On Wed, 12 Feb 2020, Matthieu Baerts wrote:

> Without this modification and if CRYPTO is not selected, we have this
> warning:
>
>  WARNING: unmet direct dependencies detected for CRYPTO_LIB_SHA256
>    Depends on [n]: CRYPTO [=n]
>    Selected by [y]:
>    - MPTCP [=y] && NET [=y] && INET [=y]
>
> MPTCP selects CRYPTO_LIB_SHA256 which depends on CRYPTO. CRYPTO is now
> selected to avoid this issue.
>
> Fixes: 65492c5a6ab5 (mptcp: move from sha1 (v0) to sha256 (v1))
> Signed-off-by: Matthieu Baerts <matthieu.baerts(a)tessares.net>
> ---
> net/mptcp/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/net/mptcp/Kconfig b/net/mptcp/Kconfig
> index 49f6054e7f4e..a9ed3bf1d93f 100644
> --- a/net/mptcp/Kconfig
> +++ b/net/mptcp/Kconfig
> @@ -4,6 +4,7 @@ config MPTCP
> 	depends on INET
> 	select SKB_EXTENSIONS
> 	select CRYPTO_LIB_SHA256
> +	select CRYPTO
> 	help
> 	  Multipath TCP (MPTCP) connections send and receive data over multiple
> 	  subflows in order to utilize multiple network paths. Each subflow
> -- 
> 2.25.0

Even though the config system prints that warning, it looks like sha256 is 
compiled and linked even without CONFIG_CRYPTO. Since we end up needing 
CONFIG_CRYPTO anyway for crypto_memneq, it's not bad to include it now, 
but it doesn't seem to be strictly necessary.

--
Mat Martineau
Intel