linux-kernel - Re: [patch 3/5] x86: headers cleanup

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Date:	Thu, 15 Jan 2009 09:55:23 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	"Metzger, Markus T" <markus.t.metzger@...el.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"harvey.harrison@...il.com" <harvey.harrison@...il.com>,
	"sam@...nborg.org" <sam@...nborg.org>,
	"jaswinderrajput@...il.com" <jaswinderrajput@...il.com>
Subject: Re: [patch 3/5] x86: headers cleanup - ptrace-abi.h

Cyrill Gorcunov wrote:
> 
> Let me explain more detailed what I've meant with my prev message.
> Markus said that ptrace_bts_config is supposed to be visible in userspace,
> right? So I've a program which is built under kernel with
> CONFIG_X86_PTRACE_BTS turned on. Then I rebuild my kernel with
> CONFIG_X86_PTRACE_BTS turned off. Then I decide to recompile
> my program for some reason _and_ compilation shouldn't fail
> because of lack of ptrace_bts_config struct. So for userspace
> kernel configuration does matter if it touches data being referenced
> from user-side. BUT all I said is valid (at least I hop so) if _only_
> ptrace_bts_config is supposed to be visible to user-space programs.
> 

CONFIG_* is not visible to userspace.  Furthermore, there is (almost) no
point in putting a structure definition under #ifdef unless it uses data
types that somehow depend on the configuration (and those data
structures would be fundamentally ineligible to be exported to
userspace!!) -- if the feature isn't configured the structure definition
just doesn't get used.

> | 
> | This would be yet another good reason why having them be always defined
> | and 0/1 instead would be such an improvement, but we're not there.
> | 
> 
> oh, that reminds me autoconf horror :) I don't know if it possible
> but we could have some common/base file with all CONFIG_ set to 0/1
> which any header being exported to userspace should include, or
> we could modify unifdef to process headers in fashion: scan the header,
> insert CONFIG_'s refered in the header with value 0/1 at top of the
> header. Not sure if it worth it (too many files are to be touched).
> 

No, no, no, no, no.

We're not exporting CONFIG_* to userspace.

The point was that if we were using #if instead of #ifdef, then -Wundef
could be used to complain instead of silently ignoring sections.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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