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Date: Mon, 25 Nov 2024 09:24:28 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>, x86-ml <x86@kernel.org>,
	lkml <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] x86/boot: Get rid of linux/init.h include
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* Borislav Petkov <bp@alien8.de> wrote:

> On Fri, Nov 22, 2024 at 05:55:52PM +0100, Ingo Molnar wrote:
> > > --- a/arch/x86/boot/compressed/head_32.S
> > > +++ b/arch/x86/boot/compressed/head_32.S
> > > @@ -24,7 +24,6 @@
> > >   */
> > >  	.text
> > >  
> > > -#include <linux/init.h>
> > >  #include <linux/linkage.h>
> > >  #include <asm/segment.h>
> > >  #include <asm/page_types.h>
> > > @@ -32,6 +31,10 @@
> > >  #include <asm/asm-offsets.h>
> > >  #include <asm/bootparam.h>
> > >  
> > > +#ifdef KERNEL_PROPER_HEADER
> > > +#error Do not include kernel proper namespace headers
> > > +#endif
> > 
> > The canonical solution in such cases is to use the existing header 
> > guard, ie:
> > 
> >  #ifdef _LINUX_INIT_H
> >  # error Do not include kernel proper namespace headers
> >  #endif
> > 
> > Then we can skip defining KERNEL_PROPER_HEADER as well, and this change 
> > will be purely to x86 code.
> 
> Yap, I know, thought about it.
> 
> However, if we have to protect against every header, then we will have to do
> a big
> 
> if defined...
> 
> which doesn't really work.
> 
> For the above example:
> 
> #if defined(_LINUX_INIT_H) || defined(_LINUX_LINKAGE_H)
> 
> and that would protect against the two headers which are included here.
> 
> If someone includes another one, it won't fire.

And if someone doesn't add the ugly KERNEL_PROPER_HEADER defines to a 
new header that somehow gets included into the decompressor build 
virally, it won't fire either. I think it's better to concentrate the 
uglies in the 'weird' code, ie. the decompressor.

Also, what's the root problem being solved? The changelog says:

   > no collisions and ugly ifdeffery when those kernel proper headers 
   > get shared.

But that's pretty vague - is there some recent build regression this is 
responding to? Which kernel headers collided with which headers used by 
the decompressor build?

Thanks,

	Ingo