linux-kernel - Re: [PATCH 1/2] boot: increase stack size for kernel boot loader decompressor

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Date:	Tue, 8 Apr 2008 10:54:15 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Alexander van Heukelum" <heukelum@...lshack.com>,
	"Mike Travis" <travis@....com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, heukelum@...tmail.fm
Subject: Re: [PATCH 1/2] boot: increase stack size for kernel boot loader decompressor

On Tue, Apr 8, 2008 at 1:23 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
>  * Alexander van Heukelum <heukelum@...lshack.com> wrote:
>
>  > I did see that the malloc space that the inflate code is using is
>  > taken from _after_ the end of the bss. I don't see how this is
>  > protected from being used/overwritten. Changing the stack size changes
>  > the memory layout a bit... maybe you were so unlucky to create a
>  > vmlinux image that was just barely smaller than some threshold and
>  > increasing the stack size made the decompression/relocation area be
>  > located somewhere else?
>  >
>  > Test patch follows.
>
>  that's a really interesting theory.
>
>  FWIIW, i've been booting allyesconfig bzImages for a long time (with
>  only minimal amount of drivers disabled - mostly old ISA ones that
>  assume the presence of the real hardware), and they boot and work fine
>  on both 32-bit and 64-bit typical whitebox PCs. That means huge bzImages
>  that decompresses into a ~41 MB kernel image. I'd expect that to be a
>  rather severe test of the decompressor.

i don't that Alexander's patch is needed.

also because Alex move heap before _end,
we may need add some extra for buffer offset

        /* Replace the compressed data size with the uncompressed size */
        subl    input_len(%ebp), %ebx
        movl    output_len(%ebp), %eax
        addl    %eax, %ebx
        /* Add 8 bytes for every 32K input block */
        shrl    $12, %eax
        addl    %eax, %ebx
        /* Add 32K + 18 bytes of extra slack and align on a 4K boundary */
        addl    $(32768 + 18 + 4095), %ebx
        andl    $~4095, %ebx =============================> need add
heap size too.
....



        /* Replace the compressed data size with the uncompressed size */
        movl    input_len(%rip), %eax
        subq    %rax, %rbx
        movl    output_len(%rip), %eax
        addq    %rax, %rbx
        /* Add 8 bytes for every 32K input block */
        shrq    $12, %rax
        addq    %rax, %rbx
        /* Add 32K + 18 bytes of extra slack and align on a 4K boundary */
        addq    $(32768 + 18 + 4095), %rbx
=============================> need add heap size too.
        andq    $~4095, %rbx

do we need to move pgtable before _end?

YH
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