linux-kernel - CONFIG_* used by user-space to figure out whether a feature is on/off

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Message-ID: <20130715154050.GA5941@phenom.dumpdata.com>
Date:	Mon, 15 Jul 2013 11:40:50 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: CONFIG_* used by user-space to figure out whether a feature is on/off

Hey Linus,

I am hoping you can help me draw an understanding and a line in sand whether:
 a) Tools should not depend on /proc/config.gz to figure out whether
    a kernel has some CONFIG_X=y feature.

 b) If they are OK to do so, what do we do when certain CONFIG_X options
    get reworked/removed. Would they be considered regressions? Aka
    is this similar to 'you shall not break user-space'?


Irrespective of that, do you have any ideas of how a user-space program (say GRUB)
can figure out whether the configuration stanze it generates is supported by
the kernel. If you don't want to answer this question - since this might
open a can of worms you prefer not to deal with - that is absolutly OK.

Folks have been tossing ideas such as:
 - Let the user deal with it and if it does not boot - oh well.
 - readelf or objdump.
 - use /boot/config-<kernel>-<version> as most (all?) distros stick that in
   there.

Thanks!
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