linux-kernel - Re: Weird Kconfig condition (IPV6 [=n] || IPV6 [=n]=n)

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Date:	Mon, 14 Sep 2015 12:57:19 +0200
From:	Andreas Ziegler <andreas.ziegler@....de>
To:	Mason <slash.tmp@...e.fr>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Weird Kconfig condition (IPV6 [=n] || IPV6 [=n]=n)

Hi,

On 09/14/2015 10:41, Mason wrote:
> Hello,
> 
> I did make menuconfig in v4.2
> 
> In the help for CONFIG_CRYPTO_HMAC, I have
> 
>   │   Selected by: IP_SCTP [=n] && NET [=y] && INET [=y] && (IPV6 [=n] || IPV6 [=n]=n) || INET_AH [=n] && NET [=y] && INET [=y] || \              │  
>   │ INET_ESP [=n] && NET [=y] && INET [=y] || INET6_AH [=n] && NET [=y] && INET [=y] && IPV6 [=n] || INET6_ESP [=n] && NET [=y] && INET [=y]\     │  
>   │  && IPV6 [=n] || SCTP_COOKIE_HMAC_MD5 [=n] && NET [=y] && IP_SCTP [=n] && SCTP_COOKIE_HMAC_MD5 [=n] || SCTP_COOKIE_HMAC_SHA1 [=n] && \        │  
>   │ NET [=y] && IP_SCTP [=n] && SCTP_COOKIE_HMAC_SHA1 [=n] || CIFS [=n] && NETWORK_FILESYSTEMS [=y] && INET [=y] || TRUSTED_KEYS [=n] && \        │  
>   │ KEYS [=y] && TCG_TPM [=n] || ENCRYPTED_KEYS [=n] && KEYS [=y] || IMA [=n] && INTEGRITY [=n] || EVM [=n] && INTEGRITY [=n] || \                │  
>   │ CRYPTO_DRBG_HMAC [=y] && CRYPTO [=y] && CRYPTO_DRBG_MENU [=m] || CRYPTO_DEV_OMAP_SHAM [=n] && CRYPTO [=y] && CRYPTO_HW [=y] && \              │  
>   │ ARCH_OMAP2PLUS [=n] || CRYPTO_DEV_QAT [=n] && CRYPTO [=y] && CRYPTO_HW [=y]                                                                   │  
> 
> 
> What does (IPV6 [=n] || IPV6 [=n]=n) mean?
> Is this a bug in my configuration?

No, this comes from the definition of IP_SCTP in net/sctp/Kconfig.
 menuconfig IP_SCTP
      tristate "The SCTP Protocol"
      depends on INET
      depends on IPV6 || IPV6=n
		 ^^^^^^^^^^^^^^

This weird-looking condition has the effect that:
- if IPV6 is disabled, IP_SCTP can be anything ("y","m" or "n")
- if IPV6 is enabled to "y", IP_SCTP can also be set to anything.
- (and this is the reason why it's there:) if IPV6 is a module ("m"),
IP_SCTP can only be "m" or "n".

In a way, the "depends on" does not only reference the symbol, but also
keeps track of the 'state' of the symbol it references, such that if the
dependency (here: IPV6) is to be built as a loadable module only, the
dependant feature can also only be built as a loadable module.

I can't find the Documentation for that at the moment, but I'm sure that
I read this at some point, maybe also only on a mailing list...

Regards,

Andreas

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