linux-kernel - Re: [PATCH RFC 1/7] x86/head/64: Mark startup_gdt and startup_gdt_descr as _

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Date:   Tue, 17 Oct 2023 15:02:27 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Hou Wenlong <houwenlong.hwl@...group.com>
Cc:     linux-kernel@...r.kernel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE 32-BIT AND 64-BIT" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Mike Rapoport <rppt@...nel.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>
Subject: Re: [PATCH RFC 1/7] x86/head/64: Mark startup_gdt and
 startup_gdt_descr as __initdata


* Hou Wenlong <houwenlong.hwl@...group.com> wrote:

> Hi Ingo,
> 
> I have sent patch #6 separately for x86. Do you have any ideas about 
> building the head code as PIE? Should I resend the patchset for the PIE 
> feature?

So I had a brief look, and despite reading 0/43 it was unclear to me what 
the precise advantages of building as PIE are.

Ie. could you please outline:

 - *Exactly* how much PIE based KASLR randomization would gain us in terms 
   of randomization granularity and effective number of randomization bits, 
   compared to the current status quo?

 - How is code generation changed at the instruction level - how does 
   kernel size change and what are the micro-advantages/disadvantages?

 - Are there any other advantages/motivation than improving KASLR?

Ie. before asking us to apply ~50 patches and add a whole new build mode 
and the maintainance overhead to support it into infinity and beyond, could 
you please offer a better list of pros and cons?

Thanks,

	Ingo