linux-kernel - About support XZ-compressed kernel on x86

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Message-ID: <20160212153407.GA2731@x1.redhat.com>
Date:	Fri, 12 Feb 2016 23:34:07 +0800
From:	Baoquan He <bhe@...hat.com>
To:	lasse.collin@...aani.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, alain@...ff.lu,
	albin.tonnerre@...e-electrons.com, phillip@...gher.demon.co.uk,
	akpm@...ux-foundation.org, keescook@...omium.org, bp@...en8.de,
	vgoyal@...hat.com
Subject: About support XZ-compressed kernel on x86

Hi Lasse,

I am checking a thread related to kaslr and setup_data issue on x86:
https://lkml.org/lkml/2015/7/7/963

Now I have a question about the commit from you:

commit 303148045aac34b70db722a54e5ad94a3a6625c6
Author: Lasse Collin <lasse.collin@...aani.org>
Date:   Wed Jan 12 17:01:24 2011 -0800

    x86: support XZ-compressed kernel

In this commit for adding support of XZ-compressed kernel on x86, you
add extra 32K to the extract_offset. In commit log you said this is
because "The XZ decompressor needs around 30 KiB of heap, so the heap
size is increased to 32 KiB on both x86-32 and x86-64." With my
understanding decompression is done in decompression stage and it uses
boot_heap in arch/x86/boot/compressed/head_64.S, and boot_heap is
assigned to free_mem_ptr which is used for decompression heap malloc.
During this decompressio stage it's still in copied ZO space, why did
you add extra 32K space to extract_offset?  If you want to increase the
decompression heap space shouldn't you decrease the extract_offset? Do I
misunderstand anything or miss things?

Please help answer, I really don't understand this.

Thanks
Baoquan