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

From: Yinghai Lu
Date: Wed Apr 09 2008 - 13:59:51 EST


On Wed, Apr 9, 2008 at 8:08 AM, Alexander van Heukelum
<heukelum@xxxxxxxxxxx> wrote:
>
> On Tue, 8 Apr 2008 10:54:15 -0700, "Yinghai Lu" <yhlu.kernel@xxxxxxxxx>
> said:
>
> > On Tue, Apr 8, 2008 at 1:23 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
> > >
> > > * Alexander van Heukelum <heukelum@xxxxxxxxxxxxx> 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.
>
> Hello Yinghai Lu,
>
> Indeed, I now think it is not needed either. The decompression is
> done in-place nowadays: the (compressed) image is moved to a high
> memory address first, then the decompression is done starting at
> the low end of the buffer. It is guaranteed that the output never
> overwrites the input, and the decompression code, the stack, and
> the heap are all at higher addresses than the input buffer. The
> same goes for the pagetables needed for x86_64.
>
>
> > 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.
> > ....
>
> No, that size is accounted for automatically: the code computes the
> buffer size needed (including slack) minus the buffer size that is
> already available (in the embedded gzip-file). The image is moved
> by this amount (rounded up to a page). So that part is fine.

just wonder if Ingo have very big vmlinux, that +32K + 18 formula still works.

YH
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