTENSIONS_BITFIELD_LEN is intended to hold the number of bytes required to assign one bit to each of the TLS extensions supported by s2n-tls. It does this by dividing the number of supported extensions (S2N_SUPPORTED_EXTENSIONS_COUNT) by the size of a char type (sizeof(char)). sizeof(char) always equals 1, so given S2N_SUPPORTED_EXTENSIONS_COUNT=21, this calculation results in 21 / 1 + 1 = 22 bytes.

The actual number of bytes needed to hold 21 extensions would be 21 / (number of bits in a byte) + 1 = 3.

How

Instead of dividing by sizeof(char) (1), divide by CHAR_BIT (8)

Callouts

Since s2n_connection has 3 uses of extension bitfields, that was resulting in a total allocation of 22 * 3 = 66 bytes for each connection. With the updated calculation of 3 bytes, that results in 9 bytes allocated for extensions, a savings of 57 bytes. Therefore, the max_connection_size in s2n_connection_size_test has been decreased by 57 bytes.

Testing

Existing tests pass

Related

release summary: each connection now uses 57 less bytes

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